Biodeterioration

of ConCrete
Biodeterioration
of ConCrete

Thomas Dyer
University of Dundee
Division of Civil Engineering
Dundee, Scotland, UK

p,
A SCIENCE PUBLISHERS BOOK
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Title:
Title:Tropical
Title: Tropical pinnipeds
Biodeterioration
pinnipedsof :: bio-ecology, threats,
threats, and
concrete / Thomas
bio-ecology, conservation
Dyer,
and University /of
conservation / editor,
Dundee
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Juan Joseì Alava,
Joseì of
Division CivilFaculty
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Oceans and
theUK. and
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 To
Judith, Angus and Oscar
Preface

Awareness of the importance of ensuring durability of concrete has been a

growing concern for engineers. There is a fairly good understanding of the
mechanisms which cause its deterioration and the ways of limiting such
damage through the use of appropriate materials and approaches to design.
Many of the deterioration mechanisms which affect concrete are
the result of interaction with the non-living environment—chlorides in
seawater, carbon dioxide in the atmosphere, cyclic freezing and thawing.
However, living organisms can also cause damage—through both chemical
and physical processes—which, under the right conditions, can be severe.
This book examines all forms of concrete biodeterioration together
for the first time. It examines, from a fundamental starting point,
biodeterioration mechanisms, as well as the conditions which allow living
organisms (bacteria, fungi, plants and a range of marine organisms) to
colonise concrete.
It also includes a detailed examination of chemical compounds
produced by living organisms with respect to their interaction with the
mineral constituents of cement and concrete, and the implications this has
for the integrity of structures.
Approaches to avoiding biodeterioration of concrete are also covered,
including selection of materials, mix proportioning, design, and the use of
protective systems.
Contents

Dedication v
Preface vii
1. Introduction 1
2. The Chemistry of Concrete Biodeterioration 7
3. Bacterial Biodeterioration 77
4. Fungal Biodeterioration 153
5. Plants and Biodeterioration 212
6. Damage to Concrete from Animal Activity 265
Index 279
Chapter 1

Introduction

Biodeterioration of Concrete
Around 1899 the outfall sewer serving Los Angeles—which had been built
only four years previously—started showing signs of severe deterioration
[1]. The concrete lining of the sewers and the lime mortar used in the
brickwork was undergoing significant expansion, before crumbling away.
The bricks themselves appeared unaffected, save for those which spalled
away as a result of the considerable pressure exerted by the expansion.
This might have been attributed to aggressive substances dissolved in the
sewage itself, were it not for the inconvenient fact that the corrosion was
occurring above the waterline.
Early in the 20th century, the construction of brick-lined sewers was
superseded by the use of concrete pipes. However, the problem of corrosion
persisted. Corrosion above the waterline suggested that a gas was causing
the corrosion and the most likely candidate was hydrogen sulphide (H2S),
a malodorous and potentially lethal gas produced by sulphate-reducing
bacteria in sewage. This theory appeared to be supported by the fact that
rates of deterioration were significantly reduced by measures taken to
prevent the release of H2S. In the Los Angeles case, modification of the
sewers to keep them fully flooded—effectively turning them into a septic
tank—solved the problem. Moreover, in 1929, reduced rates of deterioration
were observed in concrete sewers in Orange County and El Centro County,
California, after chlorination of the sewage was initiated [2].
However, H2S—whilst being extremely hazardous—is actually not
corrosive to hardened cement. The most likely explanation for the damage
was that the gas was being converted into sulphuric acid. Chemical reactions
capable of causing this conversion were proposed, but were not (and, in fact,
could not) be demonstrated experimentally. The unsatisfactory nature of
the proposed mechanism of acid formation led Guy Parker (a bacteriologist
who was part of a team facing a similar problem in concrete sewers in
2 Biodeterioration of Concrete

Melbourne, Australia) to consider the possibility that microorganisms

were playing a role not only in the formation of H2S, but in its conversion
to sulphuric acid. Through a series of experiments, he was able to isolate a
species of bacteria which was responsible, which he provisionally named
Thiobacillus concretivorus [3]. In fact, the species was an already known
species, Thiobacillus thiooxidans, but this did not depreciate Parker’s
astonishing discovery, clearly expressed in the original name: here was an
organism that ate concrete.
The above narrative is almost certainly not the first case of biodeterioration
of concrete, but it is one the most well-documented early examples.
Biodeterioration has been defined as ‘any undesirable change in the properties
of a material caused by the vital activities of organisms’ [4]. It also highlights
one of the important features often associated with biodeterioration—a
mechanism which involves multiple organisms each playing individual
and necessary roles.

Biodeterioration and Durability

One of the greatest challenges to engineers designing concrete structures
and infrastructure is that of durability: a fundamental objective of design
should be that a structure is able to satisfy its functions for the duration of its
intended working life. The factors which must be considered by a designer in
achieving this include a wide range of structural and non-structural aspects
of the shape and dimensions of each element, as well as the suitability of
the materials used and their relative proportions.
In the early years of reinforced concrete construction, it was believed
by many engineers that iron or steel encased in concrete was by its nature
invulnerable to many of the deterioration processes which challenged the
integrity of other construction materials. This turned out to be somewhat
optimistic: concrete is a porous material which will allow substances to
permeate it. These substances can cause chemical and physical deterioration
of the concrete itself, or may create chemical environments inside a concrete
element which promote the corrosion of reinforcing steel. Moreover, the
external environment of a concrete structure and the loads that are applied
to it permanently or periodically may cause physical deterioration.
Our understanding of the mechanisms of deterioration of reinforced
concrete has developed considerably over the past 150 years. It is now
reasonably safe to say that it is entirely possible for a concrete structure to
remain structurally serviceable for 150 years or more—if that is required—as
long as appropriate design procedures are followed, and that the quality of
materials and workmanship is high. Despite this, structures still regularly
become unserviceable for reasons related to compromised durability, and
when this occurs—or, ideally, before—maintenance is required.
 Introduction 3

Maintenance is a necessary aspect of the operation of any structure.

However, where it is required to rectify structural deterioration within the
intended working life, it represents a burden to the operator which was
probably not budgeted for.
Many of the processes which compromise the durability of concrete
structures are a consequence of the fact that the Earth is a living planet.
The most obvious example of this is the corrosion of steel reinforcement.
This process requires gaseous oxygen, whose presence is maintained in
the atmosphere as a result of the photosynthesis undertaken by plants.
Moreover, the presence of sulphate minerals in both seawater and soil—
which can produce sulphate attack in concrete—is partly the result of the
role played by microorganisms in the Earth’s sulphur cycle.
A proportion of atmospheric carbon dioxide—which causes carbonation
of concrete leading to the depassivation of reinforcement—is biogenic. It
should be stressed that the presence of life on this planet maintains carbon
dioxide at levels much lower than would be the case for an abiotic planet
—one need only compare the CO2 concentrations on Mars and Venus with
those on Earth. Nonetheless, around 30% of our planet’s atmospheric CO2
comes from a single species, whose current dependency on the combustion
of fossil fuels is currently generating growing environmental concern.
All of these processes are global in nature and the threat they pose to
concrete durability—and the solutions available—have been covered by
many other books. This author has chosen to concentrate on biodeterioration
processes directly caused by living organisms in close proximity to concrete.
The types of organism which can take part in the deterioration of concrete
are surprisingly varied, including bacteria, fungi, both higher and lower
plants, and a possibly surprising array of animals including molluscs and
marine worms.
This may come as something of a surprise: concrete would seem, in
theory, to be a highly sterile substance, containing little by way of the
elements required by living things for energy or growth. Furthermore,
concrete is often highly alkaline and, thus, presents a hostile chemical
environment for life. Nonetheless, the ability of life to occupy far more
challenging locations has frequently been noted, and the colonization of
concrete surfaces is no exception.
The mechanisms of biodeterioration of concrete include both chemical
processes—such as leaching of cement resulting from the formation of acidic
substances—and physical ones—for instance, from pressures exerted by
root growth. However, deterioration mechanisms can also involve fairly
complex combinations of both. An example of this is the production of citric
acid, which will form insoluble calcium citrate in contact with hydrated
cement. The resulting precipitates generate pressures within concrete pores
leading to fragmentation at the surface.
4 Biodeterioration of Concrete

Deterioration rates vary considerably and will depend on the type and
number of organisms involved, as well as the environmental conditions
and the composition and properties of the concrete. However, in cases
where long service lives are required, even slow rates of deterioration
may be unacceptable. This is certainly true in the case of much of our
infrastructure—such as bridges and sewers—but becomes fundamental
in the case of containment applications such as nuclear waste storage and
oil well decommissioning, where service lives of hundreds or thousands
of years may be required.

This Book
This book aims to examine, as widely as possible, the different ways in
which living organisms can compromise the durability of concrete, and to
also explore means by which deterioration can be prevented or controlled.
Following on from the discussion above, we might be led to assume
that the presence of life in close proximity to concrete should always lead to
problems. This is not the case—in most instances living organisms will have
little impact on the durability of a structure. Indeed, a number of instances
are discussed in this book which their presence has a protective effect.
This raises the more general issue of whether it is appropriate to attempt
to remove a particular species from around a structure simply to improve
durability. This partly depends on what sort of organism is involved, but
this is not the whole issue: the organism in question will make up part of
a much larger ecosystem, and its removal may have significant impacts
on the local environment as a consequence. Furthermore, whilst the use
of biocides as a means of protection from biodeterioration and biofouling
has been widely practiced, the secondary toxic effects to other species
potentially significant distances away from the point of use have led to this
approach being questioned. Legislation, plus a more general desire to act
in an environmentally responsible manner, is leading to a move away from
biocides. This book has been written with this trend in mind.
One aspect of the colonization of concrete structures by living organisms
which this book attempts to avoid is the matter of aesthetics. This is largely
because of the difficulties in making judgements about what constitutes an
aesthetically disagreeable state. It is notable that opinions in this respect
vary considerably. A plant climbing the side of a building is considered
attractive to many, but is disagreeable to others. The discolouration of
a facade by lichen, fungi or algae is considered by many to be ugly and
requiring cleaning, whilst to others it is the building’s ‘patina’. Thus, this
author has chosen to steer clear of adopting any position on this issue.
The book is divided into six chapters (including this introduction).
Because chemical processes play such an important role in many forms
 Introduction 5

of biodeterioration, it is necessary to understand the manner in which the

most common compounds produced by living organisms interact with
hardened cement and concrete. Chapter 2 does this by addressing the
most common compounds separately. In many ways the chapter is of a
format more suited to reference books. However, this approach is deemed
necessary on the grounds that each interaction is unique to the compound
in question and has very different effects in terms of the physical changes
imparted to concrete. Moreover, since many compounds will be encountered
in discussions of biodeterioration imparted by several different types of
organism, this approach is likely to ultimately be useful to the reader.
The current three-domain system for classifying organisms, and the
kingdoms contained within each domain is structured as follows:

Archaea Bacteria Eukara

Archaebacteria Eubacteria Protista
Fungi
Plantae
Animalia

Chapter 3 of this book deals with biodeterioration resulting from the

activities of both the Archaea and Bacteria domains together. Furthermore,
it takes the somewhat retrograde step of referring to all organisms within
these two domains as ‘bacteria’—the current system of domains has existed
since it was argued that Archaea and Bacteria were sufficiently different to
justify separate classification [5]. Whilst it is recognised that there are many
fundamental differences between these two domains, within the context
of concrete biodeterioration it makes more sense to discuss them together
than separately.
Chapters 4 and 5 cover fungi and plants. The format of Chapters 3 to
5 is similar. First, an overview of the life-cycle of each type of organism
is presented, with particular emphasis on energy sources, growth and
reproduction. Second, the metabolism of the organism is examined,
particularly with regards to the manner in which chemical compounds are
produced, and the benefits and challenges presented by concrete as a host
environment considered. Potential deterioration mechanisms are discussed
and factors influencing rates of deterioration examined. Such factors include
both those related to concrete characteristics and environmental conditions.
Finally, various approaches for limiting damage from biodeterioration are
systematically considered and appraised.
Chapter 6 examines biodeterioration from animals, with emphasis
placed on marine animals, where examples of damage are more widely
reported. However, the potential for bird droppings to damage concrete
surfaces is also evaluated.
6 Biodeterioration of Concrete

References
[1] Olmstead FH and Hamlin H (1900) Converting portions of the Los Angeles outfall sewer
into a septic tank. Engineering News and American Railway Journal 44: pp. 317–318.
[2] Goudey RF (1928) Odor control by chlorination. California Sewage Works Journal 1:
87–101.
[3] Parker CD (1945) The corrosion of concrete. 1. The isolation of a species of bacterium
associated with the corrosion of concrete exposed to atmospheres containing hydrogen
sulphide. Australian Journal of Experimental Biology and Medical Science 23: 81–90.
[4] Hueck HJ (1965) The biodeterioration of materials as part of hylobiology. Material und
Organismen 1: 5–34.
[5] Woese C, Kandler O and Wheelis M (1990) Towards a natural system of organisms:
proposal for the domains Archaea, Bacteria, and Eucarya. Proceedings of the National
Academy of Science 87: 4576–4579.
Chapter 2

The Chemistry of Concrete

Biodeterioration

2.1 Introduction
Many of the biodeterioration processes described in subsequent chapters
involve the production of chemical substances by organisms. These
substances are, in some cases, products of excretion processes, whilst in
other cases are produced with the purpose of modifying an organism’s
environment to its benefit. The influences of these substances on concrete
are diverse, since reactions between cement hydration products in the
concrete yield products whose characteristics vary widely.
Many of these substances are produced by more than one kingdom or
domain, and so it makes sense to discuss the chemistry of these substances
in the context of concrete first, and to subsequently use this chapter as a
reference for subsequent discussion. Thus, this chapter provides information
on the aqueous chemistry of various biogenic products in the presence of
the most relevant elements present in concrete. It then examines, in generic
terms, the various mechanisms that can lead to deterioration of concrete
through chemical reaction.
Before introducing specific substances, a brief introduction is provided
to the reader regarding some of the concepts used to characterize their
behaviour. These are acidic strength, the formation of metal complexes in
solution and the solubility product. The chapter employs a form of solubility
diagram to illustrate the influence of the substances on cementitious systems.
For this reason, an introduction to these diagrams is included, along with a
brief description of the method used to construct such diagrams for this book.

2.1.1 Acidic strength

The strength of an acid is dependent on its ability to deprotonate—shed

protons (H+)—in solution. Thus, a strong acid, HA, will undergo the reaction
8 Biodeterioration of Concrete

HA ⇌ A– + H+
to completion when it is dissolved in water.
For many acids, this reaction will reach equilibrium with only a
proportion of molecules deprotonated. The extent to which deprotonation
occurs for a given acid is expressed in terms of the acid dissociation constant
(Ka). This constant is defined by the equation:
[H+] [A–]
Ka =
[HA]
With square brackets denoting concentration. For convenience, Ka is
often expressed as its negative logarithm (pKa). A strong acid is one with a
pKa value of less than –1.74.
The pH of the resulting solution is given be the equation:
pH = –log[H+]
pH and pKa are intrinsically related, and the pH of a solution containing
acid HA is described by the equation:
[A–]
pH = pKa + log
[AH]

Thus, if a base is progressively dissolved in the water in which the acid

is dissolved, as the pH of the solution increases, the degree of deprotonation
will increase: [A–] will increase at the expense of [AH]. When pH = pKa the
concentrations of protonated and unprotonated acid molecules will be the
same and the ratio [A–]/[AH] will equal one. As pH increases further, the
solution will gradually approach a state in which all of the acid molecules
are deprotonated.
The proportion of molecules of an acid in a given state of deprotonation
is expressed as the abundance ratio (α). This is the concentration of the species
expressed as a ratio of the total concentration of all species present. Where
the species is the deprotonated acid, the abundance ratio is referred to as
the dissociation ratio, described by the equation:
[A–]
α=
[A ] + [AH]
–

The change in abundance ratios of the deprotonated and protonated

species of acetic acid is illustrated in Figure 2.1 using acetic acid (pKa =
4.76) as an example.
Acids may possess multiple protons, each of which will have an acid
dissociation constant. For example, the pKa values for citric acid are 3.13, 4.76
and 6.40. Each of these dissociation constants are associated with a specific
proton on the molecule. The influence pH on the distribution of citric acid
species at different degrees of deprotonation is shown in Figure 2.2.
 The Chemistry of Concrete Biodeterioration 9

1.0

0.8

0
i=
<{
0:: 0.6
L1J
u
z
<{
0
z 0.4
::::l
aJ
<{

0.2

0.0
2 4 6 8 10 12 14

pH

Figure 2.1 Distribution of protonated and deprotonated acetic acid species

as a function of pH.

s':(
citrate3-
H(citrate}'-
0.8 H2(citrater
; ""' H3 (citrate)
0 I \

~
'-'- 0.6
I I
I \
\ > o"'}-o
L1J I. / I
I '
0

u
z
<{
)' I
'I \I
~ 0.4 I.
::::l 1 I I
aJ I . I
<{ / I 1
I I I
0.2 I I
I \ I
I \
\ \
0.0 +--""----,-"~--".=::o-~----,-'-'-"~----,-----,-----1
2 4 6 10 12 14

pH

Figure 2.2 Distribution of citric acid species as a function of pH.

10 Biodeterioration of Concrete

2.1.2 The solubility product

The dissolution of a solid ionic compound comprising a cation Ay+ and an

anion Bx– in water can be described by the equation:
AxBy ⇌ xAy+ + yBx–
In a case where a compound dissolves to form a solution approaching
infinite dilution, the extent to which this reaction occurs is defined by the
solubility of the compound which can be described using its solubility
product (Ksp):
Ksp = [Ay+]x[Bx–]y
The nature of this equation means that the units of the solubility product
depend on the nature of the reaction. Where x and y are 1, the units will
be mol2/l2. However, if x were 2 and y 3, the units would be mol6/l6. For
convenience the solubility product is usually expressed as log Ksp with a
larger value denoting greater solubility.
By way of example, if we consider the compound calcium hydroxide
—Ca(OH)2 —the dissolution reaction will take the form:
Ca(OH)2 ⇌ 2OH– + Ca2+
and so the solubility product of calcium hydroxide is:
Ksp = [Ca][OH]2.
In the case of calcium hydroxide, log Ksp is –5.05, and so Ksp is 8.91 ×
10–6 mol3/l3. Its solubility in mol/l would therefore be the cube root of Ksp:
0.02 mol/l.
In the case of hydroxide compounds, the solubility product can be
expressed in a different form. This is because the dissolution of such
compounds can be expressed as an alternatively arranged equilibrium
equation in the form shown here for ferrihydrite:
Fe(OH)3 + 3H+ ⇌ Fe3+ + 3H2O
An alternative solubility product K*sp may be used, which takes the form:
K*sp = [Fe3+][H+]–3
For this reason it is usually advisable to always provide the equilibrium
equation from which the solubility product has been obtained.
In less dilute solutions where ionic species are involved, the use of
concentration to define the solubility product becomes less appropriate.
This is because ions will electrostatically interact with each other and
neighbouring water molecules. As a result, it is necessary to consider
concentration in terms of the ‘effective’ concentration accounting for these
 The Chemistry of Concrete Biodeterioration 11

interactions. This effective concentration is called the activity (ɑ) of the ion.
The activity can be defined as:
[i]
αi = γi [iθ]

where ɑi = the activity of ionic species i;

[i] = the concentration of species i (mol/l);
γi = the activity coefficient; and
[iθ] = the standard molar concentration (1 mol/l).
The standard molar concentration is required to render the activity
dimensionless.
Before it is possible to discuss how the activity coefficient is obtained, it
is necessary define one other characteristic of a solution—its ionic strength.
Ionic strength (i) is a measure of concentration which takes account of the
valence of the ions dissolved, with units of concentration. It is defined by
the equation:

I = 12 ∑[i]zi2

where zi is the charge on ionic species i. The ionic strength equation must
include all ions present in a solution.
At low ionic strengths (I < 0.001 M) the Debye-Hückel equation can be
used to obtain a value for the activity coefficient:
log γi = –Az 2i √I
where A is a constant which is independent of the ion involved, but
which changes with temperature and pressure. At 25°C and 1 atmosphere
pressure, its value is 0.5085 [1]. At higher ionic strengths, different forms
of the Debye-Hückel equation must be used. Where the ionic strength is
in the range 0.001 ≤ I < 0.1, the extended Debye-Hückel equation applies:

–Az 2i √I
logγi =
1 + Ba0 √I
where B = a constant dependent on temperature and pressure; and
a0 = the theoretical hydrated radius of the ion (Å).
The hydrated radius of an ion is the radius within which water
molecules are closely bound to the ion.
Within the range 0.1 ≤ I < 0.7, the Davies equation [2] should be used:

–Az 2i √I
logγi = + bI
1 + √I
12 Biodeterioration of Concrete

The original form of the Davies equation used a fixed value of b, and
this value has been revised several times, but is commonly 0.3. Using a
fixed value will introduce some inaccuracy at ionic strengths lower than
0.1 and also for ions with valencies of more than 1. This has subsequently
been addressed through the development of ion-specific values of b. Where
such an approach is adopted, the Davies equation is referred to as the
Truesdell-Jones equation [3].
A number of approaches for obtaining activity coefficients for solutions
of ionic strength, including the Pitzer ion interaction and the B-dot equation.
These are of lesser relevance to the issues under discussion, but of great
value when dealing with environments containing solutions containing
brine-like concentrations of highly soluble salts.
Log Ksp is influenced by crystal size, temperature and pressure. It is
often the case that the influence of crystal size, rightly or wrongly, is often
ignored. Pressure usually has a relatively minor influence over the solubility
product. Temperature, however, has potentially a much greater influence
and, in systems where temperature may vary, this effect will usually need
to be accommodated in solubility calculations.
The influence of temperature on solubility is the result of Le Chatelier’s
principle, which states that a change to a system of concentration,
volume, temperature or pressure will cause that system to oppose the
change. Dissolution reactions can either be exothermic (giving out heat)
or endothermic (taking in heat) and this will change the temperature of
the fluid in which the reaction is occurring. Thus, in accordance with Le
Chatelier’s principle, as the temperature of a system increases, the solubility
product of a compound which undergoes an exothermic reaction will
decline, whilst the solubility of a compound with an endothermic reaction
will increase.
The influence of temperature (expressed in Kelvin) is described by the
van’t Hoff equation:

∆HR0 1 1
logKsp,T2 – logKsp,T1 = ( –
2.303R T1 T2 )
where Ksp,T2 = the solubility product at a temperature of T2;
Ksp,T1 = the solubility product at a reference temperature T1;
∆HR0 = the standard enthalpy of reaction (J/mol); and
R = the gas constant (J/K mol).
In the context of aqueous chemistry, the reference temperature is
frequently 25ºC. ∆HR0 is dependent on temperature, but where temperatures
are close to the reference temperature (± 20 oK), the value for the reference
temperature may be assumed with little compromise in the accuracy of the
predicted change in log Ksp.
 The Chemistry of Concrete Biodeterioration 13

The solubility of a substance is influenced by the concentration of other

substances dissolved in the solution, where ions present in the substance
are already present in solution. If we return to calcium hydroxide, this
effect can be understood if we consider its dissolution in a solution of
sodium hydroxide (NaOH). If the concentration of NaOH is 0.001 mol/l,
the solubility product will be:
Ksp = [B][B+0.001]2.
where [B] is the concentration of calcium ions in mol/l deriving from the
dissolution of calcium hydroxide, and is therefore also the solubility of
calcium hydroxide. Therefore, substituting S for [B] and rearranging gives
a quadratic equation:
0 = S2 + 0.001S + 0.000001 – Ksp.
Since we have already seen that Ksp is 8.91 × 10–6 mol3/l3
0 = S2 + 0.001S – 0.00000791.
Solving this equation gives a value for the solubility of 2.36 × 10–3 mol/l,
meaning that the solubility has reduced by an order of magnitude. This
effect is known as the ‘common ion effect’.
It should be noted that Ksp is also dependent on the quantity of other
ions dissolved in the solution—the ‘salt effect’. Generally, an increase in
dissolved ions will act to reduce the solubility product.

2.1.3 The formation of metal complexes in solution

When a metal ion dissolves in water in the manner described in the previous
section it may simply remain in solution as a cation. However, where other
molecules or ions are present in solution, there exists the possibility that the
cation will form a metal complex with one or more of these entities, which
are referred to as ligands when interacting in this way. A complexation
reaction can be written in the form
M + L ⇌ ML
The complex ML may not be the only complex which can form. For
instance, a second ligand may be involved:
ML + L ⇌ ML2
In a similar manner to the acid dissociation constant, the equilibrium
of the first reaction can be described using the equation:
[ML]
K1 =
[M][L]
where K1 is said to be a stability constant, and is specifically the association
constant of the complex. Similarly, the equilibrium of the second reaction
is described thus:
14 Biodeterioration of Concrete

[ML2]
K2 =
[ML][L]
These association constants are described as stepwise constants: they
describe the individual steps required to move from M to ML2. Of course,
the reaction could be described in terms of both stepwise reactions occurring
together:
M + 2L ⇌ ML
In which case the equilibrium of the reaction can be characterized by
the cumulative constant, β12:
[ML] [ML2] [ML2]
β12 = K1K2 = ∙ =
[M][L] [ML][L] [M][L]2
It is, again, often convenient to express stability constants in terms of
log K.
The ionic form that a molecule adopts in solution will define to a large
extent what complexes it can form with metals. For instance, calcium will
form a complex (of the ML type) with the acetate ion (C2H3O2–), but not
with its protonated form. This has significant implications, since it means
that the concentration of metal complexes in solution will vary depending
on pH. This is shown in Figure 2.3 which shows the distribution of the
Ca(acetate) complex only appears in significant quantities once the pH is
sufficiently high to yield large concentrations of deprotonated acetic acid
(as previously seen in Figure 2.1). Another point to note about this plot is

10~ .---------------------------------------------------,

8x1o-•
""'=
0
E
z
0 6x10 9
i=
<(
0::
1- Ca(acetate(
z / ------------------,
UJ
0
z
4x10-9
/ / ' \
0
0
! \
I
2x1o-• I
I
I
I
/
--"'
0
2 4 6 8 10 12 14

pH
Figure 2.3 Concentrations of complexes formed by calcium in an acetic acid solution
as a function of pH. Concentrations: acetic acid: 100 mmol/l; Ca: 0.0001 mmol/l.
 The Chemistry of Concrete Biodeterioration 15

that calcium also forms a complex with hydroxide ions (CaOH+), whose
stability constant is higher than that of Ca(acetate), but whose appearance
–
in significant quantities is observed only once OH concentrations are high.
As for solubility, stability constants are also influenced by temperature
and the effect can again be described by the van’t Hoff equation. The salt
effect also influences stability constants and, as a result, it is usually the
convention to state the ionic strength at which a solubility product was
measured.
Particularly stable complexes are formed where the ligand is able to
form multiple bonds through different co-ordinating atoms with the same
metal ion. This type of interaction is known as chelation, and the stability
of the resulting complex is dependent on the size and number of ‘chelate
rings’ formed. A chelate ring is the closed structure formed by the metal
ion, a co-ordinating atom in the ligand molecule, through the sequence of
bonded atoms that link the co-ordinating atoms, and back to the metal ion.
A higher number of chelate rings is likely to increase the stability of the
surface complex formed. Moreover, the number of ring members plays an
important role in complex stability, with relative stability usually following
the sequence:
5-membered > 6-membered > 7-membered/4-membered.

2.1.4 Solubility diagrams and their construction

This chapter describes how some of the substances produced by living

organisms interact with the elements present in cement and, to a large extent,
concrete. The way in which this has been done is through the use of solubility
diagrams. Solubility diagrams are sometimes referred to as predominance
plots and define the predominant chemical species in a system for a given
pH and concentration of a given species, regardless of whether the species is
in solid form or dissolved as an aqueous species. An example of a solubility
diagram can be seen in Figure 2.4 for calcium. In the plot, the x-axis is pH,
whilst the y-axis is the log of concentration of calcium. The plot shows the
predominance of three species: Ca2+, Ca(OH)+ and portlandite (Ca(OH)2).
Portlandite is in the solid state, which is indicated by ‘(s)’. Thus, we are able
to see that when the pH is 13 and the concentration of calcium is 0.01 mol/l
(i.e., log[Ca] = –2), then portlandite is the predominant species in the system.
It should be stressed that the predominant species will usually not be
the only species present. This is a potential weakness of solubility diagrams,
since they do not present a comprehensive depiction of the entire system at
a given set of conditions. However, as a means of getting an approximate
idea of the main changes which occur as, for instance, pH changes in a
system, these diagrams are an extremely useful tool.
16 Biodeterioration of Concrete

\
0
~
.2l
'0
c
C1l
~
0
-2 0..

ro
~ -4
0>
0
....J
Ca2 +
1\
+I
0
C1l
-6 0

2 3 4 5 6 7 8 9 10 11 12 13 14

pH
Figure 2.4 Solubility diagram for calcium.

The solubility diagrams used in this chapter have been drawn using
the computer program MEDUSA which uses the stability constants and
solubility products defined for a system. These constants have been
obtained from various sources in the literature. Unless otherwise stated
they correspond to a temperature of 25ºC. In the case of stability constants,
wherever possible the values for an ionic strength of 0 have been selected.
Where the effect of acidic species is investigated, all solubility diagrams
have been constructed for an acid concentration of 0.1 mol/l.

2.2 Cementitious Systems in the Presence of Water

Before the influence of different acids involved in biodeterioration processes
can be discussed, it is necessary to examine the nature of cementitious
systems under more conventional chemical conditions. First, the chemical
nature of hardened cement paste will be briefly examined, before the manner
in which changes in pH affect their constituents.

2.2.1 Introduction to cement chemistry

The clinker that is used to make Portland cement consists of four main
phases: tricalcium silicate (3CaO.SiO2), dicalcium silicate (2CaO.SiO2),
tricalcium aluminate (3CaO.Al2O3) and tetracalcium aluminoferrite (4CaO.
Al2O3.Fe2O3). This clinker is ground with a quantity of calcium sulfate—
usually either as gypsum (CaSO4.2H2O) or anhydrite (CaSO4).
 The Chemistry of Concrete Biodeterioration 17

Portland cement sets and hardens as the result of a series of hydration

reactions with water. These reactions occur at notably different rates and the
nature of each reaction is influenced to some extent by the other reactions
occurring simultaneously. Whilst the details of these reactions do not require
examination in any great detail, the products of the reactions do.
The most important product of Portland cement hydration is the
poorly crystalline calcium silicate hydrate (CSH) gel. This product has a
composition which is able to vary quite considerably. In particular, its Ca/
Si ratio can vary from between 0.6 and 1.7 under normal conditions [4]. In
addition, other elements can be incorporated into its structure.
Another important constituent is calcium hydroxide (portlandite,
Ca(OH)2). This compound acts as the main source of hydroxide ions, and
consequently the high pH of water in the pores of hydrated Portland cement.
It should be noted, however, that the solubility of portlandite is relatively
low. Thus, the high pH is achieved through the release of OH– ions from
portlandite to balance the charge of potassium and sodium ions in solution
which were originally present in the clinker as sulfate salts.
Additionally there are two groups of compounds containing aluminium
and iron known as the AFt (‘aluminoferrite-tri’) and AFm (aluminoferrite-
mono) phases. The most commonly encountered AFt phase in plain
hydrated Portland cement is ettringite, which has the formula 3CaO.
(Al,Fe)2O3(CaSO4)3.32H2O. The most commonly encountered AFm phase
is monosulphate (3CaO.(Al,Fe)2O3.CaSO4.12H2O). These phases contain
sulphate, which derives from the calcium sulfate added to the clinker.
However, under different chemical conditions the sulphate can be replaced
with a wide range of different anions, and the quantity of chemically
combined water can also vary.
It has become increasingly rare for Portland cement to be used as
the sole cementitious constituent in concrete. Instead Portland cement is
combined with other materials, most of them by-products from industrial
processes. The most common of these materials are ground granulated
blastfurnace slag (GGBS), fly ash (FA) and silica fume (SF). As will be seen
in later chapters, these cement components can sometimes be used to impart
greater resistance to acids.
GGBS is a byproduct of iron manufacture and has a chemical
composition not entirely unlike Portland cement, but slightly enriched
with regards to silica and aluminate. The high pH conditions encountered
when used in combination with Portland cement causes it to undergo a
latent hydraulic reaction which produces products similar to those formed
by the Portland cement. The composition of the slag means that the Ca/
Si ratio of the CSH gel formed will be lower, and that larger quantities of
aluminoferrite phases will be formed.
FA is ash remaining from the combustion of pulverized coal during
electrical power generation and can be encountered in both calcareous
18 Biodeterioration of Concrete

(higher CaO) and siliceous (low levels of CaO) forms. Both forms are mainly
composed of SiO2 and Al2O3. Whilst some of the minerals found in FA
(notably quartz, mullite, hematite and magnetite) are of very low solubility,
and react to a limited extent, a large proportion of the material consists of a
glassy substance which reacts with portlandite formed by Portland cement
to form CSH and aluminoferrites. This reaction is known as a pozzolanic
reaction, and the high SiO2 and Al2O3 content of the material mean that,
again, the Ca/Si ratio of the CSH gel is reduced and more aluminoferrite
phases are formed.
Silica fume is a by-product of the manufacture of silicon for the
electronics sector. It is usually almost 100% SiO2 and undergoes a pozzolanic
reaction similar to FA, leading to a modification in the composition of CSH.
The use of all of these materials in combination with Portland cement
will also lead to a reduction in the quantity of portlandite formed, since the
Portland cement is diluted, and may be reduced further if the material in
question undergoes a pozzolanic reaction. As a result, the pH of the pore
solutions of hardened cement pastes containing GGBS, FA and SF will be
lower than that for Portland cement alone.
Another type of cement which is gaining popularity in many parts
of the world are the calcium aluminate cements. These cements contain
mainly Al2O3 and CaO, although the proportions vary from product to
product, unlike Portland cement whose composition has gradually evolved
to a relatively uniform composition globally. After hydration, hardened
calcium aluminate cements consist largely of CaO.Al2O3.10H2O and 2CaO.
Al2O3.8H2O, with some Ca3Al2O9.6H2O and amorphous Al(OH)3. The cement
will gradually undergo a process of conversion whereby CaO.Al2O3.10H2O
and 2CaO.Al2O3.8H2O decompose to the last two products. Conversion has
been a cause of concern in the past, since the conversion leads to a loss in
volume of the cement, leading to the formation of porosity and a loss in
strength. However, better understanding of the material means that design
processes can take into account conversion. Moreover, as will be seen in
later chapters calcium aluminate cements can potentially play a role in
achieving resistance to certain forms of biodeterioration.
A variant of calcium aluminate cements are the calcium sulfoaluminate
cements. These cements also contain a quantity of sulfate which means
that the hydration products formed are usually ettringite, monosulfate and
amorphous Al(OH)3 [5].

2.2.2 Redox potential

A number of chemical elements are said to display ‘redox’ behavior—they

are capable of existing in more than one oxidation state. These elements
include carbon (which can exist in 4+, 0, 2– and 4– states in natural waters
and minerals), hydrogen (1+, 0), iron (3+, 2+), manganese (4+, 3+, 2+),
 The Chemistry of Concrete Biodeterioration 19

oxygen (0, 2–), nitrogen (3–, 0, 3+, 5+) and sulphur (6+, 4+, 0, 1–, 2–). A
number of other trace metals also display redox behavior.
The transition from a high oxidation state (e.g., Fe3+) to a lower one
(Fe2+)—reduction—requires the species to accept electrons (e–). This process
can be summarized by the reaction:
Fe3+ + e– ⇌ Fe2+
The Fe3+ here acts as an oxidizing agent—it accepts electrons. This
reaction is referred to as a redox couple, but is not a complete chemical
reaction, since there is no explanation of the origin of the electron. The
electron must come from a reducing agent. In aqueous solution the reducing
agent may be water:
H+ + ¼O2 + e– ⇌ ½H2O
This is also a redox couple, and combining the two gives:
Fe3+ + ½H2O ⇌ Fe2+ + H+ + ¼O2
This is now a complete reaction, with Fe3+ and O2 acting as oxidizing
agents and H2O and Fe2+ acting as reducing agents.
The equilibrium state of a redox couple gives a useful measure of
whether a system is oxidizing or reducing. Since redox reactions involve
transfer of electrons, it is common for this equilibrium to be expressed in
terms of the theoretical voltage associated with the reaction. This voltage is
known as the redox potential (Eh). If we consider a solution containing two
oxidized species (A and B) undergoing reduction to produce two reduced
species (C and D):
aA + bB + ne– ⇌ cC + dD
Eh can be determined using the equation:
a
Eh = E° + RT In [A]c [B] d
b

nF [C] [D]
where E° = the standard potential of the reaction (V);
R = the gas constant (8.3145 J/molK);
T = temperature (K); and
F = the Faraday constant (96,485 J/V g eq).
It is also common for the redox potential to be expressed in terms of
the negative logarithm of the electron concentration (pE):
Eh
pE = – log10[e –] =
0.05916
The redox potential of a solution is dependent on what redox couples are
present, and their concentration. Each redox couple has a characteristic Eh
and the redox potential of the solution will reflect the predominant couple.
20 Biodeterioration of Concrete

If the solution experiences conditions which will alter the Eh—for instance,
the introduction of a reducing agent—the concentration of the predominant
couple will determine the solution’s ability to resist this change, with a
high concentration providing greater resistance. Where there is a large
concentration of a particular redox pair, the solution is said to have a high
redox capacity, or to be ‘well-poised’. Once the capacity of the predominant
pair is exceeded, the redox potential will shift with relative ease to that of
the redox couple with the nearest Eh.
The redox potential can play an important role in determining the
oxidation state of elements present in smaller quantities. For instance, if
chromium is present in small quantities alongside iron in larger quantities,
the redox reaction
Fe2+ + Cr3+ ⇌ Fe3+ + Cr6+
will be established. Thus, the relative proportions of Fe2+ and Fe3+ will
dictate the relative proportions of Cr3+ and Cr6+. This, in turn, has significant
implications for solution chemistry, since—as is the case for chromium—the
oxidation state of an element will often influence its solubility.
Redox potential is also influenced by pH, with the redox potential
rising with increasing pH.
In Portland cement, there are usually only a small number of dissolved
redox pairs present, in relatively small concentrations. These couples include
O2/H2O, Fe3+/Fe2+, SO42–/SO32–.
This means that the pore solution within Portland cement is poorly-
poised. The redox potential of Portland cement is typically around Eh = +100
– +200 mV. However, where quantities of sulfides are present—for instance,
if ground granulated blastfurnace slag has been used in conjunction with
Portland cement—the redox potential may be much lower: –400 mV [6]. In
constructing solubility plots in this chapter, an Eh of +150 mV was used.

2.2.3 Calcium in water

Calcium makes up a considerable proportion of Portland cement, and so

its behaviour in solution plays a very important role in the deterioration
of concrete under conditions of changing pH.
In hydrated Portland cement which has not undergone any chemical
interactions with its surrounding environment, calcium will be present as
CSH gel, portlandite and calcium aluminate (and ferrite) hydrates. For the
purposes of summarizing the effect of water on the integrity of hardened
Portland cement, the solubility diagram for calcium alone is initially
sufficient. In such a case, the only complex which may be formed is CaOH+,
whose stability constant is provided in Table 2.1. The only solid phase of
relevance is portlandite, whose solubility product is provided in Table 2.2.
The solubility diagram constructed using this information is shown in Figure
 The Chemistry of Concrete Biodeterioration 21

Table 2.1 Stability constants of complexes formed by calcium, aluminium

and iron(II) and (III) ions in water.

Complex Reaction Stability Reference

Constant
Ca

CaOH+ Ca2+ + H2O → CaOH+ + H+ –12.697 [7]

Al

AlOH2+ Al3+ + H2O ⇌ AlOH2+ + H+ –4.997 [7]

Al(OH)2+ Al3+ + 2H2O ⇌ Al(OH)2+ + 2H+ –10.094 [7]

Al(OH)3 Al + 3H2O ⇌ Al(OH)3 + 3H

3+ +
–16.791 [7]

Al(OH)4– Al3+ + 4H2O ⇌ Al(OH)4– + 4H+ –22.688 [7]

Fe(II)

FeOH+ Fe2+ + H2O ⇌ FeOH+ + H+ –9.397 [7]

Fe(OH)2 Fe2+ + 2H2O ⇌ Fe(OH)2 + 2H+ –20.494 [7]

Fe(OH)3 Fe2+ + 3H2O ⇌ Fe(OH)3– + 3H+ –28.991 [7]

Fe(III)

FeOH2+ Fe3+ + H2O ⇌ FeOH2+ + H+ –2.187 [7]

Fe(OH)2+ Fe3+ + 2H2O ⇌ Fe(OH)2+ + 2H+ –4.594 [7]

Fe(OH)3 Fe3+ + 3H2O ⇌ Fe(OH)3 + 3H+ –12.56 [7]

Fe(OH) 4
–
Fe + 4H2O ⇌ Fe(OH)4 + 4H
3+ – +
–21.588 [7]

Fe2(OH)24+ 2Fe3+ + 2H2O ⇌ Fe2(OH)24+ + 2H+ –2.854 [7]

Fe3(OH)45+ 3Fe3+ + 4H2O → Fe3(OH)45+ + 4H+ –6.288 [7]

Table 2.2 Solubility and molar volume data for compounds relevant to the interaction of
hydrated Portland cement with water.

Compound Formula/Reaction Solubility Reference Molar Reference

Product, Volume,
log ksp cm3/mol
Portlandite Ca(OH)2 –5.05 [9] 32.9 [10]
Ca(OH)2 → Ca2+ + 2OH–
Gibbsite Al(OH)3 –1.40 [9] 32.2 [10]
Al(OH)3 + OH– ⇌ Al(OH)4+
Ferrihydrite Fe(OH)3 3.191 [7] 28.1 [11]
Fe(OH)3 + 3H+ ⇌ Fe3+ +
3H2O
22 Biodeterioration of Concrete

2.4 which covers a pH range of 2 to 14. The concentration of calcium in the

diagram varies between 0.00001 to 1000 mmol/l.
At high pH and at relatively high concentrations of calcium, solid
portlandite is the dominant form of calcium. At lower concentrations (below
the solubility limit of portlandite) CaOH+ is dominant. It is also evident that
even a minor drop in pH below that which is typical for Portland cement
will lead to the dissolution of portlandite, with the dissolved calcium ion
—Ca2+—becoming dominant. Thus, the portlandite in Portland cement is
extremely vulnerable under acidic conditions.
Calcium is, of course, not only present as portlandite. Where it is present
as calcium aluminate and ferrite hydrates, a similar vulnerability exists,
although this is illustrated best in the section relating to sulphuric acid. The
situation for CSH gel is a little more complicated, since this will undergo
loss of calcium (decalcification), ultimately leaving a silica gel behind.

2.2.4 Aluminium and iron

Aluminium and iron are in many ways very similar elements, and as a result
are able to substitute for each other in the structures of many compounds.
This is certainly true of the AFm and AFt phases of cement. One of the key
distinguishing features is that, whilst both elements can exist in a number
of oxidation states, iron is commonly encountered in two—Fe2+ and Fe3+—
whereas aluminium usually exists in only the Al3+ state.
Figure 2.5 shows the solubility diagram for aluminium. A variety of
complexes are formed between aluminium and hydroxide ions—as shown
in Table 2.1—and most of these are encountered in the diagram. A significant
proportion of this diagram is occupied by the mineral gibbsite (Al(OH)3),
which only becomes soluble under very high and very low pH conditions.
Crystalline gibbsite is, in fact, seldom encountered in hydrated Portland
cement, meaning that it cannot be identified using techniques such as X-ray
diffraction. Instead, an amorphous precipitate is formed.
The solubility diagram for iron is shown in Figure 2.6. As for aluminium,
much of the diagram’s area is occupied by a solid phase, in this case
ferrihydrite. Ferrihydrite is an oxyhydroxide compound with a variable
composition. The official formula ascribed to it by the International Mineral
Association is 5Fe2O3.9H2O, but the water content varies considerably.
Because of this, the formula Fe(OH)3 is often used for simplicity. It has been
proposed that ferrihydrite may, in fact, be a mixture of multiple phases,
although a single phase structure has also been proposed. The reason for
this uncertainty is that ferrihydrite is precipitated as nano-scale particles
and so appears amorphous when studied using X-ray diffraction.
The persistence of ferrihydrite even at relatively low pH means that
concrete and cement undergoing acid attack will, for many acids, develop a
 The Chemistry of Concrete Biodeterioration 23

0.--.----------------------------------------------,

Gibbsite (s)
-2

~
Cl -4
0
...J

AI(OH)4-
-6

-8+---,---,---~--~~,_--,---,---,---,---,---,-~

2 3 4 5 6 7 8 9 10 11 12 13 14

pH

Figure 2.5 Solubility diagram for aluminium.

0.-----------.-------------------------------,

-2

Qi'
~
Cl
-4 Ferrihydrite (s)
0
...J
Fe'•

-6

Fe(OH);

-8
2 3 4 5 6 7 8 9 10 11 12 13 14

pH

Figure 2.6 Solubility diagram for iron.

24 Biodeterioration of Concrete

red/brown colour, which is sometimes mistakenly interpreted as evidence

for steel reinforcement corrosion.

2.3 Inorganic Acids

2.3.1 Sulphuric acid
Sulphuric acid (H2SO4) is a diprotic acid (meaning it can lose two protons
during dissociation). Its first proton is shed readily (see the extremely
low pKa1 value in Table 2.3), making it a strong acid. However, the second
deprotonation does not occur until a much higher pH is reached. The
stability constants of complexes formed between calcium, aluminium and
iron are given in Table 2.4.
The interaction of Portland cement with sulphuric acid requires
examination both in terms of interaction of the acid separately with
calcium, aluminium and iron ions, and with a combination of the two. This
is because one potential reaction product is ettringite, 3CaO.(Al2O3,Fe2O3)
(CaSO4)3.32H2O, whose characteristics relevant to this discussion are shown

Table 2.3 Acid dissociation constants for sulphuric acid.

Acid Formula Acid Dissociation Constant Reference

pKa1 pKa2
Sulphuric acid H2SO4 –6.38 1.99 [8, 9]
HSO4 –
SO4 2–

hydrogen sulphate sulphate

Table 2.4 Stability constants of complexes formed by calcium,

aluminium and iron(II) and (III) ions in water containing sulphuric acid.

Complex Reaction Stability Reference

Constant
Ca
CaSO4 Ca2+ + SO42– ⇌ CaSO4 2.36
Al
AlSO4+ Al3+ + SO42– ⇌ AlSO4+ 3.89
[7]
Al(SO4)2 –
Al + 2SO4 ⇌ Al(SO4)2
3+ 2– –
4.92
Fe(II)
FeSO4 Fe2+ + SO42– ⇌ FeSO4 2.39
Fe(III)
FeSO4+ Fe3+ + SO42– ⇌ FeSO4+ 4.05
Fe(SO4)2– Fe + 2SO4 ⇌ Fe(SO4)2
3+ 2– –
5.38
 The Chemistry of Concrete Biodeterioration 25

in Table 2.5, along with other solid compounds potentially formed between
calcium, aluminium, iron and sulphate ions.
The solubility diagram of the system comprising calcium, aluminium
and sulphate is shown in Figure 2.7. At high calcium concentrations
the dominant component of the system is portlandite. As the pH falls,
portlandite is replaced by solid gypsum (CaSO4.2H 2O). At calcium
concentrations below the solubility limit of gypsum, CaSO4 in aqueous
solution is predominant, except at high pH, where ettringite is stable. In
the absence of aluminium, the solubility diagram essentially remains the

Table 2.5 Solubility and molar volume data for compounds relevant to the interaction of
hydrated Portland cement with sulphuric acid.

Compound Formula/Reaction Solubility Reference Molar Reference

Product, Volume,
log ksp cm3/mol

Gypsum CaSO4.2H2O ⇌ Ca2+ + SO42– + –4.43 [10] 74.5 [11]

2H2O

Ettringite Ca6[Al(OH)6]2(SO4)3.26H2O ⇌ –44.6 [10] 703.6 [11]

6Ca2+ + 2Al(OH)4– + 3SO4–2 +
26H2O + 4OH–

Ettringite (Fe) Ca6[Fe(OH)6]2(SO4)3.26H2O ⇌ –44.0 [13] 717.4 [11]

6Ca2+ + 2Fe(OH)4– + 3SO42– +
26H2O + 4OH–

Monosulphate 3CaO∙Al2O3∙CaSO4∙12H2O ⇌ –29.4 [10] 308.9 [11]

4Ca2+ + 2Al(OH)4– + SO42– +
4OH– + 6H2O

Monosulphate 3CaO∙Fe2O3∙CaSO4∙12H2O ⇌ –33.2 [13] 321.3 [11]

(Fe) 4Ca2+ + 2Fe(OH)4– + SO42– +
4OH– + 6H2O

Hydrogarnet 3CaO.Al2O3.6H2O ⇌ 3Ca2+ –22.5 [10] 149.5 [11]

2Al(OH)4– + 4OH–

Hydrogarnet 3CaO.Fe2O3.6H2O ⇌ 3Ca2+ + (≥) –26.3 [14] 155.2 [11]

(Fe) 2Fe(OH)4– + 4OH–

Basaluminite Al4(OH)10SO4.5H2O ⇌ 4Al3+ + –117.3 [15] 218.9 [12]

SO42– + 10OH– + 5H2O

Jurbanite Al(OH)SO4.5H2O ⇌ Al3+ + –17.23 [16] 128.9 [12]

SO42– + OH– + 5H2O
26 Biodeterioration of Concrete

0
ortlandite (s

f::ttri
CaS04 .2H 20 (s) >?.g,i:
('!(.s;

-2

ro
~ -4 CaS04
Cl
0
_J Ca 2+

-6

-8 +--------,~-----.-------,--------,-------.-------~

2 4 6 8 10 12 14

pH

Figure 2.7 Solubility diagram for Ca and Al in the presence of sulphuric acid, with
respect to calcium-bearing species and phases. The concentrations of Al and SO42–
are both fixed at 100 mmol/l.

same, with the exception that ettringite and monosulphate are, of course,
absent. Exchanging iron for aluminium, the solubility diagram retains a
similar form (Figure 2.8).
Figure 2.9 examines the Ca-Al-SO4-H2O system from the perspective
of the predominant forms of Al. Ettringite is, again, evident under high
pH conditions. As for the case when no sulphate is present, gibbsite is
dominant at intermediate pH values, whilst the solid phase Jurbanite
(Al(OH)SO4.5H2O) is, in theory, precipitated under more acidic conditions.
However, in reality, the formation of this phase has not been reported. It has
been proposed that in natural soils, a mixture of amorphous Al(OH)3 and
basaluminite (Al4(OH)10SO4.5H2O) is actually formed [17]. Whether this is
the case for hydrated Portland cement is not known, and it is feasible that
Al3+ and SO42– ions are, in fact, present within the silica gel remaining from
the decalcification of CSH gel.
In the Ca-Fe-SO 4-H 2O system (Figure 2.10), the dominance of
ferrihydrite is somewhat reduced relative to the situation where sulphate
is absent. This is due to the formation of FeSO4 in solution. At very low pH,
and in the absence of oxygen, at least in theory, solid iron (II) sulfide forms.
 The Chemistry of Concrete Biodeterioration 27

0
I ~ortland1te (s

C•SO, 2H,0 {') ~ Fe Ettnng1te (s)

-2

rn Ca2 CaS04
~ -4
Cl
0
....J

-6

-8+--l----,------,-------,------,-------,-----~
2 4 6 8 10 12 14

pH

Figure 2.8 Solubility diagram for Ca and Fe in the presence of sulphuric acid,
with respect to calcium-bearing species and phases. The concentrations of Fe and
SO42– are both fixed at 100 mmol/l.

-2
'i-0
.. ,/~
~
Gibbsite (s)

~
Cl
-4
0
_J
Ettringite (s)

Also;
-6

AI(OH)4 -

-8
2 4 6 8 10 12 14

pH
Figure 2.9 Solubility diagram for Al and Ca in the presence of sulphuric acid,
with respect to aluminium-bearing species and phases. The concentrations of
Ca and SO42– are both fixed at 100 mmol/l.
28 Biodeterioration of Concrete

,CD
N(f)

-2 :§:

Ferrihydrite (s) Fe Ettringite (s)

'iil
!:S -4
Cl
0
___J
Fe'•

-6

Fe(OH)4 -

-8
2 4 6 8 10 12 14

pH

Figure 2.10 Solubility diagram for Fe and Ca in the presence of sulphuric acid,
with respect to iron-bearing species and phases. The concentrations of Ca and
SO42– are both fixed at 100 mmol/l.

2.3.2 Nitric acid

Nitric acid is a strong, monoprotic acid with the formula HNO 3 .

Table 2.6 gives the dissociation constant of the compound, which dissociates
to give the nitrate ion—NO3–. Table 2.7 gives the stability constants of
complexes formed by the nitrate ion with various elements relevant to
cement chemistry. Note that it is possible for the nitrate ion to be reduced
to the nitrite ion (NO2–) or the ammonium ion (NH4+) under the appropriate
conditions.
The solubility products of the nitrate salts of calcium, aluminium
and iron are shown in Table 2.8. It is evident that these compounds are
all very soluble. This, along with the fact that nitric acid is only weakly
complexing–indicated by the relatively low stability constants of complexes
formed between iron and calcium ions, and the absence of a complex with
aluminium—means that it is essentially unnecessary to present solubility
diagrams for nitric acid in combination with single metal ions, since the
more generic diagrams in Figures 2.4–2.6 suffice.
However, when calcium and aluminium are present together, a
nitrate AFm phase may be precipitated. The solubility diagram for the
calcium/aluminium/nitrate system is shown in Figure 2.11, showing that
the formation of this phase has the effect of limiting the solubilisation of
calcium until relatively low pH values. No evidence for an iron analogue
 The Chemistry of Concrete Biodeterioration 29

Table 2.6 Acid dissociation constants for nitric acid.

Acid Formula Acid Dissociation Constant Reference
pKa
Nitric acid HNO3 –1.3 [18]
NO3 –

nitrate

Table 2.7 Stability constants of complexes formed by calcium and iron (III) ions
in water containing nitric acid.

Complex Reaction Stability Reference

Constant
H+

NO2– NO3– + 2H+ + 2e– ⇌ NO2– + H2O 28.57

NH4 +
NO3– + 10H+ + 8e– ⇌ NH4+ + 3H2O 119.077 [7]
Ca

CaNO3+ Ca+2 + NO3– ⇌ CaNO3+ 0.50

Fe(III)

FeNO3+2 Fe+3 + NO3– ⇌ FeNO3+2 1.00

Table 2.8 Solubility and molar volume data for compounds relevant to the interaction of
hydrated Portland cement with nitric acid.

Compound Formula/Reaction Solubility Reference Molar Reference

Product, Volume,
log ksp cm3/mol
Calcium nitrate Ca(NO3)2.4H2O ⇌ Ca2+ + 3.30 [8] 124.3 [23]
tetrahydrate 2NO3 + 4H2O
–

Aluminium Al(NO3)3.9H2O ⇌ Al3+ + 2.45 [19] 220.9 [24]

nitrate 3NO3– + 9H2O
nonahydrate

Nitrate AFm 3CaO∙Al2O3∙Ca(NO3)2∙10H2O –28.67 [20] 148.3 [21]

⇌ 4Ca + 2Al(OH)4 +
2+ –

2NO3– + 4OH– + 4H2O

Iron (II) nitrate Fe(NO3)2.6H2O ⇌ Fe2+ + 2.86 [22] unknown –

hexahydrate 2NO3– + 6H2O

Iron (III) nitrate Fe(NO3)3.6H2O ⇌ Fe3+ + 2.92 [8] 197.6 [25]

hexahydrate 3NO3– + 6H2O

Iron (III) nitrate Fe(NO3)3.9H2O ⇌ Fe3+ + 3.56 [22] 224.4 [25]

nonahydrate 3NO3– + 9H2O
30 Biodeterioration of Concrete

\
\
\ \ Nitrate I Nitrite AFm (s)
-2
\
\
\
ro Ca2 • \
~ -4
Cl \
0
....J \
\
\
\

"1 ""-- ..__

-6

2 3 4 5 6 7 8 9 10 11 12 13 14

pH

Figure 2.11 Solubility diagram for Ca and Al in the presence of nitric acid
and nitrous acid, with respect to calcium-bearing species and phases. The
concentrations of Ca and NO3–/NO2– are all fixed at 100 mmol/l. Solid line =
nitric acid; dashed line = nitrous acid.

of the nitrate AFm phase has been found in the literature, but its existence
is possible.

2.3.3 Nitrous acid

Nitrous acid is considerably weaker than nitric acid (Table 2.9), dissociating
to form nitrite ions. It forms even weaker complexes with calcium
than nitric acid, and does not form complexes with aluminium or iron
(Table 2.10). In the presence of calcium, nitrite ions can form the salt calcium
nitrite monohydrate (Table 2.11), although this compound is relatively
soluble. Thus, as for nitric acid, Figures 2.4–2.6 act as solubility diagrams
for nitrous acid with single metal ions. A nitrite AFm phase also exists, with
similar characteristics to the nitrate AFm phase. A calcium/aluminium/
nitrite solubility diagram is also plotted in Figure 2.11, alongside that for
nitrate. As for nitrate AFm, it is possible that an iron-bearing form of the
nitrite AFm phase exists, but this has yet to be documented.

2.3.4 Carbonic acid and ‘aggressive’ carbon dioxide

When water is in contact with an atmosphere containing carbon dioxide

(CO2), the gas will dissolve to some degree, depending on its partial pressure
 The Chemistry of Concrete Biodeterioration 31

Table 2.9 Acid dissociation constants for nitrous acid.

Acid Formula Acid dissociation constant Reference
pKa
Nitrous acid HNO2 3.3 [26]
NO3 –

nitrite

Table 2.10 Stability constants of complexes formed by calcium in water

containing nitrous acid.

Complex Reaction Stability Reference

Constant

Ca

CaNO2+ Ca2+ + NO2– ⇌ CaNO2+ –0.28

[27]
Ca(NO2)2 Ca2+ + 2NO2– ⇌ Ca(NO2)2 –1.03

Table 2.11 Solubility and molar volume data for compounds relevant to the interaction of
hydrated Portland cement with nitrous acid.

Compound Formula/Reaction Solubility Reference Molar Reference

Product, Volume,
log ksp cm3/mol

Calcium nitrite Ca(NO2)2.4H2O ⇌ Ca2+ + 2.30 [22] 67.3 [22]

tetrahydrate 2NO2 + 4H2O
–

Nitrite AFm 3CaO∙Al2O3∙Ca(NO2)2∙10H2O –26.87 [20] unknown

⇌ 4Ca2+ + 2Al(OH)4– +
2NO2– + 4OH– + 4H2O

and the temperature. This dissolved CO2 is often referred to as carbonic acid
(H2CO3), although in reality it will be dissociated to bicarbonate or carbonate
ions to a degree determined by pH and temperature. The dissociation
constants for carbonic acid at 20°C are given in Table 2.12.
The relationship between the concentration of dissolved carbonic acid
—[H2CO3]—and the partial pressure of CO2–PCO2—is given by the equation:

[H2CO3] = KCO2 PCO2

where KCO2 is an equilibrium constant whose value is 1 × 101.41 at 20°C, and
whose magnitude increases with temperature [28].
32 Biodeterioration of Concrete

Carbonic acid undergoes very little by way of interaction with ions

encountered in Portland cement, as shown in Table 2.13. However, its
interaction with calcium is of great importance. Under higher pH conditions,
calcium carbonate (CaCO3) is formed (Figure 2.12). The most stable form of
calcium carbonate at normal ambient conditions is calcite, although vaterite
is often also formed, gradually converting to calcite. The aragonite form may
be precipitated at elevated temperatures. At lower pH values, calcium forms
CaHCO3+, with the charge balanced by a bicarbonate ion. This means that
calcium in this configuration can be viewed as being present as Ca(HCO3)2
—calcium hydrogencarbonate (also referred to as calcium bicarbonate).
Calcium hydrogencarbonate is essentially a hypothetical entity, since it
implies that a salt with this composition can be precipitated from solution,
which is not actually the case. However, it is useful in explaining the effect
of the presence of calcium in a solution exposed to CO2. The reaction which
converts calcium carbonate to calcium hydrogencarbonate can be written as:
H2CO3 + CaCO3 ⇌ Ca(HCO3)2
The equilibrium of this reaction is shifted to the right by a drop in pH,
such as through the dissolution of more carbonic acid. However, calcium
hydrogencarbonate associates two molecules of CO2 to one calcium ion,
compared to CaCO3, where the ratio is 1:1. Therefore, the formation of

Table 2.12 Acid dissociation constants for carbonic acid.

Acid Formula Acid Dissociation Constant Reference

pKa1 pKa2
Carbonic acid H2CO3 6.38 10.38 [28]
HCO3– CO32–
bicarbonate carbonate

Table 2.13 Stability constants of complexes formed by calcium and iron(II)

and (III) ions in water containing carbonic acid.

Complex Reaction Stability Reference

Constant
Ca
Ca+2 + H+ + CO32– ⇌ CaHCO3+ 11.60
Ca2+ + CO32– ⇌ CaCO3 3.20 [34]
Fe(II)
Fe2+ + H+ → CO32– ⇌ FeHCO3+ 11.43
 The Chemistry of Concrete Biodeterioration 33

0
"{ortlandite (s)

-2
Calcite (s)

ro
~ -4

~"'---
Cl
0
_J Ca'·

+
00
-6
I
0 "' CaC03

-8
2 4 6 8 10 12 14

pH
Figure 2.12 Solubility diagram for Ca in the presence of carbonic acid.

calcium hydrogencarbonate acts to limit the decrease in pH as more CO2

dissolves into water.
This effect is shown in Figure 2.13, which plots the pH of two quantities
of water exposed to increasing partial pressures of CO2. Where only water
is present there is rapid decrease in pH as partial pressure increases, with
pH levelling out at higher partial pressures. This behavior is to be expected
given the logarithmic nature of pH. However, when a small quantity of
portlandite is introduced into the water, the shape of the curve is altered,
with higher pH values persisting to greater partial pressures. Thus, in
systems containing Portland cement, dissolution of the cement matrix will
only occur at relatively high CO2 concentrations where enough is present
to exceed the capacity for calcium hydrogencarbonate to act as a ‘sink’. The
resulting ‘free’ CO2 is sometimes referred to as ‘aggressive CO2’.
Table 2.14 provides solubility products for calcium and iron carbonates,
but also includes calcium aluminate and ferrite phases which contain
carbonate ions. These include the AFt carbonate phase—which is structurally
analogous to ettringite—and monocarbonate and hemicarbonate phases,
which are members of the AFm group of cement hydration products along
with monosulfate. Figures 2.14 and 2.15 show solublity diagrams for
systems in which Ca and Al are present. The dominant calcium aluminate
phase in both diagrams is monocarbonate, although it should be noted that,
overall, the solubility diagrams are not altered much.
34 Biodeterioration of Concrete

14

12 1:---- __
\
10 I \
\
\

--- -- ------
I 8 I
c.
\

ll
Water + 0.001 moles portlandite

Water
4

I
2 ,_-------,--------,--------,--------,-------~
0.0 0.2 0.4 0.6 0.8 1.0

PARTIAL PRESSURE C02 , atm

Figure 2.13 Change in pH of water versus the partial pressure of CO2 in contact
with it for two scenarios: (i) pure water; (ii) water in contact with portlandite.

Figure 2.16 is a solubility diagram for iron in the presence of carbonic

acid. The main features of this plot that differentiate it from the diagram
obtained in the absence of carbonic acid is the presence of the FeHCO3+
complex and the mineral siderite (FeCO3). Figures 2.17 and 2.18 show the
situation where both Ca and Fe are present. As for Al, the presence of the iron
analogue of monocarbonate is the only significant change to the diagrams.

2.4 Organic Acids

2.4.1 Formic acid

Formic acid is a carboxylic acid with the formula HCOOH, making it the
simplest of carboxylic acid compounds. It is monoprotic and is relatively
weak (Table 2.15). It is capable of forming complexes with Ca, Al and
Fe(III) ions (Table 2.16). Although the acid mainly forms weak complexes,
Al3(OH)2(CHO2)6+ is the exception to this. Formic acid forms salts with Ca, Al
and Fe(II), all of which are relatively soluble in water (Table 2.17). Aluminium
formate and iron (III) formate have previously been thought to have the
formulae Al(CHO2)3∙3H2O and Fe(CHO2)3∙H2O respectively. However, a
recent crystallographic study has concluded that the compounds have the
formulas Al(CHO2)3(CH2O2)0.25(CO2)0.75∙0.25H2O and Fe(CHO2)3(CH2O2)0.25
(CO2)0.75∙0.25H2O, where CH2O2 is a fully protonated formic acid molecule
 The Chemistry of Concrete Biodeterioration 35

Table 2.14 Solubility and molar volume data for compounds relevant to the interaction of
hydrated Portland cement with carbonic acid.

Compound Formula/Reaction Solubility Reference Molar Reference

Product, Volume,
log ksp cm3/mol

Vaterite CaCO3 ⇌ Ca2+ + CO32– –7.91 [29] 38 [11]

Aragonite CaCO3 ⇌ Ca2+ + CO32– –8.30 [7] 34 [11]

Calcite CaCO3 ⇌ Ca2+ + CO32– –8.48 [7] 37 [11]

Siderite FeCO3 ⇌ Fe2+ + CO32– –10.24 [7] 29 [30]

AFt carbonate Ca6Al2(CO3)3(OH)12.24H2O –49.19 [31] 652 [11]

⇌ 6Ca + 2Al(OH) + 3CO3 +
2+
4
– 2–

4OH– + 24H2O

Monocarbonate Ca4Al2(CO3)(OH)12.5H2O ⇌ –31.47 [32] 262 [32]

4Ca2+ + 2Al(OH)4– + CO32– +
4OH– + 5H2O

Monocarbonate Ca4Fe2(CO3)(OH)12.6H2O ⇌ –34.59 [32] 292 [32]

(Fe) 4Ca2+ + 2Fe(OH)4– + CO32– +
4OH– + 6H2O

Hemicarbonate Ca4Al2(CO3)0.5(OH)13.6H2O ⇌ –29.13 [32] 285 [32]

4Ca2+ + 2Al(OH)4– + 0.5CO32– +
5OH– + 6H2O

Hemicarbonate Ca4Fe2(CO3)0.5(OH)13.4H2O ⇌ –30.83 [32] 273 [32]

(Fe) 4Ca2+ + 2Fe(OH)4– + 0.5CO32– +
5OH– + 4H2O

Thaumasite Ca6Si2(SO4)2(CO3)2(OH)12.24H2O –49.40 [31] 331 [11]

⇌ 2H3SiO4 + 6Ca + 2SO4 +
– 2+ 2–

2CO32– + 2OH– + 26H2O

and CO2 is a gaseous carbon dioxide molecule trapped in the cage-like

structures of the compounds.
AFt and AFm phases containing the formate ion have also been reported
and characterized in terms of their structure [38]. Whilst solubility data for
these phases is not available, it is likely that behaviour comparable to that
seen for sulphuric acid is probable.
The solubility diagram for calcium (Figure 2.19) includes a large area
in which the 1:1 complex between calcium and the formate ion dominates.
However, from a solubility perspective, this has little influence over the areas
36 Biodeterioration of Concrete

-2 Portlandite (s)
Calcite (s)
Monocarbonate (s)
ro
~ -4
Ol
0
...J Ca2-

OM
-6 u
I
u"' CaC03

-8
2 4 6 8 10 12 14

pH

Figure 2.14 Solubility diagram for Ca and Al in the presence of carbonic acid,
with respect to calcium-bearing species and phases. The concentrations of Al
and CO32– are fixed at 100 mmol/l.

Monocarbonate (s)

Gibbsite (s)
-2

<(
Ol -4
0
...J Al3 +

AI(OHJ;
-6

2 3 4 5 6 7 8 9 10 11 12 13 14

pH
Figure 2.15 Solubility diagram for Ca and Al in the presence of carbonic acid, with
respect to aluminium-bearing species and phases. The concentrations of Ca and
CO32– are fixed at 100 mmol/l.
 The Chemistry of Concrete Biodeterioration 37

Figure 2.16 Solubility diagram for Fe in the presence of carbonic acid.

Calcite (s)

-2 Portlandite (s)

Monocarbonate (Fe) (s)

ro
~ -4
Cl
0
_J
ca' -

-6
0
(.)
:r:
"'
(.) CaC0 3

-8
2 4 6 8 10 12 14

pH

Figure 2.17 Solubility diagram for Ca and Fe in the presence of carbonic acid,
with respect to calcium-bearing species and phases. The concentrations of Fe
and CO32– are fixed at 100 mmol/l.
38 Biodeterioration of Concrete

Siderite (s)

-2
Monocarbonate (Fe) (s)

Q) Ferrihydrite (s)
~ -4
0
_J

-6

2 3 4 5 6 7 8 9 10 11 12 13 14

pH

Figure 2.18 Solubility diagram for Ca and Fe in the presence of carbonic acid,
with respect to iron-bearing species and phases. The concentrations of Ca and
CO32– are fixed at 100 mmol/l.

Table 2.15 Acid dissociation constants for formic acid.

Acid Formula Acid Dissociation Constant Reference

pKa
Formic acid HCOOH 3.75 [33]
O H
HCO2–
H
formate
O

Table 2.16 Stability constants of complexes formed by calcium, aluminium and iron
(III) ions in water containing formic acid.

Complex Reaction Stability Reference

Constant
Ca
Ca2+ + CHO2– ֖ Ca(CHO2)+ 1.43
Al
Al3+ + CHO2– ֖ Al(CHO2)2+ 1.36
3Al + 2OH + 6CHO2 ֖ Al3(OH)2(CHO2)6
3+ – – +
19.90
[33]
Fe(III)
Fe3+ + CHO2– ֖ Fe(CHO2)2+ 3.10
 The Chemistry of Concrete Biodeterioration 39

Table 2.17 Solubility and molar volume data for compounds relevant to the interaction of
hydrated Portland cement with formic acid.

Compound Formula/Reaction Solubility Reference Molar Reference

Product, Volume,
log ksp cm3/mol

Calcium Ca(CHO2)2 ⇌ Ca2+ + 2CHO2– 0.92 [34] 64 [35]

formate

‘Aluminium Al(CHO2)3(CH2O2)0.25(CO2)0. –0.98 [34] 117 [36]

formate’ .0.25H2O ⇌ Al3+ + 3CHO2–
75

+ 0.75CO2(g) + 0.25CH2O2 +
0.25H2O

Aluminium AlOH(CHO2)2.0.5H2O ⇌ –1.17 [34] Unknown –

formate Al3+ + OH– + 2CHO2– +
hemihydrate
(monobasic) 0.5H2O

Formate Ca4Al2(CHO2)2(OH)12.5H2O Unknown – Unknown –

AFm ⇌ 4Ca2+ + 2Al(OH)4– +
2CHO2– + 4OH– + 5H2O

Formate AFt Ca6Al2(CHO2)6(OH)12.26H2O Unknown – Unknown –

⇌ 6Ca2+ + 2Al(OH)4– +
6CHO2– + 4OH– + 26H2O

Iron (II) Fe(CHO2)2.2H2O ⇌ Fe2+ + –1.30 [34] 88 [37]

formate 2CHO2– + 2H2O
dihydrate

‘Iron (III) Fe(CHO2)3(CH2O2)0.25(CO2)0. ‘soluble’ [8] 126 [36]

formate’ .0.25H2O ⇌ Fe3+ + 3CHO2–
75

+ 0.75CO2(g) + 0.25CH2O2 +
0.25H2O

in which solid phases dominate. In the case of aluminium, the formation of

the Al3(OH)2(CHO2)6+ complex has the effect of assuming dominance over
solid gibbsite in the lower pH region of the plot (Figure 2.20).

2.4.2 Acetic acid

Like formic acid, acetic acid is a carboxylic acid. The acetic acid molecule
contains two carbon and is slightly weaker than formic acid (Table 2.18).
The acetate ion forms weak complexes with Ca, Al and Fe(II) ions,
but forms stronger complexes with Fe(III) (Table 2.19). Many of the salts
formed by acetic acid with Ca, Al and Fe are soluble (Table 2.20). Aluminium
40 Biodeterioration of Concrete

\ ~
2
'0
c

~"'
o._
-2

ro
~ -4
Cl
0
....J
Ca2 + Ca(Formatet
\
'I
0
-6 "'
(.)

2 3 4 5 6 7 8 9 10 11 12 13 14

pH
Figure 2.19 Solubility diagram for Ca in the presence of formic acid.

0.--.----------------------------------------------~

Gibbsite (s)
-2

:l
Cl -4
AI3 (0H)2(Formate)6 '

0
....J

A I(OH)4 -
-6

-8 +---~-------,---.--L,----,---.---,---,----,---.--~

2 3 4 5 6 7 8 9 10 11 12 13 14

pH

Figure 2.20 Solubility diagram for Al in the presence of formic acid.

 The Chemistry of Concrete Biodeterioration 41

Table 2.18 Acid dissociation constants for acetic acid.

Acid Formula Acid Dissociation Constant Reference

pKa
Acetic acid CH3COOH 4.756 [8]
H O H CH3CO2 –

H
acetate
H O

Table 2.19 Stability constants of complexes formed by calcium, aluminium

and iron(II) and (III) ions in water containing acetic acid.

Complex Reaction Stability Reference

Constant
Ca
Ca2+ + CH3CO2– ֖ Ca(CH3CO2)+ 1.18
Al
Al3+ + CH3CO2– ֖ Al(CH3CO2)2+ 1.51 [33]
Fe(II)
Fe2+ + CH3CO2– ֖ Fe(CH3CO2)+ 1.40
Fe(III)
Fe3+ + CH3CO2– ֖ Fe(CH3CO2)2+ 4.02
Fe + 2CH3CO2 ֖ Fe(CH3CO2)2
3+ – +
7.57
Fe3+ + 3CH3CO2– ֖ Fe(CH3CO2)3 9.59

diacetate (Al(CH3CO2)2OH) and iron (III) diacetate (Fe(CH3CO2)2OH) are

identified as being insoluble in the literature, but quantitative solubility
data is currently unavailable. For this reason, in generating the solubility
diagrams for Al and Fe, a solubility of 0.05 g/100 g solution—which is
generally considered to be the mid-point of the range of solubilities for
substances described as being ‘practically insoluble’—has been assumed.
It should be noted that there also exists an anhydrous calcium acetate
salt—Ca(CH3CO2)2—although this is of little relevance in the context of
aqueous systems, since it reacts with moisture rapidly to give the hydrated
form of the salt.
Figure 2.21 shows a solubility diagram for calcium in the presence
of acetic acid, with the Ca(CH3CO2)+ complex occupying a substantial
proportion of the plotted area, but with little change with regards to the solid
phases present. The same is true of aluminium (Figure 2.22) with only the
presence of the Al(CH3CO2)2+ complex distinguishing it from the solubility
plot obtained in the absence of acetic acid. The acetate ion forms relatively
42 Biodeterioration of Concrete

Table 2.20 Solubility and molar volume data for compounds relevant to the interaction of
hydrated Portland cement with acetic acid.

Compound Formula/Reaction Solubility Reference Molar Reference

Product, Volume,
log ksp cm3/mol
Calcium Ca(CH3CO2)2.H2O ⇌ 1.38 [39] 117 [40]
diacetate Ca2+ + 2CH3CO2– +
monohydrate
H 2O
Calcium Ca(CH3CO2)2.2H2O ⇌ 1.76 [41] unknown –
diacetate Ca2+ + 2CH3CO2– +
dihydrate
2H2O
Calcium CaH(CH3CO2)3.H2O 3.07 [41] 158 [42]
hydrogen ⇌ Ca2+ + CH3COOH +
triacetate
2CH3CO2– + H2O
monohydrate
Aluminium Al(CH3CO2)3 ⇌ Al3+ + ‘Soluble’ [43] unknown –
triacetate 3CH3CO2–
Aluminium Al(CH3CO2)2OH ⇌ Al3+ ‘Insoluble’ [8] unknown –
diacetate + 2CH3CO2– + OH–
Aluminium Al(CH3CO2)OH2 ⇌ Al3+ Assumed – unknown –
monoacetate + CH3CO2– + 2OH– to be
soluble
Iron (II) Fe(CH3CO2)2.4H2O ‘Soluble’ [8] 127 [44]
acetate ⇌ Fe2+ + 2CH3CO2– +
4H2O
Iron (III) Fe(CH3CO2)2OH ⇌ Fe3+ ‘Insoluble’ [8] unknown –
diacetate + 2CH3CO2– + OH–

stable complexes with Fe (III), but, again, this has a negligible influence over
the manner in which solid phases are precipitated (Figure 2.23).

2.4.3 Lactic acid

Lactic acid is a carboxylic acid with the formula CH3CH(OH)COOH.

The second carbon atom in the sequence described by the formula is an
asymmetric carbon atom, meaning that it is attached to four different groups
of atoms (–OH, –CH3, –H and –COOH). Where such a structural feature
is present in a molecule, it can be certain that the compound is chiral—its
molecules can exist as more than one optical isomer. In the case of lactic
acid two isomers exist—the D- and L-forms which are mirror images of
each other. It is the L form which is produced by living organisms. Where
a mixture of the two forms is present, it is referred to as DL lactic acid.
Differences exist between many of the properties of these two forms, and
 The Chemistry of Concrete Biodeterioration 43

~
~
Jll
uc
'2 "'
0
-2 ()_

rn
Q
Ol
0
_J
-4
Ca2 + Ca(Acetate(
1\
+I
0
-6 "'
0

2 3 4 5 6 7 8 9 10 11 12 13 14

pH

Figure 2.21 Solubility diagram for Ca in the presence of acetic acid.

0,--,----------------------------------------~

Gibbsite (s)
-2

~
Ol -4
0
_J

AI( Acetate)'•

AI(OH)4-
-6

2 3 4 5 6 7 8 9 10 11 12 13 14

pH

Figure 2.22 Solubility diagram for Al in the presence of acetic acid.

this extends to their behaviour in solution with metal ions. Thus, wherever
possible, constants relating to lactic acid have been obtained for the L-form.
Lactic acid contains two –OH groups, but only the carboxylate group
normally undergoes deprotonation (Table 2.21). However, interaction with
44 Biodeterioration of Concrete

0,-----------.---------------------------------,

-2

'ID Ferrihydrite (s)

~ -4
0
_J

-6 Fe(Acetatet

2 3 4 5 6 7 8 9 10 11 12 13 14

pH

Figure 2.23 Solubility diagram for Fe in the presence of acetic acid.

Table 2.21 Acid dissociation constants for lactic acid.

Acid Formula Acid Dissociation Constant Reference

pka
Lactic acid CH3CH(OH)COOH 3.86 [46]
–
HO OH CH3CH(OH)CO2
lactate
H3C O

some metal ions allows further deprotonation, as evidenced by the existence

of the Al(CH3COCO2)(CH3C(OH)CO2) complex (Table 2.22). The complexes
formed between lactic acid with calcium and aluminium ions are weak. It is
likely that iron also forms complexes with lactic acid, but reliable stability
constants have not been reported in the literature [45].
Lactic acid forms salts with calcium, aluminium and iron. However,
they are all of relatively high solubility (Table 2.23). This, coupled with
the nature of the complexes formed, means that the solubility diagrams
remain largely unchanged, with the exception of aluminium, where the
Al(CH3COCO2)(CH3C(OH)CO2) complex prevails over gibbsite under more
acidic conditions (Figure 2.24).
 The Chemistry of Concrete Biodeterioration 45

Table 2.22 Stability constants of complexes formed by calcium and aluminium ions in water
containing lactic acid.

Complex Reaction Stability Reference

Constant
Ca
Ca2+ + CH3CH(OH)CO2– ⇌ Ca(CH3CH(OH)CO2)+ 0.90
Ca2+ + 2CH3CH(OH)CO2– ⇌ Ca(CH3CH(OH)CO2)2 1.24 [33]

Al
Al3+ + CH3CH(OH)CO2– ⇌ Al(CH3CH(OH)CO2)2+ 1.21
Al3+ + 2CH3CH(OH)CO2– ⇌ Al(CH3CH(OH)CO2)2+ 2.72 [47]
Al + 3CH3CH(OH)CO2 ⇌ Al(CH3CH(OH)CO2)3
3+ –
4.92 (DL-Lactate)

Al3+ + 2CH3CH(OH)CO2– ⇌ Al(CH3CHOCO2) 6.17

(CH3CH(OH)CO2) + H+

Table 2.23 Solubility and molar volume data for compounds relevant to the interaction of
hydrated Portland cement with lactic acid.

Compound Formula/Reaction Solubility Reference Molar Reference

Product, Volume,
log ksp cm3/mol

Calcium Ca(CH3CH(OH) –0.97 [48] – –

lactate CO2)2.5H2O
pentahydrate
⇌ Ca2+ + 2CH3CH(OH)
CO2– + 5H2O

Aluminium Al(CH3CH(OH)CO2)3 1.16 [48] 212 [49]

lactate ⇌ Al3+ + 3CH3CH(OH)
CO2–

Iron (II) Fe(CH3CH(OH) –1.87 [48] – –

lactate CO2)2.3H2O ⇌ Fe2+ +
trihydrate
2CH3CH(OH)CO2– +
3H2O

Iron (III) Fe(CH3CH(OH)CO2)3 ⇌ ‘soluble’ [8] – –

lactate Fe3+ + 3CH3CH(OH)CO2–
46 Biodeterioration of Concrete

0.--.----------------------------------------------~

Gibbsite (s)
-2

~
Cl -4
.3 AI(CH 3CHOC02)(Lactate) AI(OH);

-6

-8+---.---.---.---.---.---.---+---.---.---.---.-~
2 3 4 5 6 7 8 9 10 11 12 13 14

pH
Figure 2.24 Solubility diagram for Al in the presence of lactic acid.

2.4.4 Glycolic acid

Glycolic acid has the formula CH2(OH)COOH and possesses a carboxylate

group as well as a hydroxyl group. Normally, it is only the carboxyl
group which undergoes deprotonation to form a glycolate ion (Table
2.24). However, in the presence of iron (III) the hydroxyl group may also
deprotonate (Table 2.25).
Very little data exists on the glycolate salts of Ca, Al and Fe both in terms
of their solubility and crystal structure (Table 2.26), and no evidence can be
found for Fe (II) salts. However, it would appear that all of the compounds
formed are relatively soluble. The Ca and Al solubility diagrams in the
presence of glycolic acid are shown in Figures 2.25 and 2.26. The diagram
for iron is particularly of note, since it shows that ferrihydrite is destabilized
by the formation of iron-glycolate complexes.

2.4.5 Oxalic acid

Oxalic acid—C2O4H2—is a relatively strong acid that can shed two protons
from two carboxylate groups. The acid dissociation constants of oxalic acid
are provided in Table 2.27. The stability constants of complexes formed
between the acid and Ca and Fe ions are provided in Table 2.28.
Oxalic acid is capable of forming a number of salts with calcium of
relatively low solubility, the only difference in composition being the
amount of associated water of crystallization. Three of these compounds are
 The Chemistry of Concrete Biodeterioration 47

Table 2.24 Acid dissociation constants for glycolic acid.

Acid Formula Acid Dissociation Constant Reference

pKa
Glycolic acid CH2(OH)COOH 3.83 [33]
OH CH2(OH)CO2–
HO
glycolate
O

Table 2.25 Stability constants of complexes formed by calcium and iron(II) and (III)
ions in water containing glycolic acid.

Complex Reaction Stability Reference

Constant
Ca
Ca2+ + CH2(OH)CO2– ֖ Ca(CH2(OH)CO2)+ 1.62 [33]
Fe (II)
Fe2+ + CH2(OH)CO2– ֖ Fe(CH2(OH)CO2)+ 1.33 [33]
Fe (III)
Fe3+ + CH2(OH)CO2– ֖ Fe(CH2(OH)CO2)2+ 2.90

Fe3+ + CH2(OH)CO2– ֖ Fe(CH2OCO2)+ + H+ 4.21

Fe + 2CH2(OH)CO2 ֖ Fe(CH2OCO2)
3+ –
6.61
(CH2(OH)CO2) + H+ [33]

Fe3+ + 3CH2(OH)CO2– ֖ 8.11

Fe(CH2OCO2)2(CH2(OH)CO2)2– + H+

Table 2.26 Solubility and molar volume data for compounds relevant to the interaction of
hydrated Portland cement with glycolic acid.

Compound Formula/Reaction Solubility Reference Molar Reference

Product, log Volume,
ksp cm3/mol

Calcium Ca(CH2(OH)CO2)2·H2O ֖ –2.71 [50] unknown –

glycolate Ca2+ + 2CH2(OH)CO2– +
monohydrate
H 2O

Aluminium Al(CH2(OH)CO2)3 ֖ Al3+ + Assumed to – unknown –

glycolate 3CH2(OH)CO2 be soluble

Iron (III) Fe(CH2(OH)CO2)3 ֖Fe3+ + Assumed to – unknown –

glycolate 3CH2(OH)CO2– be soluble
48 Biodeterioration of Concrete

0
Ca2 •
\ ~
l!l
'6
c:
"'
t:
0
-2 D..

rn
~ -4
Cl
0
__J
Ca(Giycolatet
1\
+I
0
-6 u "'

-8+---~-,---,---.--,---,---,--,---,------~~
2 3 4 5 6 7 8 9 10 11 12 13 14

pH

Figure 2.25 Solubility diagram for Ca in the presence of glycolic acid.

Ferrihydrite (s)

-2

/
Qj'
~
-4
Ol
0
__J Fe2-+
Fe(CH,OCO, ), (Giycolate( ~

'i:
0
Q)
lJ._
-6

-8+---.-~.---.---.---.---.---.---.---.---.---.-L-4
2 3 4 5 6 7 8 9 10 11 12 13 14

pH

Figure 2.26 Solubility diagram for Fe in the presence of glycolic acid.

naturally occurring minerals: the monohydrate (whewellite), the dihydrate

(weddellite) and the trihydrate (caoxite). The aluminium and iron (III) salts
are highly insoluble in water, with the iron (II) salt being slightly more
soluble. The solubility products are given in Table 2.29.
 The Chemistry of Concrete Biodeterioration 49

Table 2.27 Acid dissociation constants for oxalic acid.

Acid Formula Structure Acid Dissociation Constant Reference
pKa2 pKa1
Oxalic HOOCCOOH 1.23 4.19 [51]
acid (HOOCCOO)– (OOCCOO)2–

Hydrogen oxalate Oxalate

Table 2.28 Stability constants of complexes formed between the oxalate ion relevant to cement
chemistry.

Species Reaction Stability Reference

Constant,
Log K

Ca(Oxalate) Ca2+ + C2O42– ⇌ Ca(C2O4) 3.19 [52]

Ca(Oxalate)22– Ca2+ + 2C2O42– ⇌ Ca(C2O4)22– 8.10 [52]

CaH(Oxalate)+ Ca2+ + C2O42– + H+ ⇌ CaH(C2O4)+ 6.03 [53]

CaH2(Oxalate)2 Ca2+ + 2C2O42– + 2H+ ⇌ CaH2(C2O4)2 10.18 [53]

Al(Oxalate)+ Al3+ + C2O42– ⇌ Al(C2O4)+ 7.7 [7]

Al(Oxalate)2– Al3+ + 2C2O42– ⇌ Al(C2O4)2– 13.4 [7]

Al(Oxalate)33– Al3+ + 3C2O42– ⇌ Al(C2O4)33– 17.0 [7]

AlH(Oxalate)2+ Al3+ + C2O42– + H+ ⇌ AlH(C2O4)2+ 7.5 [7]

AlOH(Oxalate) Al3+ + C2O42– + H2O ⇌ AlOH(C2O4) + H+ 2.6 [7]

AlOH(Oxalate)2 2–
Al + 2C2O
3+
4
2–
+ H2O ⇌ AlOH(C2O ) 4 2
2–
+H
+
6.8 [7]

Al(OH)2(Oxalate)– Al3+ + C2O42– + 2H2O ⇌ Al(OH)2(C2O4)– + 2H+ –3.1 [7]

Fe(Oxalate) Fe2+ + C2O42– ⇌ Fe(C2O4) 3.97 [7]

Fe(Oxalate)22– Fe2+ + 2C2O42– ⇌ Fe(C2O4)2– 5.90 [7]

Fe(Oxalate)+ Fe3+ + C2O42– ⇌ Fe(C2O4)+ 9.15 [7]

Fe(Oxalate)2– Fe3+ + 2C2O42– ⇌ Fe(C2O4)2– 15.45 [7]

Fe(Oxalate)33– Fe3+ + 3C2O42– ⇌ Fe(C2O4)33– 19.83 [7]

FeH(Oxalate)2+ Fe3+ + C2O42– + H+ ⇌ FeH(C2O4)2+ 4.35 [56]

50 Biodeterioration of Concrete

Table 2.29 Solubility products of Ca, Al and Fe salts of oxalic acid.

Compound Reaction Solubility Reference Molar Reference

Product, Volume,
log ksp cm3/mol

Ca(Oxalate) Ca(C2O4) ⇌ Ca2+ + –8.56 [54] 65.2 [57]

C2O42–

Ca(Oxalate).H2O Ca(C2O4).H2O ⇌ Ca2+ + –8.69 [54] 63.8 [58]

C2O42– + H2O

Ca(Oxalate).2H2O Ca(C2O4).2H2O ⇌ Ca2+ –8.35 [54] 79.2 [58]

+ C2O42– + 2H2O

Ca(Oxalate).3H2O Ca(C2O4).3H2O ⇌ Ca2+ –8.29 [54] 95.3 [59]

+ C2O42– + 3H2O

Al2(Oxalate)3.4H2O Al2(C2O4)3.4H2O ⇌ –33.46 [56] Unknown –

2Al + 3C2O
3+
4
2–
+ 4H2O

Fe(Oxalate).2H2O Fe(C2O4).2H2O ⇌ Fe2– + –4.73 [55] 78.0 [60]

C2O42– + 2H2O

Fe2(Oxalate)3.5H2O Fe2(C2O4)3.5H2O ⇌ 2Fe3+ –38.52 [56] Unknown –

+ 3C2O42– + 5H2O

The low solubility of calcium oxalate means that this solid phase
persists even at low pH (Figure 2.27). Whilst the formation of complexes
with oxalate ions destabilizes gibbsite and ferrihydrite, the low solubility
of the Al and Fe oxalate salts means that solid phases, again, are present in
significant quantities—at least at higher metal concentrations—across the
full pH range (Figures 2.28 and 2.29).
Oxalic acid is one of the few organic compounds known to form
complexes with silicon. However, the complex is weak, having a stability
constant for the reaction H4SiO4 + C2O42– ⇌ Si(C6H5O7)(OH)42– of 0.04 [61].

2.4.6 Pyruvic acid

Pyruvic acid is a relatively strong organic acid possessing three carbon

atoms, one carboxylate group and one ketone (=O) making it the simplest
of a group of acids known as the alpha-keto acids. Only the carboxylate
group undergoes deprotonation (Table 2.30). Pyruvic acid forms a weak
complex with calcium (Table 2.31), but no evidence exists of any complexes
being formed between the pyruvate ion and either aluminium or iron.
However, using a prediction technique based on the affinity of metal ions
for the hydroxide ion (log K1(OH–)) [62], it is unlikely that the log K forms
of these stability constants would exceed a value of 3.0.
 The Chemistry of Concrete Biodeterioration 51

0,-----------------------------------~~~~~
'\{ortlandite (s)

Ca(Oxalate).H 2 0 (s)

Ca(Oxalate)t

ro
2.
Cl
-4
.3

-6

-8+-------.------.-------,------.-------,-----~
2 4 6 8 10 12 14
pH
Figure 2.27 Solubility diagram for Ca in the presence of oxalic acid.

Gibbsite (s)
Al 2 (0xalate)3 .4H,O (s)

-2

<(
Cl -4
0
_J

AI(Oxalate)/ -
AI(OH)4-

-6

AI(OH) 2 (0xalater

-8
2 4 6 8 10 12 14
pH
Figure 2.28 Solubility diagram for Al in the presence of oxalic acid.
52 Biodeterioration of Concrete

0
Fe 3 (0H) 6 (s)
Fe 2 (0xalate) 3 .5H,O (s)

-2

Ferri hydrite (s)

Q)
f::.. -4
Ol
0
_J 2Cll
roX Fe(Oxalate) 33-
0
Q)
LL
-6

Fe(OH) 4-

-8
2 4 6 8 10 12 14

pH

Figure 2.29 Solubility diagram for Fe in the presence of oxalic acid.

Table 2.30 Acid dissociation constants for pyruvic acid.

Acid Formula Structure Acid Dissociation Reference

Constant
pKa
Pyruvic CH3C(=O)COOH O 2.50 [46]
H3C
OH CH3C(=O)COO–
O
Pyruvate

Table 2.31 Stability constants of complexes formed by calcium ions in

water containing pyruvic acid.

Species Reaction Stability Reference

Constant,
Log K
Ca(Pyruvate) Ca2+ + CH3C(=O)COO– ֖ 1.08 [46]
Ca(CH3C(=O)COO)+
 The Chemistry of Concrete Biodeterioration 53

Only the salt calcium pyruvate is known to form (Table 2.32), and
quantitative data for this compound is seemingly lacking. However, on the
assumption that calcium pyruvate is slightly soluble, a solubility of 5 g/l
has been assumed, leading to the solubility diagram shown in Figure 2.30,
indicating predominance of this compound at higher Ca concentrations
over a relatively wide pH range.

2.4.7 Succinic acid

Succinic acid is structurally similar to oxalic acid in that it possesses two

carboxylate groups at each end of the molecule. However, there is a longer
chain of carbon atoms between these groups (Table 2.33). This has the effect
of reducing the solubility of the acid relative to oxalic acid.
Succinic acid forms numerous complexes with aluminium, many of
which involve additional deprotonation (Table 2.34). The succinate ion
forms relatively insoluble salts with calcium (Table 2.35 and Figure 2.31). It
also forms Fe(II) salts. Little is documented regarding the tetrahydrate, and
the other salt, whilst of low solubility, appears to be formed only through
hydrothermal synthesis techniques [68].

Table 2.32 Solubility products of Ca salts of pyruvic acid.

Compound Reaction Solubility Reference Molar Reference

Product, Volume,
Log Ksp cm3/mol
Ca(Pyruvate) Ca(CH3C(=O) ‘slightly [63] unknown –
COO)2.2.5H2O ֖ soluble’
Ca2+ + 2CH3C(=O)
COO– +2.5H2O

Table 2.33 Acid dissociation constants for succinic acid.

Acid Formula Structure Acid Dissociation Constant Reference

pKa2 pKa1
Succinic (CH2)2(COOH)2 OH 4.00 5.42 [53]
acid O
O
HOOC(CH2)2COO– [(CH2)2(COO)2]2–
HO

Hydrogen Succinate
succinate
54 Biodeterioration of Concrete

-2 ~ Ca(Pyruvate) 2 SH,O (s)

~ "%
~

ro
~ -4
Cl
[\
0 Ca2•
__J Ca(Pyruvate)
()

"'0
I+
-6

-8 +---~-------.---.---.---.---.---.---.---..-~--~

2 3 4 5 6 7 8 9 10 11 12 13 14

pH

Figure 2.30 Solubility diagram for Ca in the presence of pyruvic acid.

2.4.8 Malic acid

Malic acid possesses two carboxylate groups and a hydroxyl group, with
only the carboxylate groups normally undergoing deprotonation (Table 2.36).
It forms an extremely varied series of complexes with aluminium and iron
(III) (Table 2.37) where further deprotonation occurs. It forms iron (II) and
calcium salts. Whilst there is little data on several of these, the dihydrate
and trihydrate calcium salts are known to be of relatively low solubility
(Table 2.38) leading to its predominance at higher calcium concentrations
on the calcium solubility diagram (Figure 2.32).

2.4.9 Tartaric acid

Tartaric acid is a chiral compound, with the levotartaric (L-) form being
the naturally occurring form. In addition, the mirror-image molecule,
dextrotartaric acid (D-) and mesotartaric acid can be synthesized artificially.
The molecule possesses two carboxylate groups, and two hydroxyl groups,
although it is only the carboxylate groups which normally undergo
deprotonation (Table 2.39).
Calcium forms complexes with both the partly and fully deprotonated
molecule, whilst aluminium and iron only form complexes once the
molecule is fully deprotonated. Calcium hydrogen tartrate and calcium
 The Chemistry of Concrete Biodeterioration 55

Table 2.34 Stability constants of complexes formed between the succinate ion relevant to
cement chemistry.

Species Reaction Stability Reference

Constant,
Log K

Ca

Ca(Succinate) Ca2+ + [(CH2)2(COO)2]2– ⇌ 1.20

Ca[(CH2)2(COO)2] [53]

CaH(Succinate)+ Ca2+ + [(CH2)2(COO)2]2– + H+ ⇌ 4.59

Ca[HOOC(CH2)2COO]+

Al

Al(Succinate)+ Al3+ + [(CH2)2(COO)2]2– ⇌ 3.91

Al[(CH2)2(COO)2]+

AlH(Succinate)2+ Al3+ + [(CH2)2(COO)2]2– + H+ ⇌ 7.31

Al[HOOC(CH2)2COO]2+

Al2(Succinate H+–3)+ 2Al3+ + [(CH2)2(COO)2]2– ⇌ –5.39 [64]

Al2[OOCCCHCOO]+ + 3H+

Al2(Succinate H+–4) 2Al3+ + [(CH2)2(COO)2]2– ⇌ –9.77

Al2[OOC4OO]
+ 4H+

Al3(Succinate H+–1)23+ 3Al3+ + 2[(CH2)2(COO)2]2– ⇌ 5.81

Al3[OOCCHCH2COO]23+ + 2H+

Fe(II)

Fe(Succinate) Fe2+ + [(CH2)2(COO)2]2– ⇌ 2.4 (37°C) [53]

Fe[(CH2)2(COO)2]

Fe(III)

Fe(Succinate)+ Fe3+ + [(CH2)2(COO)2]2– ⇌ 6.88 [53]

Fe[(CH2)2(COO)2]+

tartrate are compounds of relatively low solubility, as is iron (II) tartrate

(Table 2.40).
The solubility diagram for calcium in the presence of tartaric acid
(Figure 2.33) shows calcium tartrate to persist as the dominant component
of the system over a very wide pH range. In the case of iron, similar
behaviour is observed, with solid ferrous tartrate persisting even under
acidic conditions. Only qualitative solubility data was found for aluminium
56 Biodeterioration of Concrete

Table 2.35 Solubility products of Ca and Fe salts of succinic acid.

Compound Reaction Solubility Reference Molar Reference

Product, Volume,
log ksp cm3/mol

Calcium succinate Ca[(CH2)2(COO)2].3H2O ⇌ –3.94 [74] 132.4 [65]

trihydrate Ca2+ + [(CH2)2(COO)2]2– +
3H2O

Calcium succinate Ca[(CH2)2(COO)2].H2O ⇌ –2.90 [74] 95.8 [66]

monohydrate Ca2+ + [(CH2)2(COO)2]2– +
H2O

Iron (II) succinate Fe[(CH2)2(COO)2].4H2O ⇌ unknown – 126.5 [67]

tetrahydrate Fe2+ + [(CH2)2(COO)2]2– +
H2O

Fe5(OH)2(C4H4O4)4 Fe5(OH)2((CH2)2(COO)2)4 ⇌ ‘insoluble’* [68] 334.1 [68]

5Fe2+ + 4[(CH2)2(COO)2]2–
+ 2OH2–

*probably only formed through hydrothermal synthesis.

\ l
0

"~·'fe
f'.s;
Ca(Succinate).3H 2 0 (s)
-2
---1

Cii'
~ -4
0)
0 Ca2 +
\
...J
Ca(Succinate) ·:r:
0
0"'
-6

-8 +----,---,---,L--,,---,---,----,---,---,---,----r-~

2 3 4 5 6 7 8 9 10 11 12 13 14

pH

Figure 2.31 Solubility diagram for Ca in the presence of succinic acid.

 The Chemistry of Concrete Biodeterioration 57

Table 2.36 Acid dissociation constants for malic acid.

Acid Formula Structure Acid Dissociation Constant Reference
pKa2 pKa1
Malic CH2CH(OH) HO OH 3.24 4.68 [33]
O
acid (COOH)2 HOOCCH2CH(OH) [CH2CH(OH)
O
HO COO– (COO)2]2–
Hydrogen malate Malate

tartrate, but on the assumption that it is soluble in water, tartaric acid has
the effect of solubilizing gibbsite under lower pH conditions (Figure 2.34).
In the case of iron, both the Fe(II) and Fe(III) salts are of low solubility
(Table 2.41). However, it is the Fe(II) salt which has a significant effect on
the solubility diagram (Figure 2.35)—it leads to a persistence of a solid iron
phase to a low pH for higher iron concentrations.

2.4.10 Butyric acid

Butyric acid is a monocarboxylic acid, structurally similar to acetic and

lactic acid. It has similar properties to these compounds (Table 2.42). It
forms weak complexes with calcium and aluminium ions (Table 2.43) and
soluble salts with calcium, aluminium, and iron (Table 2.44).

2.4.11 Fumaric acid

Fumaric acid is structurally similar to succinic acid, with the exception that
the central carbon-carbon bond in its molecule is a double bond (Table 2.45).
The lack of complexes identified as being formed by fumaric acid (Table
2.46) implies that this structural difference compromises the compound’s
ability to do so. It must be stressed that the absence of data partly indicates
either a lack of experimental investigation into this compound or practical
difficulties in conducting measurements. However, it is likely that the
complexes formed are weak: using the same stability constant prediction
method used for pyruvic acid, the estimated stability constant values are
2.3, 2.8 and 3.2 for 1:1 complexes of Al, Fe(II) and Fe(III).
Whilst the salts formed by fumaric acid are only sparingly to slightly
soluble (Table 2.47), they do not appear as predominant species using the
parameters used to form solubility plots in this chapter.

2.4.12 Gluconic acid

Gluconic acid possesses a single carboxylate group, but five hydroxyl

groups. Normally only the carboxylate group takes part in deprotonation
58 Biodeterioration of Concrete

Table 2.37 Stability constants of complexes formed between the malate ion relevant to cement
chemistry.

Species Reaction Stability Reference

Constant,
Log K

Ca

Ca(Malate) Ca2+ + [C4H4O5]2– ⇌ Ca[C4H4O5] 1.96

[53]
CaH(Malate)+ Ca2+ + [C4H4O5]2– + H+ ⇌ Ca[C4H5O5]+ 5.77

Al

Al(Malate)+ Al3+ + [C4H4O5]2– ⇌ Al[C4H4O5]+ 4.52

AlH(Malate)2+ Al3+ + [C4H4O5]2– + H+ ⇌ Al[C4H5O5]2+ 7.032

AlH(Malate)2 Al3+ + 2[C4H4O5]2– + H+ ⇌ Al[C4H5O5] 10.98

[C4H4O5]

Al(Malate H+–1) Al3+ + [C4H4O5]2– ⇌ Al[C4H3O5] + H+ 1.27

Al2(Malate H+–2)2+ 2Al3+ + [C4H4O5]2– ⇌ Al2[C4H2O5]2+ + 2H+ 0.56

Al2(Malate H+–3)+ 2Al3+ + [C4H4O5]2– ⇌ Al2[C4H1O5]+ + 3H+ –3.05

Al2(Malate H ) +
–2
2Al + 2[C4H4O5] ⇌ Al2[C4H2O5][C4H3O5]
3+ 2– –
1.78
(Malate H+–1) –
+ 3H+ [69]
Al2(Malate H+–2)22– 2Al3+ + 2[C4H4O5]2– ⇌ Al2[C4H2O5]22– + 4H+ –4.46

Al2(Malate H+–1) 2Al3+ + 3[C4H4O5]2– ⇌ Al2[C4H3O5] 12.79

(Malate)2 –
[C4H4O5]2– + H+

Al3(Malate H+–1)43– 3Al3+ + 4[C4H4O5]2– ⇌ Al3[C4H3O5]43– + 4H+ 10.13

Al4(Malate H+–2) 4Al3+ + 4[C4H4O5]2– ⇌ Al4[C4H2O5] 10.54

(Malate H+–1)3– [C4H3O5]3– + 5H+

Fe(II)

Fe(Malate) Fe2+ + [C4H4O5]2– ⇌ Fe[C4H4O5] 2.6 [53]

Fe(III)

Fe(Malate)+ Fe3+ + [C4H4O5]2– ⇌ Fe[C4H4O5]+ 7.13

Fe2(Malate H+–1)2 2Fe3+ + 2[C4H4O5]2– ⇌ Fe2[C4H3O5]2 + 2H+ 12.85

Fe2(Malate H ) +
–1 2
2Fe + 3[C4H4O5] ⇌
3+ 2–
17.85 [70]
(Malate)2– Fe2[C4H3O5]2[C4H4O5]2– + 2H+

Fe3(Malate)2(Malate 3Fe3+ + 5[C4H4O5]2– ⇌ Fe3[C4H4O5]2 25.97

H+–1)2(Malate H+–2)5– [C4H3O5]2[C4H2O5] + 4H5– +
 The Chemistry of Concrete Biodeterioration 59

Table 2.38 Solubility products of Ca and Fe salts of malic acid.

Compound Reaction Solubility Reference Molar Reference

Product, Volume,
log ksp cm3/mol

Calcium Ca(C4H4O5).3H2O ⇌ Ca2+ + –3.49 [73] 128.0 [74]

malate (C4H4O5)2– + 3H2O
trihydrate

Calcium Ca(C4H4O5).2H2O ⇌ Ca2+ + –2.56 [73] 115.0 [71]

malate (C4H4O5)2– + 2H2O
dihydrate

Calcium Ca(C4H5O5)2.6H2O ⇌ Ca2+ + Unknown, 249.6 [72]

hydrogen 2(C4H5O5) + 6H2O
– assumed
malate soluble
hexahydrate

Iron (II) Fe(C4H4O5).2.5H2O ⇌ Fe2+ + Unknown, unknown [73]

malate (C4H4O5)2– + 2.5H2O assumed
hydrate soluble

Iron (II) Fe(C4H5O5)2.4H2O ⇌ Fe2+ + Unknown, unknown [73]

hydrogen 2(C4H5O5)– + 4H2O assumed
malate soluble
tetrahydrate

~
Ca(Malate) 3H2 0 (s)
"IJo:·
''&(&;
-2

ro
~ -4
OJ
0 Ca2+
_J
Ca(Malate)

-6 .___ .6
"'
()

-8
2 3 4 5 6 7 8 9 10 11 12 13 14

pH

Figure 2.32 Solubility diagram for Ca in the presence of malic acid.

60 Biodeterioration of Concrete

Table 2.39 Acid dissociation constants for tartaric acid.

Acid Formula Structure Acid Dissociation Constant Reference

pKa2 pKa1
Tartaric HOOCCH(OH) HO OH 3.06 4.37 [33]
acid CH(OH)COOH HO HO OH HO O
-

O - -
O O
O OH O O
O OH O OH

Hydrogen tartrate Tartrate

Table 2.40 Stability constants of complexes formed by calcium, aluminium and iron(II) and
(III) ions in water containing tartaric acid.

Complex Reaction Stability Reference

Constant, Log K
Ca

Ca2+ + OOCCH(OH)CH(OH)CO22– ֖ 2.80

Ca(OOCCH(OH)CH(OH)CO2)

Ca2+ + OOCCH(OH)CH(OH)CO22– + H+ ֖ 5.86

Ca(HOOCCH(OH)CH(OH)CO2)+
[33]
Al Al3+ + 2OOCCH(OH)CH(OH)CO22– ֖ 9.37
Al(OOCC(OH)CH(OH)CO2)2–
Fe (II) Fe2+ + OOCCH(OH)CH(OH)CO22– ֖ 3.10
Fe(OOCCH(OH)CH(OH)CO2)
Fe (III) Fe3+ + OOCCH(OH)CH(OH)CO22– ֖ 7.78
Fe(OOCCH(OH)CH(OH)CO2)+

(Table 2.48), but the hydroxyl groups may also deprotonate in the presence
of iron(III) (Table 2.49). The salts of gluconic acid are soluble (Table 2.50).
However, the calcium salt is close to the edge of this category and, whilst
it does not appear on a solubility diagram using the parameters adopted
in this chapter, could be precipitated at higher acid concentrations than
the 0.1 mol/l used.

2.4.13 Propionic acid

Propionic acid has characteristics similar to most of the other monocarboxylic

acids: it has a similar acid dissociation constant (Table 2.51) and forms
relatively weak complexes (Table 2.52) and soluble salts (Table 2.53).
 The Chemistry of Concrete Biodeterioration 61

0,-----------------------------------~--------,

Ca(Tartrate).4Hp (s)
~
-2 \

ca>•/'-.l----
----------------------------------------1
rn
~ -4
Cl
0
...J Ca(Tartrate)

-6

-8 +--LT---,---,---,--,---,---,---,---,---,---.--~

2 3 4 5 6 7 8 9 10 11 12 13 14

pH

Figure 2.33 Solubility diagram for Ca in the presence of tartaric acid.

0,--,------------------------------------------,

Gibbsite (s)
-2

<r:
Cl -4
0
...J

AI(Tartrate) 2-

AI(OH)4 -
-6

-8 +---.----,---,---.---.~L-.---,---,---,----,---,--~

2 3 4 5 6 7 8 9 10 11 12 13 14

pH

Figure 2.34 Solubility diagram for Al in the presence of tartaric acid.

62 Biodeterioration of Concrete

Table 2.41 Solubility and molar volume data for compounds relevant to the interaction of
hydrated Portland cement with tartaric acid.

Compound Formula/Reaction Solubility Reference Molar Reference

Product, Volume,
log Ksp cm3/mol
Calcium Ca(OOCCH(OH)CH(OH) –5.98 [74] 141 [75]
tartrate CO2)∙4H2O ⇌ Ca2+ + (D-tartaric)
tetrahydrate
OOCC(OH)CH(OH)CO22– +
4H2O
Calcium Ca(HOOCCH(OH)CH(OH) –7.55 [76] unknown –
hydrogen CO2)2 ⇌ Ca2+ + 2OOCC(OH)
tartrate
CH(OH)CO22– + 2H+
Aluminium Al2(OOCCH(OH)CH(OH) ‘Soluble’ [77] unknown –
tritartrate CO2)3 ⇌ 2Al + 3OOCC(OH)
3+

CH(OH)CO22–
Iron (II) Fe(OOCCH(OH)CH(OH) –8.23 [78] unknown –
tartrate CO2)∙2.5H2O ⇌ Fe2+ +
OOCC(OH)CH(OH)CO22– +
2.5H2O
Iron (III) Fe2(OOCCH(OH)CH(OH) –14.525* unknown –
tartrate CO2)3∙H2O ⇌ 2Fe3+ +
3OOCC(OH)CH(OH)CO22– +
H 2O
*based on solubility data provided by a number of chemical suppliers, but original source
unclear.

0.-----------.-------------------------------,

-2
Fe(II)(Tartrate)2.5H 20 (s)

Q) Ferrihydrite (s)
~ -4
0
-'

-B Fe 2
Fe(II)(Tartrate)

2 4 5 6 7 8 9 10 11 12 13 14

pH
Figure 2.35 Solubility diagram for Fe in the presence of tartaric acid.
 The Chemistry of Concrete Biodeterioration 63

Table 2.42 Acid dissociation constants for butyric acid.

Acid Formula Acid Dissociation Reference

Constant
pKa
Butyric CH3CH2CH2COOH 4.82 [33]
acid H H H O H
H CH3CH2CH2CO2–
H H H O
Butyrate

Table 2.43 Stability constants of complexes formed by calcium, aluminium

and iron(II) and (III) ions in water containing butyric acid.

Complex Reaction Stability Reference

Constant, Log K
Ca
Ca2+ + CH3CH2CH2CO2– ֖ 0.94
Ca(CH3CH2CH2CO2)+ [33]
Al
Al3+ + CH3CH2CH2CO2– ֖ 1.58
Al(CH2(OH)CO2) 2+

Table 2.44 Solubility and molar volume data for compounds relevant to the interaction of
hydrated Portland cement with butyric acid.

Compound Formula/Reaction Solubility Reference Molar Reference

Product, Volume,
log Ksp cm3/mol

Calcium Ca(CH3CH2CH2CO2)2·H2O 0.04 [76] 179 [79]

butyrate ֖ Ca2+ + 2CH3CH2CH2CO2–
monohydrate
+ H2O

Aluminium Al(CH3CH2CH2CO2)3 ֖ Assumed – Unknown –

butyrate Al3+ + 3CH3CH2CH2CO2– soluble

Iron (II) Fe(CH3CH2CH2CO2)2 ֖ Assumed – Unknown –

butyrate Fe2+ + 2CH3CH2CH2CO2– soluble

Iron (III) Fe(CH3CH2CH2CO2)3 ֖ Assumed – Unknown –

butyrate 3+
Fe + 3CH3CH2CH2CO2 – soluble
64 Biodeterioration of Concrete

Table 2.45 Acid dissociation constants for fumaric acid.

Acid Formula Structure Acid Dissociation Constant Reference

pKa2 pKa1

Fumaric (CH)2(COOH)2 OH 3.053 4.494 [52]

O
acid HOOC(CH)2COO– [(CH)2(COO)2]2–
O
HO

Hydrogen fumarate Fumarate

Table 2.46 Stability constants of complexes formed by calcium ions in

water containing fumaric acid.

Complex Reaction Stability Reference

Constant
Ca Ca2+ + C4H4O42– ֖ Ca(C4H4O4) 2.00 [53]

Table 2.47 Solubility and molar volume data for compounds relevant to the interaction of
hydrated Portland cement with fumaric acid.

Compound Formula/Reaction Solubility Reference Molar Reference

Product, Volume,
log ksp cm3/mol

Calcium Ca(C4H4O4) ֖ Ca2+ + –1.77 [80] unknown –

fumarate C4H4O42–

Calcium Ca(C4H4O4)·3H2O ֖ Ca2+ –1.99 [80] 122 [81]

fumarate + C4H4O42– + 3H2O
trihydrate

Iron (II) Fe(C4H4O4) ֖ Fe2+ + –4.17 [80] 70 [80]

fumarate C4H4O42–

Table 2.48 Acid dissociation constants for gluconic acid.

Acid Formula Acid Dissociation Reference

Constant
pKa
Gluconic acid HOCH2(CHOH)4COOH 3.56 [33]
OH OH OH
OH HOCH2(CHOH)4COO–
OH OH O
gluconate
 The Chemistry of Concrete Biodeterioration 65

Table 2.49 Stability constants of complexes formed by calcium, aluminium and

iron(II) and (III) ions in water containing gluconic acid.

Complex Reaction Stability Reference

Constant,
Log K
Ca Ca2+ + C6H11O7– ֖ Ca(C6H11O7)+ 1.21 [33]

Al Al3+ + C6H11O7– ֖ Al(C6H11O7)2+ 2.4 [82]

Fe (II) Fe2+ + C6H11O7– ֖ Fe(C6H11O7)+ 1.0

Fe (III) Fe + C6H11O7 ֖ Fe(C6H11O7)

3+ – 2+
–3.1
[33, 83]
Fe3+ + C6H11O7– ֖ Fe(C6H10O7)+ + H+ –0.8

Fe3+ + C6H11O7– ֖ Fe(C6H9O7) + 2H+ 1.5

Fe3+ + C6H11O7– ֖ Fe(C6H8O7)– + 3H+ 5.5

Fe3+ + C6H11O7– ֖ Fe(C6H7O7)2– + 4H+ 18.8

Table 2.50 Solubility and molar volume data for compounds relevant to the interaction of
hydrated Portland cement with gluconic acid.

Compound Formula/Reaction Solubility Reference Molar Reference

Product, Volume,
log Ksp cm3/mol

Calcium Ca(C6H11O7)2·H2O ֖ Ca2+ –2.44 [84] unknown

gluconate + 2C6H11O7– + H2O
monohydrate

Aluminium Al(OH)(C6H11O7)2 ֖ Al3+ Assumed [85] unknown

gluconate + 3 2C6H11O7– + OH– soluble
hydroxide

Iron (II) Fe(C6H11O7)2.2H2O ֖ Fe2+ ‘Soluble’ [8] unknown

gluconate + 2C6H11O7–
dihydrate

Table 2.51 Acid dissociation constants for propionic acid.

Acid Formula Acid Dissociation Reference

Constant
pKa
Propionic CH3CH2COOH 4.874 [8]
acid
H
O
H
CH3CH2CO2–
H
O propionate
H H
H
66 Biodeterioration of Concrete

Table 2.52 Stability constants of complexes formed by calcium, aluminium and

iron(III) ions in water containing propionic acid.

Complex Reaction Stability Reference

Constant,
Log K
Ca

Ca2+ + CH3CH2CO2– ⇌ Ca(CH3CH2CO2)+ 0.93 [33]

Al

2Al3+ + CH3CH2COOH + 2H2O ⇌ –8.04 [86]

Al2(OH)2(CH3CH2CO2)3+ + 3H+
Fe(III)

Fe3+ + CH3CH2CO2– ⇌ Fe(CH3CH2CO2)2+ 4.01 [33]

Table 2.53 Solubility and molar volume data for compounds relevant to the interaction of
hydrated Portland cement with propionic acid.

Compound Formula/Reaction Solubility Reference Molar Reference

Product, Volume,
log ksp cm3/mol

Calcium Ca(CH3CH2CO2)2 ⇌ Ca2+ + 1.91 [87] Unknown –

propionate 2CH3CH2CO22–

Iron (III) Fe3O(CH3CH2CO2)7(CH3COOH) ‘soluble’ [88] Unknown –

propionate ⇌ Fe3+ + 8CH3CO2– + H2O

2.4.14 Citric acid

Citric acid is a carboxylic acid possessing three carboxylate groups and

an additional –OH group. Only the carboxylate groups normally undergo
deprotonation (Table 2.54). The acid forms strong complexes with Ca, Al
and Fe ions (Table 2.55).
It is capable of forming a number of salts with calcium, the three most
likely to be encountered shown in Table 2.56. Of these salts, the least soluble
is calcium citrate tetrahydrate (Ca3(Citrate)2∙4H2O), which is observed as
occupying the higher calcium concentration range of the calcium solubility
diagram in Figure 2.36. Aluminium and iron (III) salts are also reported in
the literature, although they are soluble.
The strength of the complexes formed between iron and aluminium
are such that they limit the normal dominance of gibbsite and ferrihydrite
(Figures 2.37 and 2.38).
 The Chemistry of Concrete Biodeterioration 67

Table 2.54 Acid dissociation constants for citric acid.

Acid Formula Structure Acid Dissociation Constant Reference

pKa3 pKa2 pKa1

Citric C6H8O7 O OH 3.13 4.76 6.40 [33]

O
acid OH O O
HO O O O
HO
OH
O O O O
O O O
-
HO HO HO HO O HO
O O O

Dihydrogen Hydrogen Citrate

citrate citrate

Table 2.55 Stability constants of complexes formed by calcium, aluminium and

iron(II) and (III) ions in water containing citric acid.

Complex Reaction Stability Reference

Constant,
Log K
Ca

Ca2+ + C6H5O73– ֖ Ca(C6H5O7)– 4.87

Ca + C6H5O7 + H ֖ Ca(C6H6O7)
2+ 3– +
9.26

Ca + C6H5O73– + 2H ֖ CaH(C6H7O7)
2+ +
12.26
[33]
Al Al3+ + C6H5O73– ֖ Al(C6H5O7) 9.97

Al3+ + 2C6H5O73– ֖ Al(C6H5O7)23– 14.80

Al3+ + C6H5O73– + H+ ֖ Al(C6H6O7)+ 12.85

Fe (II) Fe2+ + C6H5O73– ֖ Fe(C6H5O7)– 6.1

Fe2+ + C6H5O73– + H+ ֖ Fe(C6H6O7) 10.2

Fe (III) Fe3+ + C6H5O73– ֖ Fe(C6H5O73–) 13.1

Fe + C6H5O7 + H ֖ Fe(C6H6O7)
3+ 3– + +
14.4

Citric acid is known to form complexes with silicon. However, the

strength of the complex is weak: the stability constant for the reaction H4SiO4
+ C6H5O73– ֖ Si(C6H5O7)(OH)43– is 0.11 [61].

2.5 Deterioration Mechanisms

The previous section has outlined the manner in which various acids
interact chemically with the chemical constituents of conventional cements.
68 Biodeterioration of Concrete

Table 2.56 Solubility and molar volume data for compounds relevant to the interaction of
hydrated Portland cement with citric acid.

Compound Formula/Reaction Solubility Reference Molar Reference

Product, Volume,
log ksp cm3/mol

Calcium Ca3(C6H5O7)2∙4H2O ⇌ –14.94* [74] 285* [89]

citrate 3Ca2+ + 2C6H5O73– + 4H2O
tetrahydrate

Calcium Ca(C6H6O7)∙4H2O ⇌ Ca2+ –0.33 [76] 157 [90]

hydrogen + C6H5O73– + 4H2O + H+
citrate
tetrahydrate

Calcium Ca(C6H7O7)2∙3H2O ⇌ Ca2+ 0.77 [76] unknown –

dihydrogen + 2C5H5O73– + 3H2O + 4H+
citrate
trihydrate

Aluminium Al(C6H5O7) ⇌ Al3+ + 1.26 [91] unknown –

citrate C6H5O73–

Iron (III) citrate Fe(C6H5O7).5H2O ⇌ Fe3+ + ‘slightly [74] unknown –

pentahydrate C6H5O73– + 5H2O soluble’
*These values are for the Earlandite form of calcium citrate tetrahydrate. At least one other
form appears to exist. This other form is seemingly the one formed in contact with cement [92].

Ca2• \
Ca3 (Citrate) 2 .4H,O (s)
~C/ite (sj
~
(
-2

ro
~ -4
Ol + Ca(Citrater
0
_l 2 2
~
Q_
"'
.l::
i:3
I N I'
-6 "'
0 "'
0

-8
2 4 6 8 10 12 14

pH
Figure 2.36 Solubility diagram for Ca in the presence of citric acid.
 The Chemistry of Concrete Biodeterioration 69

-8+---L---,---L---,-------.-~----.-------.-----~

2 4 6 8 10 12 14

pH

Figure 2.37 Solubility diagram for Al in the presence of citric acid.

Ferrihydrite (s)

-2

~ :§'
C) -4
0
...J
~
0 Fe(Citrater
:C
())
lL

-6
Fe(OHJ;

-8
2 4 6 8 10 12 14
pH

Figure 2.38 Solubility diagram for Fe in the presence of citric acid.

70 Biodeterioration of Concrete

However, understanding the manner in which these interactions impact

on the integrity of cement and concrete make it necessary to examine the
mechanisms which are effective when acid comes into contact with such
materials. So far, the interactions we have examined have isolated certain
cement components, but we must now consider all the constituents together.
Moreover, interaction has exclusively examined interaction in chemical
terms, but there are also physical aspects to deterioration.

2.5.1 Deterioration from leaching

Concrete exposed to acids may undergo deterioration as a result of the

accelerated leaching of chemical constituents in the cement matrix, and
possibly also in the aggregate. This can occur by two mechanisms: acidolysis
and complexolysis.
Acidolysis involves the reaction of hydration products with the acid to
yield soluble ions. Thus, any of the acids discussed in the previous section
which form soluble salts will cause deterioration by acidolysis. Of most
significance is the leaching of portlandite:
Ca(OH)2 + 2H+ → Ca2+ + 2H2O.
This is because—as has been seen from the solubility diagrams—
portlandite undergoes this reaction at relatively high pH. Where acidolysis is
the main mechanism of deterioration this will normally lead to the complete
dissolution of this hydration product. This has profound implications for
the cement matrix, because portlandite tends to be present as relatively
large crystals, leaving pores which weaken the material considerably [93].
In contrast—again evident from the solubility diagrams—the acid
decomposition of the AFm and AFt phases leads to the formation of
amorphous gibbsite and ferrihydrite, which normally persist to a low pH.
CSH gel undergoes a process of decalcification: calcium is progressively
removed from the gel by the process of acidolysis, leading to a drop in the
Ca/Si ratio until only silica gel remains. This also leads to a decrease in
strength, but is generally less than that from the loss of portlandite. For
this reason, it is often observed that cements containing a lower portlandite
content (such as those containing proportions of other cementitious
materials) perform better than Portland cement alone where acidolysis is
the main mechanism of deterioration.
Calcium aluminate cements undergo a different process, with the
precipitation of amorphous gibbsite playing a major role. If it is assumed
that the cement is fully converted, then the cement will initially take part
in the following reaction:
Ca3Al2O9.6H2O + 6H+ → 3Ca2+ + 2Al(OH)3 + 6H2O
 The Chemistry of Concrete Biodeterioration 71

The gibbsite formed will eventually undergo acidolysis at much lower

pH:
2Al(OH)3 + 6H+ → 2Al3+ + 6H2O
Complexolysis is an enhanced leaching of the cement resulting from
the formation of strong complexes. Complex formation allows more of
the solid phases to dissolve, although it should be stressed that acidolysis
must occur prior to complex formation, and so the two mechanisms occur
in tandem. A good example of complexolysis is shown in Figure 2.37 where
the formation of strong complexes between citric acid and aluminium ions
leads to a destabilization of gibbsite.
For both mechanisms, the process of deterioration will also be controlled
by the movement of acid into the concrete itself. This will occur by diffusion
and the rate will be dependent on the porosity in the concrete and the extent
to which porosity is increased by the deterioration process.
The simultaneous role of both chemical reactions and mass transport
makes the acid attack process a potentially complex one. For this reason,
where deterioration by acids is discussed in subsequent chapters, the use of
a technique known as geochemical modelling will be employed to illustrate
the outcomes of attack from different acids on different cement types. This
has been done using the computer program PHREEQC [94], which uses
stability constant and solubility product data along with mass transport
calculations to simulate the processes occurring when solid minerals (i.e.,
the cement hydration products) and aqueous solutions come into contact.

2.5.2 Deterioration from expansive reaction products

Another way in which acids can cause damage to concrete is through

the precipitation of expansive solid salts. As a general rule, a salt will be
expansive if it occupies a larger volume than the substance it replaces
through chemical reaction. This can be deduced by examining the molar
volume of the reactant and product. These values, where available, have
been included in the salt formation tables provided throughout Section 2.4.
Thus, when gypsum is formed as a result of the reaction of sulfate ions
with portlandite, the reaction takes the form:
Ca(OH)2 + SO42– + H2O → CaSO4.2H2O + 2e–
one mole of portlandite (with a molar volume of 32.9 cm3/mol) will be
converted into one mole of gypsum (molar volume = 74.5 cm3/mol). The
resulting increase in volume, when occurring in the confines of pores
and cracks in concrete may be enough to cause further cracking and
fragmentation of the cement matrix.
Whilst this general rule of thumb is useful in identifying potential issues
with specific acids, the process of expansive salt formation is complex.
72 Biodeterioration of Concrete

Various factors will play a role, including the solubility of the salt and the
pH range over which it remains stable. As a result, salt formation requires
individual investigation for specific acids. Where data are available, this
is attempted in Chapter 4, which looks at fungal deterioration of concrete.
This is because fungi are largely responsible for the production of acids
involved for this type of deterioration.

2.6 References
[1] Drever JI (1997) The Geochemistry of Natural Waters. 3rd Ed., Prentice Hall, Upper
Saddle River.
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Chapter 3

Bacterial Biodeterioration

3.1 Introduction
Bacteria are single-celled organisms falling within the Prokaryota grouping.
Prokaryote cells differ from the cells of other living organisms in their
lack of a membrane-bound nucleus, membrane-bound organelles and
mitochondria. As discussed in Chapter 1, the Prokaryote grouping has been
subdivided into Archaea and Bacteria, but for the purpose of this book the
two can be discussed together.
Bacteria comprise the largest proportion of biomass on the planet, and
their presence on the Earth’s surface (and to some distance beneath it) is
practically ubiquitous. Indeed, bacteria have been found to be capable of
existing in extremely hostile conditions.
The deterioration of concrete in contact with bacterial communities
is principally through chemical reactions between cement and aggregate
and substances produced by these organisms. However, it is of benefit to
discuss the metabolism and reproduction of these organisms first, since
these aspects have important implications with regards to interactions
between bacteria and concrete.

3.2 Bacterial Metabolism

One means of categorizing bacteria is in terms of the manner in which
they obtain the substances that they need to live and reproduce. These
substances are:
• Carbon: carbon is required to manufacture the mass required for
growth and division of bacteria cells.
• Reducing equivalents: these are substances capable of being used for
the transfer of electrons through redox reactions. These are used by the
organism either in processes used to store energy, or in the synthesis
of chemical compounds in the cell.
78 Biodeterioration of Concrete

In addition, cells require a source of energy. The manner in which the

chemical and energetic requirements of bacteria can be met is discussed
below.
Bacteria may source carbon heterotrophically or autotrophically.
Autotrophic bacteria obtain carbon from carbon dioxide in the atmosphere,
whilst heterotrophic bacteria use organic compounds in the surrounding
environment, obtaining energy at the same time. Some bacteria are capable
of obtaining carbon through a combination of both means, and are known
as mixotrophic bacteria.
Reducing equivalents can either be obtained from inorganic chemical
compounds—lithotrophism—or, again, from organic compounds—
organotrophism.
Energy may be obtained phototrophically from sunlight or chemotrophically
through the oxidation of inorganic compounds. Some bacteria are able to
harness energy through other mechanisms.
Any combination of these different mechanisms is possible, with the
bacteria being identified through a naming scheme that combines each term
into one word, e.g., chemolithoheterotroph.
The harnessing of energy depends on the type of organism and
whether the environment in which the bacteria are present contains oxygen
(aerobic) or does not (anaerobic). In all cases, however, the basic reaction
is the same: a reduced organic compound (AH2) or reduced inorganic
compound—for instance, hydrogen sulphide (H2S)—donate hydrogens to
oxidized compounds which act as hydrogen acceptors. Obligate aerobes are
organisms that require oxygen to carry out this task. The basic process for
aerobic heterotrophic bacteria can be summarized as:
2AH2 + 2O2  CO2 + 2H2O + energy.
In the case of aerobic chemotrophic bacteria, an example reaction would
be the following:
H2S + 2O2  SO42– + 2H+ + energy.
Obligate anaerobes are organisms that are killed under normal aerobic
conditions. Metabolism for obligate anaerobic heterotrophic bacteria follows
the scheme:
AH2 + B  BH2 + A + energy
where A is a reduced organic compound and B is oxidized organic compound.
Obligate aerobes and anaerobes exist at two extremes with regards to the
manner in which bacteria respond to oxygen. Between these extremes exist
facultative anaerobic, microaerophilic and aerotolerant organisms. Heterotrophic
bacteria which are facultative anaerobes can metabolise both anaerobically
or aerobically. In anaerobic conditions, metabolism will often employ
hydrogen acceptors such as nitrate or sulphate as a source of oxygen:
 Bacterial Biodeterioration 79

AH2 + NO3– + H+  AH + N2 + CO2 + H2O + energy.

AH is an intermediate compound which is typically an organic acid or
an alcohol, and such bacteria are referred to as ‘acid forming’. Heterotrophic
‘acid-splitting’ bacteria are potentially capable of breaking intermediates
down further, e.g.:
2AH + SO42– + 2H+  CH4 + S2– + 2CO2 + energy.
Microaerophiles metabolise aerobically, but are unable to survive at
normal atmospheric oxygen concentrations. Aerotolerant bacteria have
anaerobic metabolisms, but are unaffected by oxygen concentration.
As well as carbon, bacteria require additional elements for the purpose
of cell growth and division in smaller concentrations. These include
nitrogen, which is used in the synthesis of amino acids, DNA and RNA.
Additionally, synthesis of nucleic acids requires phosphate, but this element
is also necessary for the synthesis of adenosine triphosphate (ATP) which
is used for energy transfer. Other elements required by bacterial cells in
still smaller quantities include potassium, sodium, calcium, iron, zinc and
magnesium. Some bacteria require even smaller quantities of other elements
(micronutrients) for more specialized purposes.
As we have seen, with certain bacteria under anaerobic conditions,
organic compounds may be formed as metabolites. However, bacteria are
also capable of releasing compounds for the purpose of dissolving minerals
to release nutrients. Bacteria colonising siliceous rock surfaces have been
found to weather the surface, seemingly to obtain phosphorous present in
trace quantities [1]. Indeed, it has been proposed that the presence of this
element is the sole reason for the bacterial colonization of such surfaces.
The compounds released include organic acids and polysaccharides, both
of which are capable of forming complexes with orthosilicate (SiO44–)
ions. Bacillus mucilaginosus has been found to excrete both organic acids
(oxalic, citric and lactic) and polysaccharides, with consequent accelerated
weathering of mica and clay [2]. The polysaccharide, besides keeping the
bacteria attached to the mineral surface, also appears to play two additional
roles. Firstly, it keeps organic acids in close proximity to the surface and
prevents them from dispersing more widely, thus maintaining a high acid
concentration [2, 3]. Secondly, it adsorbs SiO44– ions as they entered solution,
thus preventing the solution at the surface of the mineral from achieving
a state of equilibrium with the solid material, hence permitting further
dissolution [3]. Other organic acids produced by bacteria, and which form
complexes with SiO44– include oxalic acid and 2-ketogluconic acid [4, 5].
Additionally, as will be discussed later, polymeric materials produced by
bacteria in the formation of biofilms can also contain groups—such as the
catecholate groups—which form these complexes.
Dissolution of carbonate minerals for the purpose of harvesting
nutrients has also been proposed [6], and evidence of this is backed up
80 Biodeterioration of Concrete

by experimental results [7]. Dissolution of miliolite, a rock principally

comprising calcite, aragonite (both CaCO3) and quartz (SiO2), was found
to be conducted largely by three different bacterial genera: Bacillus,
Staphylococcus and Promicromonosporaceae. Analysis of the organic acids
formed identified acetic, lactic and malic acids as those having the largest
influence on calcium dissolution [8]. Other studies have also found that the
enzyme carbonic anhydrase by bacteria also has the effect of significantly
increasing the dissolution of carbonate minerals [9].

3.3 Growth of Bacterial Communities

Bacteria reproduce asexually through binary fission: a cell divides into two
cells which resemble the original. The manner in which a population of an
isolated species of bacteria grows is described by the Monod equation [10],
a version of which is:
dX µmax SX
=
dt Ks – S

where X = the mass of bacteria;

t = time;
µmax = the maximum specific growth rate, time–1;
S = concentration of the growth limiting substrate, mass/
volume; and
Ks = half saturation coefficient, mass/volume.
The growth limiting substrate is a nutrient required by the bacteria
which is present in sufficiently low quantities that its use in metabolism will
start to limit its availability, and hence limit growth. The specific growth
rate (µ) is defined by the equation:

(dX
dt
)
µ=
X
and the maximum specific growth rate is the specific growth rate at a
concentration of substrate at or above saturation. The saturation constant
is the substrate concentration required for a specific growth rate which is
half that of the maximal rate.
Thus, growth is dependent on the population of bacteria and the
concentration of the growth limiting substrate. The concentration of
substrate will decline in a manner described by the equation
dS µ SX
= max
dt Y(Ks – S)
Where Y is a parameter known as the growth yield. The mass of bacteria
will change in accordance with the plot shown in Figure 3.1. The initial
 Bacterial Biodeterioration 81

exponential declining· stationary endogenous phase

growth growth · phase

z
0

"" \ BIOMASS
SUBSTRATE
~
1-
z
(f) \ w
(f) 0
\ z
~ \ 0
Q 0
(}) w
\ I-
\ ~
\ 1-
(f)
\ aJ
:::>
\ (f)

\
\

TIME

Figure 3.1 Growth of an isolated culture of bacteria [11].

process of growth is exponential in nature where the substrate is present in

relative abundance, and the rate of growth is consequently limited only by
the bacteria’s capacity to employ it. As the substrate starts to become scarcer,
its limiting effect begins to manifest itself and growth enters a declining
phase where the rate of growth gradually declines.
Throughout the process of growth bacteria will be dying, and a
stationary phase is eventually entered in which the rate of reproduction
equals the death rate. The death of cells leads to the process of lysis where
the membrane of the cells breaks down and the contents is released into the
solution, potentially allowing the stationary phase to continue for longer.
In some cases, this may lead to some of the substrate being released back
into solution. A point is reached where the levels of substrate are so low
that the rate of cells dying exceeds that of reproduction and the mass of
bacteria declines in an endogenous phase.
A number of bacteria are capable of spore formation. Spores are
reproductive units from which a bacterium can grow. Spore formation is
not the primary means of reproduction of such bacteria. Instead, it provides
a means for bacteria to survive hostile conditions: spores are not destroyed
by dry conditions or low temperatures. Thus, spore formation acts as a
mechanism by which a bacterial community can re-establish itself after
periods of cold weather or drought.
Bacteria seldom occupy a particular environment as a single species,
although there is likely to be a dominant species which is dictated by the
nutrients and environmental conditions present. However, the metabolic
processes of the dominant strain and the process of lysis will produce
substances which can be used as nutrients by other species of bacteria, and
organisms from other domains.
82 Biodeterioration of Concrete

Many bacteria display a tendency to colonise surfaces, and the

formation of biofilms plays an important role in this process. Biofilms are
formed from biopolymers produced by the bacteria. These biopolymers
largely consist of proteins and polysaccharides and are collectively known
as extracellular polymeric substances (EPS). These polymers are initially
formed by the bacteria to attach themselves, but continued formation leads
to the formation of a film which links individual bacteria to neighbouring
cells, encases the cells and provides a degree of 3-dimensionality to the
colony, since it permits the formation of structures in which bacteria are
stacked on top of each other.
As well as playing a structural role, biofilms can carry out a number
of other functions. In many cases biofilms contain channels to provide
microorganisms beneath the surface with nutrients or oxygen [12].
Conversely, biofilms can act as a barrier to atmospheric gases such as
oxygen [13], or to sunlight [14]. It would also appear that the polysaccharide
component of biofilms acts as a store of carbon which can be accessed during
periods when nutrients would otherwise be in short supply [15]. Many
biofilm molecules possess functional groups capable of forming bonds
with metal cations in solution [16]. This mechanism has been suggested as a
means of removing dissolved heavy metal ions from the local environment
and thus avoiding the toxic effect these substances would have on bacteria
[17].
The ability of biofilm compounds to form complexes with metal ions can
allow biofilms to either accelerate the dissolution of calcium from surfaces
or to act as the nucleation sites for the precipitation of calcium carbonate
(CaCO3). The first of these processes will be discussed in greater detail
later on in this chapter. Biopolymer-induced precipitation of CaCO3 can
lead to the formation of either crystalline calcium carbonate or amorphous
precipitates. It is believed that amorphous CaCO3 precipitated in this
way contains polysaccharide molecules incorporated into the structure
[18]. It has also been proposed that enzymes released by the bacteria into
the biofilm play a role in controlling this process in terms of location and
possibly morphology [19].
A large proportion of bacteria are capable of movement. The most
common form of motility is through the use of flagella. Flagella are rotary
structures extending out of the outer membrane of the cell. When rotating,
the flagella act as propellers, driving the bacteria through the surrounding
fluid [20]. Other cellular architectures may be used for motion. These include
Type IV pili, junctional pore complexes, ratchet structures and contractimg
cytoskeletons. Type IV pili, are fibres projecting from the cell membrane
which can be used to move bacteria across a surface through a series of jerks.
This process is generally known as ‘gliding’. Junctional pore complexes are
also used as a means of gliding. These pores act as nozzles which permit
motility through the extrusion of slime. Ratchet structures function through
 Bacterial Biodeterioration 83

interaction between proteins in the cytoplasm of the cell and those in the
outer membrane, leading to a movement of the outer membrane not unlike
that of caterpillar tracks on vehicles.
The motion of bacteria is directed by local stimuli including
concentrations of dissolved chemical species (chemotaxis), light (phototaxis)
and oxygen (aerotaxis). Motion may be directed towards or away from such
stimuli, depending on the nature of the bacteria.

3.4 Concrete as an Environment for Bacterial Life

In evaluating the surface of concrete as a habitat for bacteria, it becomes
evident that, whilst many aspects of concrete offer a welcoming environment,
there are other aspects which present a challenge to colonization. One of
the more extreme characteristics of concrete—at least in its early life—is
the very high pH that water takes on when in contact with its surface or
when present within its pores. The physical nature of the surface is another
aspect. Finally, bacteria require various nutrients, only some of which are
present in concrete itself, and often in relatively small quantities. These
factors are discussed below.

3.4.1 pH

High and low pH conditions are toxic to most bacteria. Whilst some species
can survive under very extreme pH conditions, growth is fundamental to
the survival of these organisms. Most bacteria are neutrophilic—they grow
at optimal rates around a pH of 7. A small number of bacteria are able to
survive at pHs as low as 1, and are referred to as being acidophilic. Alkaliphilic
bacteria are capable of growth in the range of between 8.5 and just over 11,
but their optimal rate of growth is less than this upper limit [21]. The pore
solution of relatively young concrete will typically be in the range of pH
11–13, meaning that a new concrete surface is unlikely to support bacterial
growth of any kind.
A number of different processes can occur in concrete which will act
to reduce pH at the concrete surface. Probably the most common of these
reactions is carbonation. This reaction, between carbon dioxide gas and
cement hydration products in the presence of water, leads to the conversion
of portlandite (Ca(OH)2) to calcium carbonate:
CO2 + H2O  H2CO3

Ca(OH)2 + H2CO3  CaCO3 + 2H2O.

The reaction occurs at an optimal rate at a relative humidity of around
55%. Portlandite is only slightly soluble, but is the main source of hydroxide
ions in hydrated cement. Thus, the reaction leads to a drop in pH to around
84 Biodeterioration of Concrete

9. The reaction is diffusion-driven, with cement at the immediate surface

being neutralised first and a ‘carbonation front’ progressing to greater
depths, at an increasingly reduced rate (Figure 3.2).
When concrete is in contact with water, portlandite will be gradually
leached out (see Chapter 2) also leading to a drop in pH, albeit over relatively
long periods of time. Where acidic substances are dissolved in the water,
this process is much quicker. Since acids are often formed by the metabolic
processes of bacteria, and the presence of bacteria (or other microbes) near
to a concrete surface may well render it suitable for colonization.
Concrete in contact with seawater has been found to form brucite
(Mg(OH)2) at its surface [23] in place of portlandite. Brucite is considerably
less soluble than portlandite, and so a drop in pH is again observed.

3.4.2 Concrete as a physical habitat for bacteria

The surface of concrete is relatively rough. Roughness can be measured in

a number of ways, but one of the most common means of expressing it is
in terms of Ra, the average vertical distance from the mean line of a surface
measured as a surface is traversed. When such measurements are conducted
on concrete, values in the order of 100s of µm are typically obtained [24].
Where surfaces have been mechanically ground after hardening, this may
be an order of magnitude less [24, 25]. It might be expected that a rough
surface would present an easier foundation for the establishment of bacteria
populations. Studies with a focus on biomedical materials have found
no clear relationship between surface roughness and growth of bacterial

18

16 •
0
w/c = 0.5; PC content = 18%
w/c = 0.6; PC content = 14%

14 " w/c = 0.9; PC content = 11 %

E
12 "
E 10
I -
I-
c.. 8
LJ.J
Cl
6
"
4

0
0 4 6 8 10 12 14 16

TIME , years

Figure 3.2 Depth of carbonation front with time [22].

 Bacterial Biodeterioration 85

communities [26, 27, 28, 29]. This is also reflected in the results of studies
examining the relationship between the development of populations
of cyanobacteria (and other organisms) on concrete surfaces, where no
clear relationship was identified [30, 31]. However, the surface porosity
of concrete does appear to influence the rate at which it is populated by
bacteria, with a higher volume of porosity (via a higher water/cement ratio)
leading to higher rates of colonization (Figure 3.3) [30, 31, 32]. The reason
for this is unclear. Whilst it is possible that the pores provide features on
the surface for bacteria to become lodged, it is most likely that the porosity
simply provides a reservoir for moisture, which the bacteria need.
The previous discussion identified that many of the molecules produced
by bacteria in the formation of biofilms possess functional groups capable
of forming bonds with metal ions. This has two important implications
from the perspective of bacterial attachment to concrete surfaces. Firstly, it
is likely that this ability may well provide a mechanism for the formation
of strong chemical bonds between biofilms and cement hydration products
at the surface. One study has observed the formation of covalent bonds
between biofilms produced by Pseudomonas aeruginosa and TiO2 surfaces
[33]. The compound responsible was identified as pyoverdine, which is
a peptide siderophore. Siderophores are molecules which are capable of
chelating iron, but also capable of forming strong bonds with other metal
ions. In the case of pyoverdine, this appears to be via catecholate functional
groups. Whilst this species has not been identified as one which is involved
in deterioration of concrete, molecules with catecholate groups and similar

100

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~
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w-
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WATER I CEMENT RATIO

Figure 3.3 Coverage by a combination of cyanobacteria and algae of Portland
cement mortar surfaces after 8 weeks in conditions favourable to growth [32].
86 Biodeterioration of Concrete

are produced by many bacteria, making the formation of strong bonds with
metal oxides in concrete a very likely possibility. It is also believed that
divalent cations, such as calcium, can act as bridges between polysaccharide
molecules, rendering biofilms stronger [34].
So far discussion of colonization of concrete by bacteria has been
limited to surfaces. However, the fact that concrete is a porous material
means that the possibility of bacteria penetrating further into the material
exists. Concrete pores cover a wide range of diameters from nanometers to
hundreds of micrometers. The pores in concrete take two forms: capillary
pores and gel pores.
Gel pores are present in the CSH gel produced by the hydration of
cement and have dimensions of between 0.5 and 10 nm. Capillary pores are
the remnants of water-filled space between cement grains from when the
concrete was in its fresh state. Much of the original space will be occupied
by cement hydration products in hardened concrete, but what remains
forms a network of pores, and has dimensions in the 10 nm to 10 µm range.
Bacteria range widely in size between species, generally within the
range 0.5–1.0 µm [35]. This means that for many bacteria, much of the
porosity—and certainly all of the gel porosity—is inaccessible. It is often
tempting to imagine porosity in concrete as a series of tunnels of circular
cross-section and uniform diameter running from one end of the material
to another. The reality is much more complex—porosity consists of a
highly interconnected network with high constrictivity (i.e., rapid change
in diameter over a length of porosity). Taking this into account, cement
porosity should be even less accessible to bacteria.
However, bacteria are able to penetrate beneath the surface of concrete,
albeit gradually. One reason for this is that, as the concrete deteriorates, pore
sizes will increase as their surfaces are dissolved. Additionally, cracks may
form, leading to the interior becoming even more accessible. One study
has examined bacterial communities in the deteriorated layers of asbestos
cement pipes used for the transport of drinking water for over 50 years [36].
The researchers identified four layers—an outer layer where significant
deterioration had occurred, two intermediate layers, and a deterioration
front beyond which the cement was essentially unaffected. The bacteria
were characterised in terms of their grouping (slime-forming, iron related,
heterotrophic, acid-producing and sulphate-reducing) and populations of
bacteria in each layer were estimated (Table 3.1). The most populous layer
was the innermost layer, which contained mainly slime-forming bacteria.

3.4.3 Nutrients

There are usually no organic compounds present in concrete, other than

very small quantities of chemical admixtures, formwork release agents
and possibly cement grinding aids. Inorganic carbon may be present as
 Bacterial Biodeterioration 87

Table 3.1 Estimated populations of different bacteria in the deteriorated layers within
an asbestos cement pipe used for carrying drinking water for a period of 52 years [11].
Total thickness of deteriorated zone was between 3.1 and 4.6 mm, but thickness of
individual layers is not specified.

Estimated Population of Bacterial Group, active cells/g

Layer Slime- Iron- Heterotrophic Acid- Sulphate-

forming related producing reducing
Deterioration front 5,552,100 27,900 Nd Nd Nd
Intermediate layer I 2,150,500 34,500 115,000 Nd Nd
Intermediate layer II 18,333 40,740 8,633 29,876 Nd
Outer layer 28,840 35,560 75,460 Nd Nd

carbonate minerals, either as aggregate, included as a cement component,

or calcium carbonate minerals deriving from carbonation of the cement
matrix (Section 3.4.1). These carbonate minerals will partially dissolve in
water. The dissolved inorganic carbon will either be in the form of carbonic
acid (H2CO3), bicarbonate ions (HCO3–) or carbonate ions (CO32–), with the
dominant species dependent on pH (Figure 3.4). This inorganic carbon is
available to autotrophic bacteria.
Some autotrophic bacteria struggle to obtain inorganic carbon under
neutral pH conditions. This is because such bacteria require carbon to be
in the form of CO2, principally because it has a neutral charge and can
therefore pass easily through their lipid membranes, whereas bicarbonate is
the dominant species in these conditions. However, a very large proportion

1.0
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H,co, /co,'"
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Figure 3.4 Change in the concentrations of Predominance of carbonic

acid (H2CO3), bicarbonate (HCO3–) and carbonate (CO32–) in water
in contact with calcite.
88 Biodeterioration of Concrete

of the bacteria found on external building surfaces are cyanobacteria which

possess transporter regions in their cytoplasmic membrane which provide
a means of bringing bicarbonate ions into the cell’s interior [37].
Other autotrophic bacteria can convert carbonate ions to carbonic acid
and subsequently to water and CO2 by using enzymes extracellularly to
catalyse the reaction [38]:
HCO3– + H+ ⇌ H2CO3 ⇌ H2O + CO2.
Thus, concrete (or at least carbonated concrete or concrete containing
carbonaceous aggregates) can provide some bacteria with a source of carbon.
In examining the metabolic processes of bacteria, we have seen that both
nitrogen and sulphur can play roles in how bacteria obtain energy. Nitrogen
compounds are typically present in very small quantities in concrete.
As will be seen in subsequent sections one group of bacteria—the
sulphate-reducing bacteria—are capable of utilising sulphate as a source
of oxygen. Sulphate in concrete will come mainly from calcium sulphate
compounds (usually gypsum) added to Portland cement to control setting
times. The sulphate content of cement is limited by many industrial
standards. EN 197-1 [39] limits it to below 5% by mass as SO3, although
this can be lower for certain cement types. Assuming the highest cement
content that would be encountered in a conventional concrete mix would be
around 800 kg/m3 (a very rare situation), the maximum possible sulphate
content—using the highest sulphate content for Portland cement—would
be around 2% by mass or 0.2 mol/kg. Most typical concrete would have
a sulphate content of a third of this, or less. With the additional issue of
availability of this substance, given that access to nutrients within concrete
is largely via the surface, this represents a very limited supply.
Other compounds which are required in smaller quantities by bacteria
are usually present to some extent in concrete. Phosphate (P2O5) may be
present in both cement or aggregate. Phosphate has a detrimental influence
on strength development in Portland cement [40], although most industrial
standards do not actively limit it. In recent years the possibility for higher
phosphate levels in cement has been realized through the combustion of
waste fuels [41]. A recent survey of Slovakian cement output, where use of
such fuels was known to occur found the maximum level of phosphate in
CEM I (Portland) cement to be 3.22% by mass, with commercial cements
containing slag, fly ash and possibly other materials displaying lower
levels [42].
Industrial by-products used alongside Portland cement may also
contain phosphate. The production of fly ash in coal-fired power stations
can sometimes involve ‘co-combustion’ where other waste fuels are burnt
alongside the coal. As for Portland cement, where the wastes contain
phosphorous (for instance, sewage sludge) phosphate levels in the ash
may be relatively high. For this reason the current European standard for
 Bacterial Biodeterioration 89

fly ash for use in concrete limits total P2O5 levels to less than 5.0% by mass,
and soluble phosphate to 100 mg/kg [43].
The alkali metals potassium and sodium are present in all concrete
constituents. Regardless of the issue of biodeterioration, the alkali content of
concrete is of concern from a durability perspective, since certain susceptible
aggregates may undergo alkali-aggregate reactions at high alkali contents.
Alkalis in Portland cement are usually of greatest concern, since they are
typically present in highly soluble sulphate salt form on the surface of
cement grains, usually converting to alkali hydroxides during hydration.
In cement, alkali content is expressed as the equivalent of sodium oxide
(Na2O), which is calculated using the equation:
Na2Oeq = Na2O + 0.658K2O
A study of Portland cement manufactured in the USA and Canada in
2004 found the average level of alkalis to be 0.70% Na2Oeq, for Type I cement,
with a maximum of 1.20. Other types of cement displayed slightly lower
averages. Fly ash and silica fume typically possess alkali levels somewhat
higher than this [44, 45, 46], whilst slag generally contains lower quantities
[47].
Alkalis in aggregate are often in a much less soluble form, frequently
present in silicate minerals such as feldspars, mica and clay-forming
minerals. However, these can gradually be released under the high pH
conditions of concrete [48] and, as discussed previously, bacteria can
potentially accelerate their release through the production of organic acids
and/or complex-forming compounds. Alkali may also be introduced into
concrete dissolved in mix water.
Iron is present in Portland cement in relatively large quantities (typically
around 2–4% by mass as Fe2O3). In the hydrated state it is present largely
as the AFm and AFt phases and as a substituted ion in CSH gel. As the
pH of concrete is reduced, the AFm and AFt phases will decompose (see
Chapter 2). However, the iron will remain in a relatively insoluble form as
ferrihydrite, until much lower pH conditions are reached. The presence of
organic acids capable of forming complexes with iron may increase the pH
at which iron becomes soluble. Many aggregates will contain quantities
of iron, although commonly this is present as hematite (Fe2O3) or possibly
magnetite (Fe3O4) which are essentially insoluble. Again, complexation by
organic acids and other compounds may render iron more soluble.
Magnesium is also present in Portland cement, although its levels are
again limited by industrial standards. EN 197-1 limits MgO in Portland
cement clinker to 5.0% by mass, whilst ASTM C150 limits it to 6.0% in all
cements covered by the standard [49]. The European standard for fly ash
in concrete limits MgO to less than 4.0% by mass [43], whilst the equivalent
standard for slag limits it to 18% [50].
90 Biodeterioration of Concrete

Calcium is present in large quantities in concrete, certainly from cement,

but also potentially from aggregate. It has been observed that cyanobacteria
have a tendency to preferentially grow on surfaces containing calcareous
surfaces, with colonies of bacteria present on mortar between siliceous
stone [51, 52]. Whilst this presents the possibility that either the presence of
calcium or carbonate, or both, is the reason for this, a study of exclusively
calcareous building surfaces found that cyanobacteria preferred cement and
mortar surfaces to those of limestone, which is also rich in calcium [53]. It
must be concluded, therefore, that it is the higher porosity of the cement and
mortar which provides the favourable conditions, rather than the chemistry.
It can be concluded that whilst some of the nutrient required in small
quantities by bacteria may be present in concrete, a number of key nutrients
are in short supply, specifically nitrogen and organic carbon. This, however,
does not mean that bacteria cannot successfully colonise concrete surfaces.
Instead, it means that the types of bacteria that can colonise a concrete
surface are limited to those which are able to obtain the absent nutrients
from other parts of the surrounding environment.

3.5 Deterioration of Concrete

Bacterial deterioration of concrete is primarily the result of chemical attack
deriving from substances produced by the metabolic processes of bacteria,
and it is three such forms of this which will be examined first. However, the
presence of bacterial communities on concrete surfaces also has aesthetic
implications, and so this aspect is also explored.

3.5.1 Attack by sulphate reducing and sulphur-oxidising bacteria

By far the most documented concrete deterioration process involving

bacteria is the attack of sewage pipes and related structures by biogenic
sulphuric acid. This form of attack follows a relatively convoluted route
involving many aspects of chemistry and physical conditions which are
unique to such environments. Moreover, it requires two types of bacteria
whose metabolisms function in very different ways. The processes involved
are summarized in Figure 3.5, and described in more detail below.
The first type of bacteria involved in the process are a subset of the
group known as sulphate reducing bacteria. The main species encountered
in sewer pipes is Desulfovibrio desulfuricans [54]. These bacteria are obligate
anaerobes and can only exist in accumulations of biofilm and sludge
which occur on sewer pipe surfaces just above the waterline where oxygen
levels are sufficiently low. They obtain their energy by oxidising organic
compounds (AH in Figure 3.5 and principally lactic acid) and hydrogen
gas, and obtain the oxygen required for this by reducing sulphate ions
 Bacterial Biodeterioration 91

Sulfur oxidising bacteria:

H2S + 202 --'1- H2 S0 4 etc.

Biofilm containing sulfate re ducing bacteri a:

2AH +SO/·+ 2H' --+ CH 4 +52·+ 2C0 2 +energy

Figure 3.5 Processes occurring in a sewer pipe leading to attack

of concrete by sulphuric acid.

(SO42–) dissolved in sewage. The resulting sulphide ion (S2–) rapidly reacts
with hydrogen dissolved in the sewage to give hydrogen sulphide (H2S).
The solubility of sulphide in water is dependent on pH. This is because
H2S undergoes dissociation at higher pHs to give bisulphide (HS–) ions and
a variety of sulphide ions at even higher pHs, increasing the capacity of
the water to contain dissolved sulphide. The dissociation constants of H2S
at different temperatures are given in Table 3.2.
Where the pH of sewage is above 7, its capacity for dissolved sulphide
is relatively high. However, where the pH is lower—which is often the
case for sewage—the capacity is limited and H2S escapes as gas into the air
space above the sewage in the pipe. The reason for the low pH is also tied
to microbial activity—organic matter in the sewage is being broken down
into organic acid intermediates, which will also act as a source of carbon to
the sulphate reducing bacteria [57]. It can also be seen from Table 3.2 that
higher temperatures will also encourage the release of H2S.
The H 2S above the sewage now experiences a different chemical
environment: at the surface of the concrete pipe is usually a thin layer of
condensed water which, being in contact with concrete has a relatively high pH.

Table 3.2 Dissociation constants of H2S at different temperatures [55, 56].

Temperature, Dissociation Constant (pka)

°ºC
H2S ⇌ HS + H – + Sulphide Ions
HS– ⇌ S2– + H+ S22– S32– S42– S52– S62–
25 7.00 17.30 11.78 10.77 9.96 9.37 9.88
71 6.55 – – – – – –
93 6.46 – – – – – –
104 6.44 – – – – – –
92 Biodeterioration of Concrete

This means that the layer of moisture can dissolve relatively large quantities
of H2S. Oxygen in the atmosphere above the sewage can react with the H2S
to form a range of different solid compounds including elemental sulphur,
thiosulfates and polysulphates (tetrathionates, pentathionates, etc.) [58].
It is at this point that the second group of bacteria play a role. These
are the species Thiobacillus which are also present at the concrete surface.
Thiobacillus is one of a group of species known as colourless sulphur
bacteria, or sulphur-oxidising bacteria. These organisms are chemotrophs
and obtain their energy by oxidising inorganic compounds. Many reactions
can be used for this purpose, as outlined in Table 3.3. The products of most
of these reactions is either sulphuric acid (H2SO4) or compounds which can
subsequently by used in reactions which yield sulphuric acid. Another
common group of sulphur-bearing compounds in sewer pipes are the
mercaptans—organic compounds with a thiol (–SH) group. These, however,
do not appear to be usable by thiobacilli [60].
Bacteria identified in or on samples of concrete taken from corroded
sewer pipes from three studies are shown in Table 3.4. Common to all these
studies is Thiobacillus thiooxidans. Also included in the table are the pH
ranges within which different Thiobacillus species can grow. It should be
noted that Thiobacillus thiooxidans is only able to grow in conditions of low
pH, and that none of the other species grow under pH conditions greater
than 9 or 10. For this reason, it is necessary for the surface of the concrete,
which would otherwise have a pH of 11 or more, to become less alkaline.
The dissolution of acidic H2S in the water layer will play a small role in
the neutralization of the cement hydration products which are responsible
for the high pH. However, the largest contribution is likely to come from
carbonation—CO2 concentrations in the airspace in the pipe are in most cases

Table 3.3 Reactions used by colourless sulphur-oxidising bacteria to obtain energy [61].

Compound Reaction

H2S H2S + 2O2 → H2SO4

2H2S + O2 → 2S0 + H2O

5H2S + 8KNO3 → 4KSO4 + H2SO4 + 4N2 + 4H2O

Sulphur 2S0 + 3O2 + 2H2O → 2H2SO4

5S0 + 6KNO3 + 2H2O → 3KSO4 + 2H2SO4 + 3N2

Sodium thiosulphate Na2S2O3 + 2O2 + H2O → Na2SO4 + H2SO4

4Na2S2O3 + O2 + 2H2O → 2Na2S4O6 + 4NaOH

Sodium tetrathionate 2Na2S4O6 + 7O2 + 6H2O → 2Na2SO4 + 6H2SO4

Potassium thiocyanate 2KSCN + 4O2 + 4H2O → (NH4)SO4 + K2SO4 + 2CO2

 Bacterial Biodeterioration 93

Table 3.4 Bacteria identified from samples of corroded concrete from sewer pipes.

Organism ph for Growth [59] Carbon Fixation [59] Reference

Thiobacillus thiooxidans 0.5–4.0 Autotroph [62, 63, 64]
Thiobacillus neapolitanus 4.0–9.0 Autotroph [63]
Thiobacillus intermedius 1.7–9.0 Mixotrophic [63]
Thiobacillus novellus 5.0–9.2 Mixotrophic [63]
Thiobacillus ferrooxidans 1.5–2.5 Autotroph [64]
Bacillus [62]
Ochrobactrum anthropi [62]
Microbacterium [62]
Pseudomonas [63]

likely to be high, due to microbial activity, leading to rates of carbonation

considerably faster than under normal atmospheric conditions.
When sulphuric acid comes into contact with concrete made using
Portland cement, two processes detrimental to the integrity of the material
occur simultaneously. The first of these is acidolysis, whilst the second is
sulphate attack. Because both forms of attack influence each other, it is
necessary to discuss them as a single process.
When sulphuric acid comes into contact with the hydration products of
Portland cement it will lead to ions from these products becoming dissolved
in solution. For instance, in the case of one of the main components of
hydrated Portland cement, Portlandite (Ca(OH)2):
H2SO4 + Ca(OH)2 → Ca2+ + SO42– + 2H2O
The resulting calcium and sulphate ions will, under the right conditions,
precipitate as gypsum (CaSO4.2H2O). However, where calcium aluminate
hydrates are present in the cement, such as monosulphate (3CaO.Al2O3.
CaSO4.12H2O), a different process may occur if the pH is sufficiently high:

3CaO.Al2O3.CaSO4.12H2O + 2Ca2+ + 2SO42– + 20H2O → 3CaO.

Al2O3(CaSO4)3.32H2O

The product of this reaction is ettringite. The precipitation of both these

compounds with respect to solution pH has been examined in Chapter 2
in the section covering sulphuric acid. These two compounds are stable at
high pHs and have high molar volumes. Thus, where the pH is high these
compounds precipitate leading to expansion and cracking of the cement
matrix. However, sulphuric acid continues to ingress below the concrete
surface and as this occurs and the reactions continue, the Ca(OH)2, which
provides the main source of OH– ions, is neutralised and the pH drops. As
pH drops, gypsum and ettringite become soluble and CSH gel begins to
94 Biodeterioration of Concrete

decalcify. The overall effect of this is that considerable mass is lost from the
concrete surface and its strength is substantially reduced.
The effect of the above process is shown in Figure 3.6, which shows
the results of a geochemical model in which hydrated cement paste is
brought into contact with sulphuric acid. It can be seen that the outer part
of the cement paste is completely dissolved, followed by a layer in which
decalcified CSH is present. Beyond this layer are deposits of small quantities
of ettringite and substantial quantities of gypsum, followed by unaffected
hydrated cement. These modelled results match well with the results of
powder X-ray diffraction carried out on the acid-affected layers of concrete
specimens [65]. Under conventional sulphate attack, it would be not unusual

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z
<(
::J
a
0 .005

0 2 3 4 5 6 7 8 9 10

DEPTH FROM SURFACE, mm

Figure 3.6 Concentrations of dissolved elements (top) and quantities of solid phases
obtained from geochemical modelling of sulphuric acid attack of hydrated Portland
cement. Model conditions: acid concentration = 0.1 mol/l; volume of acid solution = 4 l;
mass of cement 80 g; diffusion coefficient = 5 × 10–13 m2/s.
 Bacterial Biodeterioration 95

to find higher quantities of ettringite. The dominance of gypsum under

sulphuric acid attack derives from the stability of this mineral down to the
relatively low pH conditions characteristic of acid attack.
An alternative mechanism has been proposed in which iron from
cement in the acid-affected zone of the concrete (where pH is low) is
dissolved (See Chapter 2), diffuses further into the concrete (where pH is
higher) and precipitates as a thin layer of ferrihydrite [66]. This process
is also evident in the geochemical modelling results in Figure 3.6. The
precipitation of ferrihydrite is well-known as having the effect of creating
expansive stress around corroding steel reinforcement, and it is proposed
that the same effect is observed in this case. If this mechanism does occur,
it is not unique to sulphuric acid attack, but would occur for any other
process of acidolysis, which does not appear to be the case.
The cracking resulting from the precipitation of expansive products
should permit the ingress of sulphuric acid at a faster rate. However, this
appears to be offset to a large extent by the precipitation of the reaction
products, leading to a reduction in porosity and a reduction in the diffusion
coefficient of sulphate into the concrete in the zone where these products
are present [64].
The effect of sulphuric acid exposure on concrete porosity is shown in
Figure 3.7. There is a reduction in total porosity with most of this reduction
occurring in the finer pore size range, which is in accordance with what

0.25

Before exposure
Sulfur bacteria
"'E
.!2 Sulfuric acid
0.20
"'E
(.)

>--
1-
U5 0.15
0
0::
0 ' ' '
[}_
'
LlJ
>
0.10 ' "\
i= \
::5 \
::::> \
~ 0.05 \
::::>
()
"' ..... ~

0.00
1x10° 10x10° 100x10° 1x103 10x103 100x103 1x106

PORE DIAMETER, nm

Figure 3.7 Cumulative pore size distributions obtained using mercury intrusion
porosimetry for concrete specimens placed in a reactor containing sulphur oxidizing
bacteria or submerged in a 0.15 mmol/l solution of sulphuric acid for 14 days [67].
96 Biodeterioration of Concrete

might be expected from the precipitation of gypsum. Also shown in this

figure is the pore size distribution obtained for concrete stored in a bioreactor
containing sulphur oxidizing bacteria. It is notably different to that obtained
for sulphuric acid exposure—there is an increase in total porosity, with a
pore size distribution curve indicative of a widening of pores of all sizes.
It should be stressed that these results were obtained using mercury
intrusion porosimetry, which is unable to distinguish between cracks and
pores. Nonetheless, these results suggest that a degree of caution should
be employed before considering sulphuric acid exposure as a model for
deterioration resulting from the activity of sulphur oxidizing bacteria.
The drop in pH that results as sulphuric acid ingress proceeds will of
course make more and more of the concrete surface and inner volume of
concrete favourable to colonisation by sulphur oxidising bacteria, and also a
shift towards more acid-tolerant species. As the pH declines, there is a shift
from an abundance of bacteria such as Thiobacillus neapolitanus towards the
more acidophilic Thiobacillus thiooxidans [63].
The process of sulphuric acid attack leads to a loss in mass from the
concrete, which consequently leads to a loss in cross-sectional area and
strength. The sulphuric-acid environment to which concrete is exposed
plays an important role in its rate of deterioration. The concentration of acid
will obviously play an important role, with higher concentrations leading
to lower pH conditions and faster rates of deterioration (Figure 3.8). pH
values as low as 1 may be measured in concrete which has undergone attack
by Thiobacillus thiooxidans [63].
Another important factor is whether the sulphuric acid is replenished,
or whether a finite quantity of acid is present. In the latter case, the acid will
gradually become depleted as it reacts with cement hydration products,
and the deterioration rate will decline with time, whereas the rate of
deterioration is steady where replenishment occurs. This is illustrated in
Figure 3.9, which shows mass loss rates from concrete exposed to both
types of regime. In the case of biogenic formation of sulphuric acid in sewer
pipes, the conditions are such that a replenishing acidic environment is the
most likely scenario, since the flow of sewage through the pipe will ensure
a constant supply of nutrients to the system.
One study has examined mass loss from concrete suspended in a reactor
containing simulated wastewater and various sulphur oxidising bacteria,
with the atmosphere above the liquid steadily replenished with H2S [70].
This led to a fairly uniform pH being established and the results of mass
loss measurements (Figure 3.10) indicate a reasonably uniform rate of mass
loss characteristic of a constant sulphuric acid concentration.
The decalcified and deteriorated surface tends to remain at least
partially in place unless it is subject to mechanical disruption. It possesses
very little strength and so is easily removed. Many studies examining the
influence of sulphuric acid utilise periodic brushing of the concrete surface
 Bacterial Biodeterioration 97

40.----------------------------------------------.
• Cement content = 570 kg/m2
- -o- Cement content= 310 kg/m2

30

::oR
0

(/)

g
(/)
20
(/)
(/)
<(
:2:
10

0
0.0 0.5 1.0 1.5 2.0 2.5 3. 0 3.5

pH
Figure 3.8 8-week mass loss from concrete exposed to sulphuric acid solutions with
different concentrations (expressed in terms of the resulting pH). Specimens were
not abraded [68].

20 , -----------------------------------------------,

•
--o-
Finite concentration
Replenished

15

::oR
0

(/)-
(/)
0 10
...J
(/)
(/)
<(
:2:

0~------------.------.------.------.-----,,-----~

0 20 40 60 80 100 120 140

TIME, hours
Figure 3.9 Mass-loss versus time plots for concrete specimens exposed to a finite
concentration of sulphuric acid (pH 1.0) and one which is replenished every 8
hours [69].
98 Biodeterioration of Concrete

2.0

• water/cement ratio = 0.331

0 /
/

I 0 water/cement rat1o = 0.42 /

D/ 0
1.5 /
/
/
/
/
/

?F. //0

,__ ,___ _.---·

0 /
Clf 1.0 /

(/) / /0
/
0 /
_J
9/
(/)

_...-- --•
/
(/) /
<( 0.5 /

~ 0 /
0/ 0

_...-·- •
o--·-·-
/
//

0.0

-0.5 - f - - - , - - - , - - - , - - - , - - - - , - - - - , - - - - , - - - - - - 1
80 100 120 140 160 180 200 220 240

TIME, days

Figure 3.10 Mass loss from concrete specimens stored in a reactor containing
simulated wastewater and sulphur oxidizing bacteria [70].

to mimic the effect of the flowing water in sewers. The extent to which
this mimics reality is debatable, given that corrosion occurs above the
water-line. However, this level will in fact vary with time and so corroded
zones are likely to experience flowing water periodically. The rate of
mass loss resulting from brushing is greater than that without brushing
(Figure 3.11). Clearly the majority of this increase is simply the result of
removal of material. However, it should also be remembered that the
reaction products may protect the underlying concrete from further attack,
and brushing will diminish this effect.
Concrete characteristics which influence the magnitude to which
sulphuric acid attack causes deterioration include cement content, water/
cement ratio, cement type and aggregate type.
Since sulphuric acid attack is largely a cement deterioration process, the
cement content plays a major role in determining the response of concrete.
As the cement content increases, it increases the availability of calcium,
which is an essential component in the formation of gypsum and ettringite.
Moreover, it is largely the cement which will be dissolved as a result of the
process of acidolysis. Thus, as cement content increases, so too does the
rate of deterioration (Figure 3.12).
The relationship between water/cement ratio and concrete durability
under sulphuric acid attack runs counter to expectation. A reduced water/
cement ratio yields higher strengths and reduced porosity, both of which
provide greater resistance against many forms of deterioration. This appears
 Bacterial Biodeterioration 99

10000

• Unbrushed
- -o - Brushed
8000

'E
0, 6000
CJi
(f)
0
--'
(f) 4000
(f)
~
::2
2000

0 20 40 60 80 100 120 140 160 180

TIME, days

Figure 3.11 Mass-loss versus time plots for concrete specimens exposed to a
pH 1.5 sulphuric acid solution with and without periodic abrasion using a
brush [71].

35

• -- ""
30 - --o -
--y--
pH 1
pH 0.6
pH 0.3
/ .....-
__ .,...- -- >r --
25 /

-
/ _-0
:oR ~
0
/ _o - - o- -
u) /
(f) 20 y"' _-er
__o-
0 o---
--'
(f)
(f) 15
~
::2
10

0
250 300 350 400 450 500 550 600

CEMENT CONTENT, kg/m 2

Figure 3.12 Mass loss from concrete specimens with different Portland cement
contents after exposure to sulphuric acid solutions for a period of 60 days [68].
100 Biodeterioration of Concrete

to not be the case for sulphuric acid, regardless of whether the concrete
undergoes abrasion [72, 73, 74, 75]. This can probably be attributed to two
factors. Firstly, the conventional means of achieving low water/cement
ratios during concrete mix design is to increase the cement content, yielding
the increase in vulnerability discussed above. Secondly, the increased
porosity at high water/cement ratios may offer a larger volume of space
within which the reaction products are able to precipitate leading to a delay
in the onset of expansive forces. Moreover, the accumulation of reaction
products in this way is likely to offset mass loss from fragmentation and
acidolysis.
However, results obtained from sulphuric acid exposure experiments
do not present the whole picture, and Figure 3.10 indicates that, where
attack derives from sulphur bacteria, a low water/cement does reduce the
rate of attack. This issue is discussed further in the chapter when measures
to limit rates of deterioration are discussed.
Cementitious materials used in combination with Portland cement tend
to provide enhanced resistance to sulphuric acid in comparison to concrete
containing only Portland cement. This is likely to be for two reasons. Firstly,
the cement fraction will possess a reduced calcium content, thus limiting the
quantities of gypsum and ettringite formed. Secondly, many such materials
have the effect of refining the pore structure of concrete either through fine
particles occupying the space between coarser Portland cement particles,
or through the formation of reaction products as a result of pozzolanic or
latent hydraulic reactions. Table 3.5 summarises the findings of studies
examining the performance of concrete containing ground granulated
blastfurnace slag (GGBS), fly ash (FA), silica fume (SF), natural pozzolanas
(NP) and metakaolin (MK) with regards to sulphuric acid resistance. GGBS
appears to have only a minor influence over resistance, presumably due
to its relatively high calcium content. Fly ash, whose presence will reduce
calcium content considerably more, improves performance to a much
greater extent (Figure 3.13). Whilst silica fume and metakaolin improve
performance in most of the studies included in Table 3.5, this cannot be
attributed to reduction in calcium content, since they are used at relatively
low levels, and so refinement of porosity is likely to be the mechanism of
enhanced resistance.
Calcium aluminate cements have also been found, in some instances,
to display enhanced resistance to sulphuric acid. However, results vary
considerably from study to study, possibly due to variation in composition
of these materials. A field study in which calcium aluminate cement (CAC)
was used in concrete mixtures in sewer pipes alongside Portland cement
concrete pipes indicated considerably less deterioration from the CAC
material when evaluated through inspection [69]. However, laboratory
experiments conducted by the same researchers did not replicate these
results. The same study also placed specimens made from sewer pipe
 Bacterial Biodeterioration 101

80

60
I
•
I
--
-·-
Mass loss, 100% PC
-o- Mass loss, 70% PC /30% FA
Expansion, 100% PC
--0- Expansion, 70% PC /30% FA
0.8

0.6

0.4
•
I
~ I ~
(/)~ 40 I z
r...rt
0.2
(/)
_.o----0 0
0_j Ui
--o-- o - z
(/) -o- -o- o- -o- -o- -o- -o-- -o- <{
(/) 0.0 [L
20
<{
::;! ><
L.U
-0.2

0
-0.4

~20 -0.6
0 10 20 30 40

TIME, months

Figure 3.13 Mass loss and expansion of Portland cement and Portland cement/
fly ash concrete specimens exposed to a 2% sulphuric acid solution with periodic
replenishment to maintain pH [78].

lining mixes made using both CAC and Portland cement in sewers and
measured mass loss. The CAC lining materials performed better than their
Portland cement equivalents. A similar study in which concrete specimens
were suspended in sewers found similar results when CAC concrete was
compared against Portland cement and Portland cement/GGBS blend
concrete [86].
One reason for enhanced resistance of CAC concrete, where this is
achieved, is most probably the lower calcium content of the material. The
results of geochemical modelling of sulphuric attack of a typical hydrated
calcium sulphoaluminate cement is shown in Figure 3.14. Comparing
these results to those obtained for Portland cement (Figure 3.6) it is evident
that the peak quantity of gypsum produced is lower for the calcium
aluminate cement. However, a substantial quantity of this mineral is
still present, and so only minor improvement in performance might be
expected. It would also appear that the high aluminate content of the
cement has the effect of suppressing bacterial growth: it has been shown
that aluminium concentrations in excess of 350–600 mg/l stop growth of
Thiobacillus thiooxidans [87]. This would certainly explain the relatively poor
performance of CAC concrete when exposed to sulphuric acid solutions,
compared to performance in the field.
The possibility of using limestone aggregates as a means of enhancing
sulfuric acid resistance has been explored, on the basis that the calcium
carbonate in these materials is capable of neutralising acids [76, 88].
The presence of limestone does tend to enhance resistance, although it
Table 3.5 Findings of research conducted to evaluate resistance of concrete containing pozzolanic and latent hydraulic materials to sulphuric acid attack. FA =
fly ash; SF = silica fume; NP = natural pozzolana; MK = metakaolin.

Cement Study

[72] [71] [73] [74] [75] [76] [78] [80] [81] [82] [83] [84] [84]

GGBS Level Not stated, 50–70%

but 36–65% with
limestone
Acid Conc. Alternating 50 1%
mg/l NaSO4
and pH 2
102 Biodeterioration of Concrete

H2SO4
Outcome Slight Negligible
improvement change
FA Level 7.5–22.5% 33% Optimum 10%–70% 13.5–50%
siliceous siliceous fly at 50% siliceous siliceous
fly ash ash, 7% SF siliceous fly fly ash fly ash
ash, 10% SF
Acid Conc. 1% 1% 1% 2% with 8.7%
replenished replenished periodic H2SO4
weekly weekly additions

Outcome Reduced Enhanced Enhanced Enhanced Slightly

enhanced
SF Level 7.5–30% 5%–30% 7.5–30% 15% 5–15%

Acid Conc. 1% 2% with 1 and 5% 1% 2%

periodic
additions

Outcome Negligible Enhanced Enhanced Enhanced Enhanced

change at > 5%
NP Level 13.5–50% 40% ‘true 28% trass
Algerian pozzolana’ + 6%
natural calcareous
pozzolana fly ash

Acid Conc. 8.7% 2:1 0.5% to

sulphuric/ 1.2% +
nitric, pH periodic
3.5, periodic additions
additions
Outcome Slightly Reduction Reduction
enhanced

MK Level 7.5–30% 5–15%

Acid Conc. 1% 2%

Outcome Negligible Enhanced

change

Others Level 2.5–20%

corn cob
ash

Acid Conc. 35%

Outcome Enhanced
up to 10%
of cement
Bacterial Biodeterioration 103
104 Biodeterioration of Concrete

100
"'= 10-1
0
E 10·2
c:i 10·3
~=::.-=~-=-~"=-~~~~-~~;_-
w \ .·
~ 10·4 .. /
0
(f)
(f)
10-5
""'' - - SO/
0 10·6 ------
I ./" ---- Ca
_J 10•7
/
~
0
10·8 1/ - · -- -
Si
AI
I- 10·9 --- Fe

. --·-- 12
0.020 pH,.. . /
10
8 I
0.
6
4
rJ)
Q)
0 .015
2
0
E
>--
I-
i= 0 010
z
<(
;:)
0
0 .005

0 2 3 4 5 6 7 8 9 10

DEPTH FROM SURFACE, mm

Figure 3.14 Concentrations of dissolved elements (top) and quantities of solid phases
obtained from geochemical modelling of sulphuric acid attack of a typical hydrated calcium
sulphoaluminate cement. Model conditions: acid concentration = 0.1 mol/l; volume of acid
solution = 4 l; mass of cement 80 g; diffusion coefficient = 5 × 10–13 m2/s.

seemingly only delays the process of deterioration (Figure 3.15)—it should

be remembered that whilst the capacity to neutralise acids is enhanced,
the calcium content of the concrete will be increased. Nonetheless, use in
combination with pozzolanic materials and possibly other measures to
improve resistance would appear to be a valid approach. When limestone
powder is used as a cement filler component, resistance is not enhanced [79].
Significantly enhanced performance has been achieved where slow-
cooled (and hence crystalline) blastfurnace slag fine aggregate has been
used in place of siliceous sand. It has been proposed that this results from
 Bacterial Biodeterioration 105

12

10
•
--o-
Siliceous aggregate
Limestone aggregate

8
;,R
0

(f)-
(f) 6
0_J
jJ
/
(f)
(f) 4
<{
:::2: /
;:f
2
_o---
--0--- ------
0 -- o---

-2
0 20 40 60 80 100 120 140 160 180
TIME, days

Figure 3.15 Mass loss from concrete specimens containing different aggregate
exposed to a 1% sulphuric acid solution [76].

the formation of a denser layer of protective gypsum in the acid-affected

zone. However, why this should be the case is not clear.
Synthetic aggregate made from calcium aluminate cement clinker has
been shown to substantially enhance sulphuric acid resistance of concrete
made using CAC, compared to dolomite aggregate [68]. It should be noted
that the particle size of the two aggregates was also different, with the
synthetic aggregate having finer particles.
Whilst the process of corrosion by sulphate reducing and sulphur
oxidising bacteria is common in sewers, it is not exclusively limited to these
environments. Deterioration of concrete by this process in outlet tunnels
in two artificial lakes in Ohio, USA has been reported in the literature [88].
In these cases, sulphate levels in the water were high (> 300 mg/l) and
attributed, at least for one lake, to strip-mining activities upstream.

3.5.2 Attack from nitrifying bacteria

Bacterial activity can also produce nitric acid. As with sulphuric acid
formation, the process is relatively complex, involving a chain of interaction
between various bacterial groups. The starting point of biogenic nitric acid
attack is the compound urea (CO(NH2)2). Urea can be used by a group of
bacteria known as urobacteria. Urobacteria hydrolyse urea for energy,
producing ammonia (NH3):
106 Biodeterioration of Concrete

CO(NH2)2 + H2O → 2NH3 + CO2

A wide range of bacteria are capable of this process.
Ammonia is employed by a group of bacteria collectively known as
ammonia-oxidising bacteria (see Table 3.6). These organisms are autotrophic
(but are capable of mixotrophism) and grow optimally at temperatures of
25–30°C at pH 7.5–8.0 [90]. They obtain energy through oxidising ammonia
in the following manner:
2NH3 + O2 → 2NO2– + 3H+ + 2e–
The simultaneous formation of nitrite ions and protons means that
nitrous acid (HNO2) is formed. Nitrite ions (NO2–) are oxidised further by
nitrite-oxidising bacteria (Table 3.7):
2NO2– + H2O → NO3– + 2H+ + 2e–
Again, because protons are produced, nitric acid (HNO3) is effectively
the product of this reaction.
The formation of nitric acid through bacterial activity has historically
been a problem in water treatment and distribution systems, where
conditions are suited to the organisms involved, and where quantities of
ammonia are typically present. It is for this reason that these environments

Table 3.6 Ammonia-oxidising bacteria found in water distribution and treatment environments.

Genus Species Location References

Nitrosomonas – Water distribution systems [91]
europaea Biofilm bioreactors [92, 94]
Wastewater treatment plants [97]
oligotropha Trickling filters [93]
Chloraminated water distribution systems [96]
Wastewater treatment plants [97]
communis Trickling filters [93]
Wastewater treatment plants [97]
uraea Chlorinated water distribution systems [95]
‘Nitrosococcus’ mobilis Biofilm bioreactors [92]
Nitrosospira - Activated sludge bioreactors [94]
Chloraminated water distribution systems [96]
Wastewater treatment plants [97]
briensis Chlorinated water distribution systems [95]
‘Nitrosovibrio’ Sandstone [98]
Sandstone, concrete [99]
 Bacterial Biodeterioration 107

Table 3.7 Nitrite-oxidising bacteria found in water distribution and treatment

environments.

Genus Species Location Reference

Nitrospira – Trickling filters [93]
Chloraminated water distribution systems [96]
moscoviensis Chlorinated water distribution systems [95]
Nitrobacter – Biofilm bioreactors [92]
Chloraminated water distribution systems [96]
Wastewater treatment plants [97]
Sandstone, concrete [99]

feature heavily in Tables 3.6 and 3.7. However, a growing problem in recent
history has been the growth in concentrations of atmospheric ammonia.
The rise in atmospheric ammonia concentrations is the result of human
activities, with such sources including coal combustion, fertilizer use and
livestock production. Ammonia released into the atmosphere will tend to
react with pollution-derived acids in the atmosphere (sulphuric acid, nitric
acid, etc.) to form ammonium salts. These salts will be precipitated either
as particulates or dissolved in precipitation [97]. As a result, high levels of
ammonia are frequently found in porous building materials with surfaces
exposed to the exterior environment [98].
Most nitrifying bacteria are obligate lithoautotrophs (with the exception
of Nitrobacter, which can grow heterotrophically [90]), and so no source of
organic matter is required. They are also obligate aerobes, needing oxygen
for the nitrifying reactions. They are, however, killed by visible blue
light and long-wavelength UV light [100]. As a result, they form biofilms
containing significant quantities of EPS to act as a barrier to light.
In sandstone (which is typically more porous), Nitrosovibrio have been
found at depths of 30 cm below the surface [99]. Nitrifying bacteria are also
capable of penetrating concrete: whilst the ‘slime forming’ bacteria found at
some depth inside asbestos cement water pipes discussed in Section 3.4.2
are not specifically identified, it is likely that they are nitrifying bacteria.
When nitric acid is brought into contact with hardened Portland cement,
the effect is more straightforward than that for sulphuric acid. The cement
hydration products will undergo acidolysis, leaving a decalcified zone at
the surface. The results of geochemical modelling of the process of nitric
acid attack are shown in Figure 3.16.
The results from the model indicate why the water pipe study discussed
in Section 3.4.2 found the most nitrifying bacteria at the interface between
the unaffected cement and the deteriorated material. The production of
nitric acid, if unchecked, will lead to a drop in pH which will eventually
108 Biodeterioration of Concrete

101
<:::: 10°
0
E 10-1
cl 1Q·2
UJ
>
_J
10-3
0 10-4
(/)
(/) 10-5
0 10-6
_J

~
10·'
Ca
Si \_ _____ _
0 10-8 - ·- - • AI
I- 10-9 --- Fe

1
12
r·-- ·--·--·--·--·--·--·
. pH
0.020 10
I
8
I I
Q_
6
I
4
If)
Q)
0 .015 I
2
0 I
E · -·- · - · -·-·-·- · - · -·- · ----~ 0
>-"
I-
i= 0.010
z hydrite
<t:
~
0

0 2 3 4 5 6 7 8 9 10

DEPTH FROM SURFACE, mm

Figure 3.16 Concentrations of dissolved elements (top) and quantities of solid phases obtained
from geochemical modelling of nitric acid attack of hydrated Portland cement. Model
conditions: acid concentration = 0.1 mol/l; volume of acid solution = 4 l; mass of cement 80
g; diffusion coefficient = 5 × 10–13 m2/s.

lead to a cessation in growth. However, where cement hydration products

come into contact with nitric acid, neutralization will occur, e.g.:
Ca(OH)2 + 2HNO3  Ca(NO3)2 + 2H2O
Thus, in the zone just ahead of the deterioration front, the pH of the
concrete pore solution will be in the range in which nitrifying bacteria
populations will undergo growth.
As for sulphuric acid, exposure of concrete to nitric acid leads to mass
loss, although this is much more marked when abrasion of the surface
is occurring simultaneously. The decalcification of the outer surface of
 Bacterial Biodeterioration 109

the concrete leads to the formation of an outer layer of amorphous silica

gel, followed by an iron-rich zone directly before the unaffected cement
which can be seen as a brown layer [101], features which are also seen
in the modelled data in Figure 3.16. The thickness of the brown layer
is dependent on the concentration of acid, with a higher concentration
leading to an increase in thickness. The silica gel layer is prone to shrinkage
and undergoes cracking. The rate of deterioration of the cement matrix is
strongly dependent on the concentration of acid present (Figure 3.17).
The change in porosity as this occurs would be expected to increase,
due to the removal of significant quantities of hydration products. However,
experiments on concrete exposed to either nitric acid or a bioreactor
environment in which nitrifying bacteria were present indicates only a
relatively small change in porosity, with total porosity decreasing in the
case of nitric acid exposure (Figure 3.18). It is conceivable that the lack of
change in the porosity distribution resulting from exposure to bacteria is
the result of pores becoming blocked with biofilm.
Unlike sulphuric acid attack, the relationship between nitric acid
resistance and water/cement ratio is the expected one: a higher ratio leads
to a microstructure through which the movement of acid is easier, leading to
a higher rate of deterioration (Figure 3.19). Additionally, as cement content
increases, resistance also increases (Figure 3.20). The results in these two
figures are inter-related as a result of the approach taken in designing the
concrete mixes: the water/cement ratio was reduced by increasing the

60

50
E
E
I 40
I-
a_
w
0
z 30
Q
(f)
0
0:: 20
0::
0
(.)

10

0
00 0.1 0.2 0.3 0.4 0.5 0.6

CONCENTRATION , mol/1
Figure 3.17 Depth of corrosion of Portland cement paste specimens exposed
for 200 days in nitric acid solutions maintained at various concentrations [101].
110 Biodeterioration of Concrete

0.16

Before exposure
0.14 Nitrogen bacteria
"'E
0 Nitric acid
"'"0E 0.12 ' -... ,.
~
1- 0.10
" "\
\
Ui \
0 \
0::: "\
0 0.08 .\
0... \
L.U \
> 0.06 \
i=
<(
--'
=> 0.04
,.
\.
I

:2
=>
u
"""
0.02 "
0.00
1x10° 10x10° 100x1 oo 1x103 10x1 03 100x103 1x106

PORE DIAMETER, nm
Figure 3.18 Cumulative pore size distributions obtained using mercury intrusion
porosimetry for concrete specimens placed in a reactor containing nitrifying
bacteria or submerged in a 0.15 mmol/l solution of nitric acid for 14 days [67].

16

-~ -------- v 60 days
14

E 12
E
:i"
1- 10
c..
L.U

___.,. - --- ---

0 ~ - -------,.. 28 days
z 8
0
(/) __ __ Q 15 days
0
0:::
0:::
6
__ o--------\Y----
0 4 0'-
u

2
... ---- - - - - · - - - - - - - - - - - - - - - - - - - · 7 days

0
0.3 0.4 0.5 0.6

WATER I CEMENT RATIO

Figure 3.19 Influence of water/cement ratio on deterioration of Portland cement

pastes exposed to nitric acid solutions maintained at 0.2 mol/l [102].
 Bacterial Biodeterioration 111

--·
14

E
12
·--- - ------ - ~- -
E
z- 10
0
Ui
0 8
0::
0::
0
u 6
LL
0
I
f- 4
()._
w
0
2

0
0.5 1.0 1.5 2.0 2.5 3.0 3.5

AGGREGATE I CEMENT RATIO

Figure 3.20 Depth of corrosion in 0.5 water/cement ratio Portland

cement mortar specimens exposed to a nitric acid solution maintained
at 0.2 mol/l after 100 days, w/c = 0.5 mortar [103].

cement content whilst maintaining a constant water content. Nonetheless,

the results make sense—a lower water/cement ratio will reduce porosity
and pore size leading to a slower rate of ingress, whilst a higher cement
content will act to neutralize the acid.
Cement type again plays a role in defining resistance to nitric acid attack.
Table 3.8 summarises the findings of experiments in which concrete with
different cement types were employed. The outcome of these experiments is
less clear cut than for sulphuric acid. Generally, the presence of silica fume
has the effect of increasing resistance to nitric acid attack. The presence of
siliceous fly ash seems to provide less protection, as do natural pozzolanas.
Calcium aluminate cements are also less susceptible to nitric acid
attack in comparison to Portland cement [105]. This is attributed both
to the stability of gibbsite to relatively low pH levels, and the enhanced
capacity of these cements to neutralize acid [86]. Figure 3.21 shows the
result of geochemical modelling of nitric acid attack on a hydrated calcium
sulphoaluminate cement. Comparison with the results obtained for Portland
cement highlights the persistence of gibbsite, additional quantities of
which are precipitated as other calcium aluminate hydration products are
dissolved by the acid. This ‘band’ of gibbsite at the interface between the
decalcified and unaffected zones of the concrete is common to all forms
of acid attack where acidolysis is the principal mechanism. It has been
proposed that precipitation of gibbsite in this way has the effect of blocking
porosity and limiting further ingress of acidic species [86].
112 Biodeterioration of Concrete

Table 3.8 Findings of research conducted to evaluate resistance of concrete containing

pozzolanic and latent hydraulic materials to nitric acid attack. FA = fly ash; SF = silica fume;
NP = natural pozzolana; MK = metakaolin.

Cement Study
[73] [103] [104] [83] [84]
Slag Level

Acid
Conc.
Outcome
FA Level 7.5–22.5%
siliceous
fly ash
Acid 0.01%
Conc.
Outcome Reduction
SF Level 7.5–30% 5–30% 0.2%
Acid All 1% 0.013% Concentrated
Conc. and 20%
Outcome Negligible Enhancement Enhancement
change
NP Level 40% ‘true 28% trass
pozzolana’ + 6%
calcareous
fly ash
Acid 2:1 0.3% to
Conc. sulphuric/ 0.8%
nitric, pH =
3.5, periodic
additions to
maintain pH
Outcome Reduction Reduction
MK Level 7.5–30%
Acid 0.01%
Conc.
Outcome Negligible
change

Whilst water distribution and treatment systems are without question

an environment in which damage to concrete from nitrifying bacteria can
occur, the effect of the same bacteria on the exterior surfaces of buildings
is less certain. In discussing the deterioration of stone masonry, it has been
proposed that damage is likely if populations of 106 cells/g of material are
 Bacterial Biodeterioration 113

101
"'
0
10°
E 10"1
cl
LU
>
...J
10·2
---
10-3 -!=~~~ ·~·--~~·--------- - - - - --
0 10·4
(/)
(/) 1Q·5
No; IL ----------
0 10·6
Ca 1
...J
10·7 ------ Si I____ _
~ 10·8 --- · AI
0
1- 10·9 --- Fe v
1
12
0.020 10
8 I
c.
6
4
en 0.015
Ql
2
0
-·-·-·-·-·
)
E -·-·- ·- - ·- - 0
~
1-
i= 0.010
z
<(
::J
a
0005

0 2 3 4 5 6 7 8 9 10

DEPTH FROM SURFACE, mm

Figure 3.21 Concentrations of dissolved elements (top) and quantities of solid phases
obtained from geochemical modelling of nitric acid attack of a typical hydrated calcium
sulphoaluminate cement. Model conditions: acid concentration = 0.1 mol/l; volume of
acid solution = 4 l; mass of cement 80 g; diffusion coefficient = 5 × 10–13 m2/s.

present [106]. However, this figure comes from a study of deterioration

of wall paintings [107], and must be viewed cautiously. Nonetheless,
populations of this magnitude can frequently be found on buildings.
A study of stone buildings in Germany found that buildings with
significant concentrations of nitrifying bacteria also possessed quantities
of nitrate salts at their surface, indicating deterioration had been occurring,
albeit potentially over long periods of time [106]. The same researchers
also conducted experiments in which concrete blocks were inoculated with
nitrifying bacteria and stored under optimal conditions for their growth
–30°C, 95% relative humidity and with an excess supply of ammonium
nitrate [108]. The concrete blocks had lost more than 3% of their mass
after a 12-month period. Deterioration of a much lesser magnitude was
114 Biodeterioration of Concrete

observed when identical blocks were exposed to nitric acid at comparable

concentrations. However, it should be stressed that this process of
biodeterioration was an accelerated one—the provision of more than
sufficient substrate in the form of ammonium nitrate does not reflect the
situation in reality, where the rate of delivery of ammonia to the building
surface will be limited by air pollution levels.
Many nitrifying bacteria can survive and grow using very small
quantities of ammonia and nitrite. For instance, Nitrosospira from terrestrial
locations have half saturation coefficients (Ks, see Section 3.2) as low as
0.1 mg/l NH3 [108], whilst those of Nitrobacter can be as low as 0.06 mg/l
NO2– [109]. Whilst this means that the bacteria can exist in an environment
where very small quantities of ammonia are supplied, it also means that
nitrous and nitric acids will be produced in very small quantities, limiting
the rate at which damage can be done.
One example of nitrifying bacteria undoubtedly causing damage to a
cement-based building surface is that of the deterioration of asbestos-cement
roofs on stables [111]. Nitrifying bacteria on the surfaces of such roofs were
established as being the cause of deterioration. It should be noted that the
circumstances of this case study were favourable to such a process—the
presence of horses would have provided a significant source of ammonia,
whilst the researchers established that evaporation of moisture within the
stable and its subsequent condensation on the underside of the roof acted
to leach nitrate salts. Moreover, the elevated temperatures within the stable
would probably have encouraged bacterial growth.

3.5.3 Attack by organic acids

When heterotrophic bacteria metabolise organic compounds, the

metabolites will often take the form of organic acids. The nature of these
acids will depend on the bacteria, environmental conditions and the organic
compounds that are being metabolized. There are two main ways in which
concrete can potentially be damaged by the bacterial production of organic
acids. Firstly, the concrete may be brought into contact with volumes
of solutions in which heterotrophic bacteria are breaking down organic
material. This is the most likely manner in which deterioration can occur.
Secondly, there exists the potential for damage resulting from the growth
of bacteria present on the surface of buildings. The former scenario will be
explored first.
A number of agricultural activities provide environments where
heterotrophic bacteria metabolise organic matter in a volume of liquid. Such
processes can either occur circumstantially or intentionally. An example of
the intentional formation of organic acids includes the production of silage.
The production of silage—the anaerobic fermentation of grass and other
plants for the provision of winter feed for livestock—leads to the formation
 Bacterial Biodeterioration 115

of organic acids. Traditionally, silage production relied on bacteria already

present in the plant matter. Today, however, it is usual for inoculant bacteria
to be added to silage to ensure rapid fermentation. The bacteria used fall
into two categories: homo- and heterofermentative. Homofermentative
bacteria include species such as Lactobacillus plantarum, Enterococcus faecium
and the Pediococcus species, and convert the plant sugars exclusively into
lactic acid [112]. Heterofermentative bacteria produce a range of metabolites
including lactic acid, acetic acid and ethanol. The main heterofermentative
species is Lactobacillus buchneri.
The mixing of manure with water is an agricultural practice which
yields a fluid rich in plant nutrients which can be evenly spread on land
with ease. Manure is a substance within which bacterial activity has already
occurred, but the storage of liquid manure will usually permit bacteria to
continue the process of breaking down the organic compounds, yielding
organic acids including propionic, acetic, butyric and isobutyric acid [113].
The production of foodstuffs also generates solutions containing
potentially substantial concentrations of organic acids deriving from
bacterial activity. Effluents from the dairy industry contain quantities
of lactic acid, and often butyric acid [113]. Whey generated from the
manufacture of cheese is notably acidic, with pH values around 6, due to
the presence of lactic, butyric, propionic and citric acid [114, 115]. The first
three of these are caused by bacteria breaking down lactose from milk and
are present at concentrations of tens to hundreds of mg/l, whilst citric
acid is added to the cheese-making process to curdle the milk and is often
present in higher concentrations. Bacteria involved in the formation of
the biogenic acids include the Lactobacillus, Streptococcus, Pediococcus and
Leuconostoc genera.
Thus, the primary organic acids produced by bacteria in applications
likely to lead to contact with concrete are acetic, lactic, propionic and butyric
acid. The reactions of these acids individually with hydrated Portland
cement are discussed below.

Acetic acid

Consulting the data in Chapter 2 regarding the interactions of acetic acid

with metal ions in cement, acetic acid forms weak complexes with Ca, Al
and Fe(II), and stronger complexes with Fe(III). However, precipitation of
salts is typically not an issue, and so the process of deterioration resulting
from acetic acid exposure is principally that of acidolysis.
Figure 3.22 shows mass loss from hardened Portland cement pastes
exposed to acetic acid solutions, alongside the depth to which deterioration
was observed, with both sets of result showing parity with each other, and
also fairly substantial damage in a relatively short period. Whilst being
acidolysis-based, and hence producing results comparable to nitric acid
116 Biodeterioration of Concrete

100 10

• Mass loss
Deteriorated depth
I
I 0

80 ---o 8 E

----
E

-- 0
::C

--
;,R 1-
0 9- ll.
(/)- 60 6 w
(/) /
/ 0
0 / 0
....1 -()
/
w
(/) / 1-
~
/
___________ _.
(/)
<{ 40 4
a' / o 0
~
I - · --- -
a:w
cj _.----- 1-
w

/r
20 I
I
_.
... 2 0

0 t
0 100 200 300
0

TIME, days

Figure 3.22 Mass loss and depth of deterioration of Portland cement paste specimens
exposed to acetic acid [116]. Acid concentration = 0.28 mol/l, with pH adjusted to
4.0 using sodium hydroxide.

attack, deterioration will typically occur at a slower rate with acetic acid,
since it is a weaker acid.
Since acidolysis is the main mode of attack, deterioration takes the
form of decalcification at the surface, leading to a thick layer of what is
essentially silica gel being the only remaining material at the outside of the
cement paste. Figure 3.23 shows micro-CT scans through various cement
paste specimens exposed to acetic acid solutions. The dark outer layer is
the silica gel layer. In the case of the Portland cement specimen, there is a
second layer between the wholly-decalcified material and the unaffected
cement, where partial decalcification has occurred, which is not present
in the case of the other cements in this figure, due to their lower calcium
content. It should be noted that the silica gel layer has cracked substantially
in all cases. This is not the direct result of acid attack, but of shrinkage
during drying. However, the magnitude of the cracking shown in these
images clearly illustrates the notable susceptibility of the layer to shrinkage.
Taking the diameter of the wholly unaffected cement as a measure of
resistance, the figure indicates that there is some enhancement in resistance
to acetic acid attack through the use of fly ash. This has also been observed
with concrete [117], with enhanced performance with silica fume reported
frequently, also (Table 3.9). Calcium aluminate cements a sulfoaluminate
cement—CSA—in the case of Figure 3.23 typically provide enhanced
resistance, for the same reasons as for nitric acid.
 Bacterial Biodeterioration 117

Figure 3.23 micro-CT scans from cement paste specimens exposed to a 0.1 M
solution of acetic acid for 90 days [118].

Table 3.9 Findings of research conducted to evaluate resistance of concrete containing

pozzolanic and latent hydraulic materials to acetic acid attack. FA = fly ash; SF = silica fume;
NP = natural pozzolana; MK = metakaolin.

Cement Study
[117] [103] [73] [80] [81]

FA Level 5%
Acid
conc. 1.7%
Outcome Enhancement
SF Level 10% 5–30% 7.5–30% 78.5–30% 15%
Acid
conc. 1.7% 14% 5% 5% 5%
Outcome Enhancement Enhancement Reduction Enhancement Enhancement
MK Level 7.5–30%
Acid
conc. 5%
Outcome Negligible
change/
Reduction

Lactic acid

Lactic acid does not form insoluble salts with calcium, aluminium or iron,
and forms weak complexes with these ions (although the aluminium
complexes are somewhat stronger than those with other ions). For this
reason, the deterioration of concrete in contact with lactic acid is almost
purely a process of acidolysis.
An example of mass loss characteristics from lactic acid attack are
shown in Figure 3.24, whilst micro-CT scans from cement paste specimens
118 Biodeterioration of Concrete

made from PC, PC/FA and CSA cements are shown in Figure 3.25. It should
be noted that the specimens in this figure show less deterioration than
those exposed to acetic acid. From a theoretical perspective, this seems
improbable, since lactic acid has a lower acid dissociation constant (see
Chapter 2) and is, thus, the stronger of the two. A study examining the
composition of organic acid solutions brought into contact with hardened
Portland cement paste [120] has found that, despite the lower pH of the
lactic acid solution, the concentration of calcium is lower than that of acetic
acid (as well as butyric, isobutyric and propionic acids).

100,-----------------------------------------------~

80
I ~ PC concrete
85% PC / 15% Silica fume concrete
I

0~

(/)- 60
(/)
0
-'
(/)
(/)
<( 40
~

20

0
0 50 100 150 200

TIME , days

Figure 3.24 Mass loss from concrete specimens made with Portland cement
and a Portland cement/silica fume blend exposed to lactic acid [81]. Specimens
were scrubbed periodically with a wire brush.

Figure 3.25 micro-CT scans showing cross-sections through cement paste

specimens exposed to a 0.1 M solution of lactic acid for 90 days [119].
 Bacterial Biodeterioration 119

The reason for this is almost certainly related to the fact that the lactate
ion forms complexes with Al, leading to an increase in the concentration of
this element in solution. Higher Si concentrations were measured compared
to the other acid, which is at least partly the result of the lower pH obtained
with lactic acid. However, neither of these effects directly explain the lower
Ca concentrations. It is possible that, since CSH gel will normally contain a
quantity of aluminium, the removal of this element from the gel as a result
of complexation means that calcium takes its place.
Both figures show that the use of pozzolanic material and CSA enhances
resistance to lactic acid attack. Slag also gives improved resistance, as will
be discussed later in this section.

Propionic acid

Very little research has been conducted into the interactions of propionic
acid with cement and concrete. However, experiments using of a number
of the common organic acids formed by bacteria have been conducted to
examined the pH and dissolved species detected after exposure of crushed
PC paste specimens to acid solutions of the same molar concentration [120].
It was found that there was very little difference between the behaviour of
propionic acid compared with either acetic, butyric or isobutyric acid. This
is not surprising, since all of these acids are structurally similar and have
very similar acid dissociation constants (see Chapter 2).

Butyric acid

As previously stated, butyric acid’s chemical similarity to acetic acid means

that the nature of its interaction with hardened cement is comparable. This
can be seen in Figure 3.26, which shows very similar cross-sections through
cement paste specimens as those seen in Figure 3.23.

Organic acid mixtures

Whilst it is useful to understand the effects of these organic acids on

concrete individually, it must be remembered that they commonly occur
as mixtures. For this reason, a number of studies have been carried out
using such mixtures. Because of the similar chemical behaviour of acetic,
butyric, isobutyric and propionic acid [120], the approach taken has been to
use acid solutions comprising acetic and lactic acid. Cement types studied
have included PC, PC/GGBS [121], PC/fly ash and PC/silica fume cements
[122]. A comparison of mass loss from PC and PC/GGBS mixes are shown in
Figure 3.27, with clearly enhanced performance where GGBS is used. Also
illustrated in this figure is the influence of cement content, which appears
120 Biodeterioration of Concrete

Figure 3.26 micro-CT scans showing cross-sections through cement paste specimens
exposed to a 0.1 M solution of butyric acid for 90 days [118].

600

N
E
0
- Gravel
c=J Limestone
1
0, 500 1
E
<(
w
0::: 400
<(
!:::
z ,---
::J 300
0:::
w
a..
C/) 200
C/)
0
.....J
C/)
C/) 100
<(
~

Figure 3.27 Mass loss from concrete specimens made using either PC, PC/GGBS
or calcium aluminate cement (CAC) as the cement fraction exposed to a solution
containing both lactic acid (32 mg/l) and acetic acid (4 mg/l). All water/cement ratios
are 0.45, except the CAC mixes (w/c = 0.40). PC/GGBS proportions unstated, but by
definition must be 36–95% by mass [121].
 Bacterial Biodeterioration 121

to have little effect in the case of the PC-only mixes, but which produces
lower levels of deterioration for lower cement content where GGBS is
present.
Figure 3.28 compares performance in volume reduction terms of PC,
PC/FA and PC/SF mixes of the same water/cement ratio. Silica fume
is more effective than fly ash, although both give higher resistance to
deterioration compared to PC [122]. Generally, performance increases in
the sequence PC–PC/FA–PC/SF–PC/GGBS [123].
Calcium aluminate cement also seemingly provides greater resistance
to attack (Figure 3.27), although it should be noted that the CAC mixes in
this figure had lower water/cement ratios, which is also likely to play a
part. Conversion of the cement (the result of carbonation, leading usually
to a loss in strength) reduces resistance slightly, but the converted material
still performs better than the PC-only mixes [121].
The influence of aggregate type is also shown in Figure 3.27. The use of
limestone aggregate provides greater resistance when compared to siliceous
gravel for the PC and PC/GGBS mixes. This is the result of the calcium
carbonate present in limestone increasing the neutralising capacity of the
concrete. This effect is not evident in the case of calcium aluminate cement.
In a similar study, a solution mimicking the composition of pig
manure was used [124]. This comprised 0.21 mol/l of acetic acid, 0.04
mol/l propionic acid, 0.02 mol/l butyric acid, 0.01 mol/l isobutyric acid

400 ,------------------------------------------------,

-PC
c:::::J FA
- SF
w· 300
0
<(
u..
0:::
=>
(j)

!:::
z 200
=>
0:::
w
o.._
(j)
(j)
100
0_J
(j)
(j)
<(
::::
0+-----~~------~------~~----J,L------.~~--~

100% 10% 22% 38% 10%

LEVEL, % by mass
Figure 3.28 Mass loss from concrete specimens made using either PC, PC/FA or
PC/SF as the cement fraction exposed to a solution containing both lactic acid (30
mg/l) and acetic acid (30 mg/l). All water/cement ratios are around 0.40 [122].
122 Biodeterioration of Concrete

and 0.003 mol/l of valeric acid. The solution was regularly refreshed to
maintain these concentrations. Figure 3.29 shows how the development of
deteriorated depth settles into what approximates to a linear relationship
with respect to time under these conditions. The pH of these solutions was
adjusted to 4 and 6 using sodium hydroxide, and it is evident that pH plays
an important role, with a high pH solution yielding less damage. GGBS was
found to improve performance. 10% Silica fume also enhanced resistance,
albeit to a slightly lesser extent [125].

Heterotrophic bacterial attack in the laboratory

An interesting example of bacterial deterioration observed in the laboratory

involved bacterial samples taken from a deteriorating sewer [126]. The sewer
was suspected of being attacked under the action of sulphate reducing and
sulphur oxidising bacteria. Bacteria were isolated from samples taken from
surfaces in a sewer system and transferred to bioreactors in which mortar
specimens were held in previously sterilized growth culture solution.
Initially, sulphate reducing bacteria were introduced, followed by sulphur
oxidising organisms. The concrete underwent deterioration, but analysis
of the deteriorated material found an absence of gypsum and ettringite.
However analysis of the growth culture medium found high concentrations
of acetic, propionic and, by inference, carbonic acid. Examination of the
isolated bacteria indicated that the sulphate reducing bacteria population

6,------------------------------------------------,
e PC. pH4
· 0· 68% GGBS / 32% PC. pH4
E 5 __ .....,.._ __ PC. pH6
E - -? - 68% GGBS I 32% PC, pHS
I"
I-
o._
4
L.U
0
0
L.U 3
I-
<(
0::
0
0:
__.--- - - - ____
2
L.U
1-- 0
.,
L.U _ ..xv
0
0
------.. .- -:.
---:-JJ~- · · - · · - · Y
--v----

~- - ·
0~~~-v,_·--.----.----.---.----.----.---.----.--~
0 2 4 6 8 10 12 14 16 18 20
TIME, weeks

Figure 3.29 Deteriorated depth versus time for PC and PC/GGBS pastes in
a mixed acetic/propionic/butyric/isobutyric/valeric acid solution [124].
 Bacterial Biodeterioration 123

consisted of two different species, one of which could not be identified and
either a species of Flavobacterium or Xanthomonas. The sulphur oxidizing
bacteria were Thiobacillus intermedius. It was concluded that the concrete
in the bioreactors was deteriorating as a result of attack from organic acids
deriving from aerobic bacteria (and, thus, presumably Flavobacterium or
Xanthomonas). It should be stressed that this does not mean that the sewer
pipes themselves were undergoing this form of deterioration, simply
that the samples taken and the conditions of the bioreactor favoured the
development of aerobic heterotrophic organisms.
Studies into the effect of heterotrophic bacteria on reinforced concrete
have observed considerable deterioration of properties and accelerated
de-passivation of the steel [127, 128]. Concrete specimens submerged in a
growth medium solution in which heterotrophic bacteria were present were
found to display lower strengths than specimens submerged in sterilised
water from the same source [127]. The disparity between the strength was
as high as 36%, with the greater difference in strength corresponding to the
weaker concrete mixes, although the researchers do not state the period
of time over which this occurred, or whether the differences correspond
to losses in strength or arrested strength development. Electrochemical
measurements made on the steel reinforcement bars embedded in other
specimens found that the corrosion potential in the presence of bacteria
increased at early ages and, in most cases, remained high, whereas the
sterile specimen potentials remained low. However, potentiodynamic
polarisation measurements indicated passivation of the steel affected by
bacteria, and it was proposed that this was the result of biofilm formation
at the steel surface.
A similar study did not use a nutrient solution, but instead introduced
sodium citrate into the concrete as an admixture at the mixing stage [128].
The citrate ion can be used as a nutrient by heterotrophic bacteria. Sodium
citrate is used as a corrosion inhibitor, but not normally in concrete,
since it will lead to the formation of calcium citrate leading to expansion
and cracking. Consequently, the 28 day strengths of the concrete mixes
were reduced in direct proportion to the admixture dosage. The concrete
specimens were submerged in volumes of water removed from a pond
—to ensure the presence of heterotrophic bacteria—for a period of 90
days. The strength of all specimens increased over this period, although
strength gain was proportionally less in the specimens containing higher
quantities of citrate. Corrosion potential measurements indicated a rise in
corrosion potential with citrate dosage, and gravimetric measurements on
the reinforcement indicated a higher rate of corrosion above sodium citrate
dosages of 1.0%.
Whilst both these last two studies suggest that the presence of
heterotrophic bacteria have the potential to cause significant damage to
concrete and its reinforcement, features of the experimental design and
124 Biodeterioration of Concrete

interpretation of results in both cases means that this aspect still needs
further investigation.

3.5.4  Biofilm formation as a form of deterioration

The growth of biofilms on building surfaces also potentially has aesthetic

implications for concrete surfaces. The main bacterial contribution towards
discolouration of construction material surfaces comes from cyanobacteria.
Often the most significant proportion of biomass on a building surface
will comprise cyanobacteria. A study of building surfaces in both Europe
and Latin America found cyanobacteria as the dominant microbe at many
sites [129]. The families of cyanobacteria identified were Scytonemataceae (the
most common), Microchaetaceae, Rivularaceae, Oscillatoriales and Nostocaceae.
Cyanobacteria have also been identified as the main component of stains on
concrete walls in Toulouse in France. In this case, the genera of Gloeocapsa-
Chroococcus (family: Chroococcaceae), Microspora (family: Microsporaceae)
and Trentepohlia-Gloeocapsa (Family: Microcystaceae) were identified [32]. A
programme of sampling from building faҫades around France identified
the cyanobacteria genera Aphanocapsa, Aphanothece, Calothrix, Chroococcus,
Cyanosarcina, Gloeocapsa, Gloeothece, Leptolyngbya, Microcoleus, Nostoc,
Phormidium and Scytonema [130]. Analysis of DNA from concrete surfaces
from sites in Georgia, USA has also identified cyanobacteria as major
occupiers of these surfaces [30].
Cyanobacteria, as the name suggests, often have a blue-green
pigmentation, which is the result of the presence of phycobilisomes—
regions in the inner membranes of the bacteria which capture energy from
light. These contain phycobiliproteins and carotenoids, which give the
bacteria their colour [3]. Colours are not limited to blue-green: carotenoids
and a group of phychobiliproteins known as phycoerythrins impart a
reddish-brown colour.
The presence of substantial quantities of cyanobacteria on the surface of
a building material is therefore likely to be visible, potentially in a manner
which is undesirable. Cyanobacteria from the genus Gloeocapsa-Chroococcus,
Microspora and Trentepohlia-Gloeocapsa have been observed to produce black,
green and red stains respectively [32].
Both environmental and material characteristics influence colonisation
by cyanobacteria. The previously mentioned study of European and
Latin American cyanobacteria found that the dominance of cyanobacteria
was more pronounced for concrete, mortar and brick surfaces in the
Latin American locations, and it was proposed that this was the result of
these organisms ability to resist dehydration and higher temperatures.
Nonetheless, moisture is an important factor and the high humidity of the
Latin American locations was thought to favour growth of cyanobacteria.
This is also reflected in the tendency of cyanobacteria to grow preferentially
 Bacterial Biodeterioration 125

on the façades of buildings which are facing the dominant wind direction
(which are consequently most likely to be the dampest side of a building)
[130]. The sides of building receiving less sunlight during the day will also
tend to retain moisture longer, promoting growth of cyanobacteria.
The material characteristic which has most influence over cyanobacterial
colonisation is the porosity of concrete, with higher levels of porosity
(corresponding to higher water/cement ratios) encouraging growth [30,
31, 32]. Surface roughness, as discussed earlier in this chapter, appears to
have less of an influence [30, 32]. Portland cement composition and the
use of pozzolanic and latent hydraulic materials have little influence over
colonization rates [30].
It should be stressed that biofilms are frequently not exclusively
composed of bacterial communities, and can contain a range of different
organisms. For this reason, biofilms will be revisited in subsequent chapters.

3.5.5 Aggressive CO2

As seen earlier, heterotrophic bacteria will ultimately oxidise organic

compounds to CO2, potentially creating an environment in which aggressive
CO2 is present. This is certainly possible in the lower level of water (the
hypolimnion) in lakes. In such environments, the decomposition of organic
matter leads to the dissolution of calcium carbonate in the sediment by
aggressive CO2, leading to the sediment becoming enriched in elements
such as iron [131].
Attack of concrete used in the gate structure of a shipway in Virginia,
USA is thought to be the result of combined sulphate attack and attack by
aggressive CO2 [132]. Samples taken from the water indicated high levels of
aggressive CO2 (up to 57 mg/l) and a pH as low as 6.9. Reference to Chapter
2 indicates that such a pH is sufficient to significantly increase the solubility
of calcium ions. Whether the high concentrations of CO2 are ascribable
to bacterial activity was not explored, but given that conditions in such
waterways may well be similar to those in natural lakes, it is reasonable to
suppose that this is a possibility.
In the discussion of organic acid formation by heterotrophic bacteria,
the study of bacteria isolated from a deteriorating sewer produced not only
organic acids, but also carbonic acid [126]. This was established as a result
of the formation of large quantities of calcium carbonate at the surface of
mortar specimens in contact with the bacteria. Whilst the concentration
of carbonic acid was not established, the low pH obtained (as low as 6.2)
suggested that this was partly the result of the presence of carbonic acid.
The influence of carbonic acid on the loss of mass from concrete is
shown in Figure 3.30. The figure shows the effect of periodic brushing of the
specimens in comparison to specimens which experienced no abrasion, with
mass loss from the abraded specimens being considerably higher. The results
126 Biodeterioration of Concrete

1.2.-------------------------------------------------,

• 100% PC unbrushed
o 50% PC /50% FA unbrushed
1.0 " 70% GGBS I 30% PC unbrushed
-v 100% PC brushed
• 50% PC I 50% FA brushed
::R 0.8 D 70% GGBS I 30% PC brushed
0

(/)-
(/)
0 0.6
...J
(/)
(/)
<t:
:2 0.4

0.2

0.0
0 20 40 60 80 100

TIME, days
Figure 3.30 Mass loss from concrete specimens exposed to a carbonic acid
solution with a pH of 4.1 [133].

from three different concrete mixes are shown—PC, and 50% PC/50% FA
and 70% GGBS/30% PC. The GGBS and FA mixes show greater resistance
to attack, with the GGBS mixes performing particularly well when abrasive
conditions are effective. It should be noted, however that rates of mass loss
are considerably smaller than for other acids discussed previously.

3.5.6 Long-term processes

Most of the issues relating to bacterial deterioration of concrete that have

been discussed in this chapter concern timescales pertinent to the built
environment—years to hundreds of years. However, in some applications,
persistence of integrity of cement-based materials over much longer periods
are necessary. The most obvious of these is in nuclear waste repositories
where low and intermediate level wastes may be encapsulated in a
cementitious matrix within a metal container for storage for periods of
many thousands of years. Moreover, the repository is normally backfilled
with cementitious material after the waste is deposited to create a high pH
environment that limits the likelihood of leaching of radionuclides.
Engineers involved in planning such activities clearly need to consider
potential damage to such ‘waste packets’ by bacterial activity, since even
small-scale bacterial activity over these timescales presents the potential
for significant deterioration.
 Bacterial Biodeterioration 127

From a nutrient perspective, low and intermediate level nuclear waste

will frequently contain organic carbon in the form of paper, wood and cotton
[134]. Whilst the compounds in these materials are not directly useable by
bacteria, they are able to produce enzymes which break the compounds
down to smaller molecules [135]. Additionally, the high pH conditions have
the potential to lead to alkaline hydrolysis of cellulose to form compounds
that can be metabolized by bacteria [134].
Another organic material that may be present in a nuclear waste
repository is bitumen, which is used in some cases as the encapsulation
medium. Leaching of this material has been found to release organic
compounds including alcohols, carbonyl compounds, glycols, aromatic
compounds, nitrogen compounds and carboxylic acids, which could
potentially be used by bacteria [136]. Moreover, exposure to radioactivity
can yield a greater quantity of these compounds in leachate [137]. Where the
leachant has a composition comparable to water which has been in contact
with Portland cement, levels of leaching are also higher. Bituminized waste
packages will also frequently contain a source of nitrogen in the form of
sodium nitrate, which is highly soluble [138].
A study has examined the effects of simulated leachate from a
bituminized waste package coming into contact with a CEM V cement
(containing a combination of Portland cement, slag and fly ash) [139].
It found that where the acids formed included oxalic acid, this was not
available for bacterial use, since it was rapidly rendered insoluble in contact
with cement (see Chapter 2). However, acetic acid and nitrate remained
available to bacteria.
For heterotrophic bacteria, the often relatively abundant presence of
carbon means that the limiting substrates in repositories are likely to be
inorganic substances used as sources of oxygen, such as sulphate and nitrate.
Since repositories are designed to wholly isolate their contents from the
external environment, they will be sealed to the external atmosphere. Thus,
oxygen will gradually be used up by aerobic bacteria, to be replaced with
CO2. Once oxygen is exhausted, bacteria will begin to reduce any available
nitrate to obtain oxygen, followed by sulphate when this is also used up
[140]. Analysis of gases in sealed containers in which such processes are
occurring indicates an accumulation of methane once sulphate is exhausted,
indicating a shift towards anaerobic respiration [134]. The resulting reducing
atmosphere is considered to be favourable for repository safety, since this not
only limits the extent to which corrosion of the steel containers can occur, but
also tends to significantly reduce the solubility of many radionuclides [141].
Models of the growth of bacteria in repository environments is able
to reproduce the features described above [134, 140], with one model
predicting bacterial activity for around 400 years after the repository is
completed [134]. On the basis that the majority of bacterial activity involves
128 Biodeterioration of Concrete

heterotrophic bacteria, the formation of organic acids presents the main

threat from bacteria in repositories.
Experiments where fluid was percolated through columns of simulated
intermediate level waste and crushed cement paste have found that bacterial
activity led to concentrations of between 0.07 and 0.7 mmol/l of acetic,
propionic and butyric acid [134]. Whilst these concentrations are relatively
low, it must be remembered that this does not mean that damage will not
be done. Taking the example of mixed organic acid solutions shown in
Figure 3.29, a deterioration depth of around 5.25 mm is obtained after 18
weeks. If a linear relationship between the depth of deterioration and time
is assumed, and the rate of movement of this front is assumed to be in
direct proportion to the concentration of acid, using a concentration of 0.7
mmol gives a depth of deterioration of around 15 mm after 400 years. This
represents a worst-case scenario, since it would be expected that bacterial
activity would fall off as sources of carbon, nitrate and sulphate become
scarcer. Nonetheless, it is clear that the use of cementitious backfill is a
prudent strategy to act as an additional protective measure.
Another area in which long-term deterioration by bacterial activity is a
potential problem is in the decommissioning of oilwells. The production of
hydrogen sulphide by sulphate reducing bacteria is frequently encountered
process in oilwells. The resulting ‘sour’ conditions are of concern because
they are associated with the corrosion of steel casings, etc. The corrosion
process has been proposed to be the result of the reaction [142]:
(x–1)Fe + Sy–1 ∙ S2− + 2H+ → (x–1)FeS + H2S + Sy–x
This reaction undoubtedly occurs because layers of iron sulphide can
be detected at steel surfaces. However, an alternative theory is that, for
corrosion to occur at the rates that can be observed, it is more likely that
elemental sulphur forms sulphuric acid. This can occur to some extent
through hydrolysis [143]:
S8 + 8H2O → 6H2S + 2H2SO4
However, research into the effect of adding nitrates to sour wells to
limit sulphide production has found that, despite limiting sulphide levels,
the rate of corrosion of steel increases considerably with nitrate addition
[144]. This suggests that bacterial activity is the cause of the corrosion.
Specifically, it implies that sulphur oxidising bacteria are utilising nitrate
as a source of oxygen, forming sulphuric acid which is the source of the
steel corrosion. Sulphur oxidising bacteria are undoubtedly present in such
environments [145].
The significance from a concrete (or more accurately, cement)
perspective relates to the plugging of oilwells once they have ceased to
be viably productive. Such plugging activities require sealing the well for
geological periods of time, as is the case for nuclear waste storage. The
 Bacterial Biodeterioration 129

material of choice for the plug (or ‘barrier’) is currently cement, although
other materials can potentially be used [146]. Whilst the process of cement
plug deterioration by sulfuric acid attack is one that is a potentially real one,
it should be put into perspective—multiple barriers with combined lengths
of 50 to 100 m are typical. Additionally, a source of oxygen is necessary to
produce sulphuric acid. This will normally derive from nitrate, but since
concentrations of nitrate in such environments are typically limited, it is
likely that sulphuric acid formation would only occur for relatively short
periods of time. Nonetheless, in the UK, evaluation of barrier materials
requires testing under conditions which mimic downhole conditions to
establish rates of deterioration, to allow prediction of performance [146].
The organic acids found in waters associated with oil formations are
characteristic of those produced by heterotrophic bacteria. However, their
origins are most probably related to the exposure of oil hydrocarbons to
elevated temperatures, rather than any biological degradation [147].

3.6 Limiting Bacterial Deterioration

When considering options available for protecting concrete from bacterial
biodeterioration, some measures are specific to the type of bacteria involved.
This is particularly true of attack by sulphur bacteria where behaviour of
concrete is, in some respects, different than for other forms of bacterial
attack. Moroeover, the processes which lead to attack of concrete by sulphur
bacteria may be addressed not only through engineering of the concrete, but
through manipulation of the environment in which the bacteria live. For this
reason, approaches to enhancing the durability of concrete are discussed
below, firstly in terms of those which are apply solely to attack by sulphur
bacteria, and, secondly, in terms of those which are generic.

3.6.1 Sulphur bacteria

Two different approaches are available with regards to limiting the damage
from sulphur bacteria. Firstly, the environment in which sulphur bacteria
are likely to establish themselves can potentially be engineered such that the
concentrations of substances which play a part in the deterioration process
are limited. Secondly, concrete itself can be designed and possibly treated
in such a manner that it possesses enhanced resistance.

Environmental control

The manner in which sewers are designed and operated can be used to limit
damage from sulphur bacteria. The removal of H2S before it comes into
contact with the sewer walls is clearly a desirable condition, and this can be
achieved through ventilation of the air space in the sewage pipe. Ventilation
130 Biodeterioration of Concrete

is normally achieved through natural ventilation processes which utilise

wind, air pressure differences within the system, and air movement resulting
from frictional drag from moving wastewater or suction resulting from a
fall in wastewater level [148]. Where issues of H2S accumulation are serious,
mechanical ventilation may also be employed. Ventilation has additional
beneficial side effects. Firstly it maintains high concentrations of oxygen in
the sewer atmosphere, which will be seen later to be beneficial. Secondly,
the movement of air will often have a drying effect on sewer walls, leading
to an environment low in moisture, thus creating conditions unfavourable
for colonisation by sulphur oxidising bacteria.
Since accumulation of H2S in the air-space above the waste water is a
key aspect of the attack process, one means of preventing this happening,
in theory, is to simply remove the airspace by running the sewer full [89].
This essentially entails the use of force mains, siphons and surcharged
sewers. However, the alternative viewpoint is that this will create anaerobic
conditions which promote sulphide formation by sulphate reducing
bacteria, and this approach is therefore not wholly ideal [149].
The alternative is to ensure that the conditions within the sewer are
sufficiently aerobic to limit the growth of sulphate reducing bacteria, and to
increase the redox potential, allowing for oxidation of sulphide to sulphate.
This is normally done by ensuring that the rate of flow is sufficiently high.
The rate of increase in the concentration of sulphide in a pipe which is
running partially full can be described using the empirical equation [150]:
d[S] M[EBOD] N[S](su)3 ⁄8
= –
dt r dm
where [S] = sulphide concentration (g/l);
M,N = empirical coefficients;
[EBOD] = effective biochemical oxygen demand (g/l);
r = the radius of the pipe (m);
u = stream velocity (m/s);
s = the slope of the pipe, expressed as a fraction; and
dm = hydraulic depth (m).
Biochemical oxygen demand (BOD) is the amount of oxygen needed
by aerobic microbes to break down the organic matter in a sample of
water, expressed as a mass of oxygen per volume of water. This is also
dependent on temperature, and so effective BOD (EBOD) includes a means
of describing the effect of temperature:
[EBOD] = [BOD](1.07)T – 20
where T is temperature expressed as ºC. A higher [EBOD] will increase
the rate at which oxygen is exhausted, thus creating anaerobic conditions
appropriate for sulphate reducing bacteria.
 Bacterial Biodeterioration 131

Thus, from the rate equation it is evident that increasing the velocity
of the stream and/or increasing the slope of the pipe reduces the rate of
development. This is believed to be for two reasons. Firstly, an increased
rate of flow will increase the rate at which re-aeration occurs, thus providing
adequate oxygen to satisfy the BOD and prevent aerobic conditions
establishing. Secondly, faster flow will tend to act to scour biofilms and
solids at the bottom of the pipe, limiting their development as a habitat for
sulphate reducing bacteria. However, some doubts have been expressed
with regards to how important this process is [148].
Where turbulence is created, re-aeration rates are further enhanced,
although it must be noted that where turbulence occurs and sulphide is
already present, this tends promote the release of H2S as gas [149].
Dissolved oxygen can be increased further by injecting air or pure
oxygen into the water, usually in features within a sewer system such as
pressurized mains pipes (‘force’ or ‘rising’ mains) or the sumps of gravity
flow sewers (wet wells) [152]. The biofilm (or slime layer) on the sewer wall
in which sulphate reducing bacteria reside will act as a barrier to oxygen.
However, increasing the dissolved oxygen concentration to above 0.5 mg/l
usually has the effect of increasing the depth of penetration sufficiently to
limit bacterial growth and allow sulphide oxidation [149]. The effect of air
injection is shown in Figure 3.31.
Given that this is the result of a process of oxygen diffusion through the
biofilm, increasing the concentration of dissolved oxygen in the water will

4 .-------------------------------------------------,
_.
I ~ Without air injection
With a1r injection
I
/
/
/
/

/
3 /
'"E /

/.
0, /
/
u.i /
0
/
LL
_J 2
::::J
(/) /
/
;::
_J
/
/
0 /
t- /
/
/
/
~ .........
..........

0~~~~-~-~-----~-~
----_-_-_-__-_-_-_&__-,-_-__-_-_-_-__-_-_~------------~
0 2 3 4

RETENTION TIME , hours

Figure 3.31 Effect of air injection on sulphide concentrations in a sewer
at different points long its length (expressed in terms of retention time
wastewater has spent in system) [152].
132 Biodeterioration of Concrete

accelerate the reaction, since it will create a greater concentration gradient.

Thus, there is some justification in using pure oxygen in comparison to
air, although given the considerable quantity of energy required to purify
oxygen from air, the environmental credentials of this are likely to be poor.
The rate at which sulphide is produced by bacteria can be reduced by the
addition of soluble nitrate salts (typically sodium nitrate) to wastewater. A
number of theories exist as to why this is effective. When nitrates are added
to wastewater, development of populations of chemolithotrophic bacteria
which oxidise sulphide whilst reducing nitrate is observed [153]. However,
the alternative explanation is that the addition of nitrate promotes the
growth of heterotrophic nitrate reducing bacteria which, whilst not having
a direct influence on sulphide production, compete with sulphate reducing
bacteria to limit its formation [154]. Regardless of which process occurs (and
it is conceivable that both occur simultaneously) nitrate is observed to be
reduced in preference to sulfate in oxygen-poor sewer environments [155].
Various oxidant compounds can potentially be introduced into the
sewer systems to oxidise hydrogen sulphide to sulphur. Hydrogen peroxide
(H2O2) is one of these compounds. It undergoes the reaction:
H2O2 + H2S → S + 2H2O
From the equation, the minimum molar ratio of H2O2 to H2S is required
to be 1, but may need to be as high as 5 to ensure thorough removal
[149]. Chlorine in the form of chlorine gas or a solution of a hypochlorite
compound can be used in a similar manner:
Cl2 + H2S → S + 2HCl
Despite a theoretical need for a molar ratio of Cl2 to H2S of 1.0, ratios
between 10 and 15 are required, which can make the cost excessive. Chlorine
is a hazardous gas, and the risk of leakage into the wider environment is
a valid concern which limits its use. Chlorine and hypochlorite also have
a biocidal effect. However, their ability to kill bacteria is limited by the
effectiveness of biofilms in protecting resident bacteria, again requiring high
concentrations to be effective. A number of instances of bacterial resistance
deriving from the presence of biofilms have been observed, albeit not for
sulphate reducing bacteria [151, 156, 157].
Another oxidant, potassium permanganate, has been identified as being
suitable. Under acidic conditions it takes part in the following reaction:
2KMnO4 + 3H2S → 3S + 2H2O + 2KOH + 2MnO2
Where conditions are alkaline, potassium sulphate (K2SO4) is also
formed. Potassium permanganate is relatively expensive and must be used
at KMnO4 to H2S ratios of 6–7, making it a less common practice [149].
In any sewer, a proportion of sulphide will be present in insoluble form
as metal sulphide salts. Metals which form insoluble sulphides include iron
 Bacterial Biodeterioration 133

(II), manganese, zinc, copper and nickel. One possible means of reducing
hydrogen sulphide formation is to add additional of soluble metal salts
to wastewater which will precipitate sulfide salts. The most appropriate
candidates are iron (II) salts, since they are relatively cheap and are likely
to have lesser environmental implications, due to their lower toxicity in
comparison to the other suitable metals.
The addition to wastewater leads to a reaction of the form:
Fe2+ + HS–  FeS + H+
However, addition of Fe (III) salts is as effective, if not more so
(Figure 3.32), since in wastewater Fe (III) will be reduced to Fe (II):
2Fe3+ + HS–  2Fe2+ + S0 + H+
Suitable soluble iron (II) compounds are iron (II) chloride (FeCl2),
iron (II) nitrate (Fe(NO3)) and iron (II) sulphate (FeSO4). Suitable iron (III)
compounds are the ferric equivalents: FeCl3, Fe(NO3)3 and Fe2(SO4)3.
It has already been shown that the formation of H2S requires acidic
conditions, and so raising the pH will limit its formation. A pH of greater
than 9 is required, since this will ensure that the vast majority of dissolved
sulphide is present as the dissociated HS– rather than H2S (Table 3.2).
This can be achieved through the addition of sodium hydroxide (NaOH).
Continuous dosing is prohibitively expensive [159], but an alternative
approach is to add large doses to achieve a pH of 12.5–13.0 for periods of

2.5

• Fe (II) addition

• •
0 Fe (Ill) addition

2.0
'§,
E
UJ-
0
.,
\
\
LL 1.5
•\•
<
...J
::J

.,
"'• ,.• •
(f)

..
0 \
LlJ
1.0 cP \
~ qCb 0
'
0
(f)
o<f o ••• • •
"••"" ,.•__
(f)
0 ~cg
0.5
or!} •
()<f3""8&
0

0 • • ~---- •
0.0
8 •
0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0

Fe:S RATIO
Figure 3.32 Effect of the addition of Fe (II) and Fe (III) salts on dissolved sulphide
concentrations in wastewater samples [158].
134 Biodeterioration of Concrete

less than an hour [160]. These conditions will reduce H2S formation, but also
have the effect of severely reducing the population of sulphate reducing
bacteria in a sewer system for a number of days, after which the process
must be repeated (see Figure 3.33). One problem with this approach is that
the high pH wastewater resulting from this ‘shock’ dosing cannot undergo
the microbial digestion treatments that sewage will normally undergo
downstream, and so must be diverted and dealt with by alternative means.
Magnesium hydroxide (Mg(OH)2) has been used in a similar manner
to sodium hydroxide [161]. It has the benefit of being ‘self-buffering’—it
maintains pH at around 9.5 to 10.0 despite perturbations in the form of
additions of acidic or alkaline substances. This is because magnesium
hydroxide precipitates as a solid at a pH above around 10.0. Thus, if the
pH begins to rise, magnesium hydroxide will precipitate, removing OH–
ions from solution and reducing the pH. As the pH falls below 10.0, solid
magnesium hydroxide dissolves again, maintaining the pH above 9.5. This
characteristic is beneficial in that it prolongs the high pH conditions, thus
extending the period over which dosing is effective.
Cleaning of sewer pipes has the potential to remove both the biofilms
and deposits of solids in which sulphate reducing bacteria reside, along with
sulphur oxidizing bacteria on pipe surfaces above the waterline. A range
of different techniques can be used, which are selected on the basis of the
size of the sewer, the extent to which deposits have established themselves,
and the types of deposits present [163].

~ 120
0

L.LJ- ::::~:::: pH10.5for05h

pH 10 5 f . ours
1- · or 6 hours
<{
100
0:::
z
0
i= 80
()
::::>
0
0
0::: 60
0...
L.LJ
0
u:::
_J 40
::::>
(/)
L.LJ
> 20
~
_J
L.LJ
0:::
0
-2 0 2 4 6 8 10 12

TIME , days
Figure 3.33 Reduction in sulphide levels following shock dosing of a pressure
main sewage system with sodium hydroxide for two different durations [162].
 Bacterial Biodeterioration 135

Hydraulic cleaning techniques employ the movement of water, and

include flushing, jetting and balling. Flushing simply involves increasing the
rate of flow through pipes for a short period of time. In the case of jetting,
a high power spray of water is directed at the pipe sides. Balling involves
pulling a ball that is slightly smaller than the pipe diameter through the
sewer whilst water is made to flow past it. This has the effect of causing the
ball to spin round within the confines of the pipe, scraping material from
the surface in the process.
Mechanical cleaning techniques include power rodding and winching.
Power rodding employs a drive unit to push a long flexible rod through the
pipe. At the end of the rod is a blade configuration which rotates to loosen
debris. Winching employs a bucket, which consists of a cylinder closed at
one end, with two opposing jaws at the other end which can be opened
up such that when pulled through a pipe they dislodge debris, which are
then collected in the bucket.
Cleaning is not only conducted to reduce bacterial attack, but is a more
general maintenance activity employed as a means of clearing blockages
and as a measure to prevent such blockages occurring. However, it has
been found to be effective in reducing the rate of sulphide oxidation.
The regularity with which cleaning must be conducted to be effective in
controlling bacterial deterioration will depend on the conditions within
a sewer. However, one study found that sulphide oxidation rates were
restored after only 10–20 days [164].
In the case of H2S production in oil wells, other chemicals can be
introduced into sewer systems which are specifically used for their ability
to kill bacteria—biocides. Two of the most commonly used biocides
are diamines, most commonly cocodiamine, and glutaraldehyde [165].
However, bacteria are well-known for their ability to rapidly adapt to
hostile conditions, and the development of resistance to diamine biocides
has been observed [166].
Molybate compounds (such as ammonium heptamolybdate,
(NH4)6Mo7O24.4H2O) and nitrite compounds (such as sodium nitrite,
NaNO2) act as metabolic suppressors to sulphate reducing bacteria [165,
167]. Research has identified that their use in combination is potentially the
most economic way of employing them (Figure 3.34) [165]. This approach
has also been successfully applied to reduce hydrogen sulphide emissions
from manure slurries arising from livestock production [168].
Research has also examined the possibility of using genetically-
engineered bacteria to produce biocidal substances to limit the growth of
sulphate reducing bacteria [169]. The bacteria used for this purpose were
strains of Bacillus subtilis which were genetically engineered to secrete the
peptide antimicrobials indolicidin and bactenecin. When introduced into
cultures containing the sulphate reducing bacteria Desulfovibrio vulgaris this
population was reduced by 83%.
136 Biodeterioration of Concrete

""'0
20 •0
Control
0.05 mmolll nitrite
E ~ 0.01 mmoll l molybdate
E \1 0.05 mmolll nitrite+ 0.01 mmolll molybdate
z-
0 15
i= ~--·---~-·-~--~~*-~~
,/..,
<(
0::: /e • // o ~
I- I I
z • I
w

.
u 10
I
I I
)f/
z d;
0
u
+I I
I J/
w I I ~
/
0 I I /
5 I
I
p /
l.L. ~__,.
....1 I
=>
(f) _.~/.:
I y ---~ / ~
~="Po~- :::r)V'k~ - I;Z. -v- -'V
0
0 100 200 300 400 500

TIME, hours
Figure 3.34 Effect of using nitrite and molybdate compounds on sulphide
formation by a pure culture of Desulfovibrio sp. sulphate reducing bacteria [165].

Material factors

Attack of concrete by sulphuric acid differs somewhat from other forms of

acid attack in that a low water/cement ratio is usually observed to increase
rates of mass loss. Whilst the superficial response to this is to conclude that
concrete exposed to attack by sulphur bacteria should have high water/
cement ratios, the reality is, in fact, more complex.
The first point to note on this issue is that there may be differences in
the behaviour of concrete exposed to sulphuric acid compared to concrete
exposed to sulphur bacteria. This is illustrated in Figure 3.10, where a lower
water/cement ratio appears to provide greater protection in experiments in
which the concrete specimen is held in an environment containing sulphur
bacteria. It should be noted that the concrete with the lower water/cement
ratio in these experiments also had a slightly lower cement content (see
discussion below). However, it is also conceivable that the lower ratio is
limiting the extent to which water is absorbed by the concrete surface,
providing a less habitable environment for sulphur oxidizing bacteria, as
discussed in Section 3.4.2.
The second point is that concrete mix design conventions typically
achieve lower water/cement ratios through an increase in cement content.
Thus, in most of the studies in which the effect of water/cement ratio has
been explored, the concrete mixes are, at least in part, more vulnerable
to attack from sulphuric acid as a result of a higher quantity of calcium
available to form ettringite, but more importantly gypsum. Thus, it is
 Bacterial Biodeterioration 137

likely that greater protection of concrete can be realised through the

reduction of water/cement ratio without increasing cement content. Water
in concrete plays two fundamental roles—it acts as one of the reactants in
the hydration of cement, and also as the fluid component which defines the
ease with which the fresh material flows and compacts (its ‘consistency’ or
‘workability’). If water/cement ratios are to be reduced without increasing
cement content, this means a reduction in water content which may reduce
the consistency of the mix unacceptably. Thus, it is likely that water-reducing
admixtures (plasticizers and super-plasticizers) are required to play a role in
achieving this goal. One study which has explored this approach found that
a reduced water/cement ratio with constant cement content did, indeed,
have greater resistance to attack from sulphuric acid [74].
There is, however, a strong incentive for taking the above approach a
step further by reducing rather than maintaining the cement content, since
it is the calcium in the cement matrix which is responsible for the formation
of gypsum. This also explains why the inclusion of siliceous fly ash and
silica fume as part of the cement are effective in reducing sulphuric acid
attack, and the inclusion of slag less so: siliceous fly ash and silica fume
have low calcium contents and will consequently reduce the total calcium
content of the cement.
The use of cement/asbestos fibre mixtures in place of concrete in sewer
pipes has been demonstrated to impart greater durability, although the
precise reason for this was not explored by the researchers [68]. Asbestos is a
hazardous substance and its use in new pipes is now uncommon. However,
the possibility of using other fibres is one that may be worth exploring.
Whilst the protection afforded to concrete by protective coatings is
essentially similar regardless of the nature of the acidic substances involved,
some surface protection technologies are specifically used in pipes, and so
should be discussed here. In many parts of the world, the use of concrete
sewer pipes with internal polymer linings has become common practice.
Pipe linings are typically made from polymeric materials. Historically, this
was normally PVC, but high density polyethylene (HDPE) is now commonly
used. Such liners can be incorporated in precast concrete pipes or installed
in cast in situ pipes.
The installation of liners inside pipes after construction can be achieved
through spray-on and brush-applied formulations. One such liner is
discussed in Section 3.6.2. Another means of installing liners in situ is
using rehabilitation systems in which a felt liner is saturated with epoxy
resin and fed into the pipe. A rubber bladder is then inserted into the liner
and inflated to stick the liner against the sides of the pipe. The bladder is
deflated and the liner left to cure.
An undesirable side-effect of the use of inert liners is that there is
no longer a ‘sink’ for hydrogen sulphide. Where exposed concrete is
undergoing attack from sulphur bacteria, sulphur is effectively being
138 Biodeterioration of Concrete

captured in the form of poorly-soluble gypsum. Thus, hydrogen sulphide

levels will be higher in liner sewer systems, leading to issues relating to
odour [164]. Whilst this is not reason enough to reject the use of liners, it
does mean that there may be additional costs associated with hydrogen
sulphide control.

3.6.2 General approaches to protection

Constituents and mix proportions

From the experimental results presented throughout this chapter, other

than for attack from sulphur bacteria, a reduced water-cement ratio
unambiguously enhances resistance to attack from acids deriving from
bacterial activity. Not only does a lower water/cement ratio reduce the rate
at which the acid-deteriorated front progresses into the material, but it also
limits the extent to which bacterial communities can establish themselves
on concrete surfaces. It is also probable that a higher residual strength
in the acid affected layer imparted by a lower water/cement ratio will
enhance resistance to abrasion where this type of deterioration is occurring
simultaneously to bacterial attack.
A lower cement content will also, generally, reduce susceptibility
to attack. It should be noted that cement possesses a finite capacity for
neutralizing acids. Thus, where a relatively small and finite quantity of acid
is present, a higher cement content may, in fact, provide greater resistance.
However, by its nature, this is an unlikely scenario in the case of bacterial
deterioration. It has also been proposed that the higher volume of aggregate
that arises from a lower cement content also enhances resistance to acid
attack in other ways. Firstly, because aggregate particles tend to act as
barriers to crack growth, their presence in higher number leads to a reduced
crack density in the acid-deteriorated layer [170]. Additionally, aggregate
acts as a restraint to shrinkage. It has been stated previously that cement
which has undergone decalcification is prone to shrinkage and cracking
resulting from this. Thus, the presence of aggregate acts to limit the extent
to which cracking occurs in this zone [103].
Aggregates containing carbonate minerals impart a significantly greater
neutralizing capacity in comparison to largely inert siliceous aggregate.
It has also been seen already that the inclusion of GGBS, fly ash and silica
fume will typically enhance resistance to acids whose main mode of attack
is acidolysis (which is the case for most of the main organic acids produced
by bacteria). Generally, GGBS provides the highest level of resistance and
silica fume the least. Other cementitious materials used in combination
with Portland cement appear to perform less well. High alumina cement is
also more resistant to attack from both nitric acid and organic acids formed
by bacteria. The hydration products of these cements are generally less
 Bacterial Biodeterioration 139

susceptible to acidolysis (specifically, there is an absence of portlandite),

and they also possess a higher acid-neutralization capacity.
Admixed biocides appear to be of limited value with regards to bacterial
colonisation. One study has examined the potential of including fibres
impregnated with the biocide Microban in concrete [171]. The study also
evaluated the performance of concrete containing zeolites containing silver
and copper whose antimicrobial characteristics are already established.
The concrete specimens were initially exposed to a hydrogen-sulphide
environment prior to being placed in a vessel containing sulphur oxidising
bacteria and nutrients. In both cases, there was very little improvement in
resistance against attack. Another study, using a biocide based on silver,
copper and zinc (presumably not in the form of zeolites) also found little
improvement in the performance of concrete specimens stored in a manhole
in a sewer for 17 months [69].

Coatings

Coatings are employed against bacterial degradation processes to prevent

acid coming into contact with the concrete surface, and subsequently
penetrating further into the pores of the material. For this reason, the most
appropriate surface coatings for these applications are organic pore-blocking
coatings [172]. These are formulations based on organic polymers which
are applied in a liquid form to the surface. The liquid subsequently sets
leaving a continuous solid barrier at the concrete surface. Such formulations
include epoxy resins (possibly modified with coal tar) and polyurethane,
polyester, vinyl and acrylic polymer materials.
Evaluation of coatings for concrete for protection against attack from
bacterial deterioration and acid attack in the literature is largely focused
on resistance to sulphur bacteria and sulphuric acid. However, since the
purpose of all such coatings is to prevent the penetration of acid below the
surface, performance can be expected to be similar for other acids.
Figure 3.35 is a bar chart showing the mass loss from concrete specimens
(taken from commercial sewer pipes) exposed to sulphur oxidising bacteria
after initially being exposed to an H2S-bearing environment [170]. The
surfaces of the specimens had previously been treated with a range of
surface treatments with potential to protect against sulphuric acid. These
were a cementitious coating, an epoxy coating and a sprayable polyurethane
lining intended for the treatment of internal sewer pipe surfaces created by
first spraying the surface with an epoxy primer, followed by hot-spraying
with polyurethane. Both the lining and epoxy coating performed extremely
well in isolating the concrete from the acid produced by the bacteria. The
cementitious coating, presumably because it was made from cement and
was, thus, essentially as vulnerable to attack as the concrete itself, performed
poorly.
140 Biodeterioration of Concrete

50

40

Ol
Cl) 30
(/)
0_J
(/)
(/)
<( 20
~

10

0 l l
-iy ~

Figure 3.35 Mass loss from samples taken from concrete sewer pipes, treated with
various surface coatings and exposed to sulphur oxidizing bacteria [170].

Measurements of the pH of concrete from the surface of concrete

from sewer pipes found much less reduction in pH where the surface had
been treated with epoxy resin, compared to untreated surfaces [173], thus
indicating protection was achieved. Visual evaluation of concrete surfaces
treated with epoxy and acrylic coatings and then exposed to a sulphuric
acid solution with a pH of –0.9 concluded that the acrylic surface treatment
was superior [174].
It should be stressed that the performance of surface treatments is
very much dependent on the quality of workmanship during application.
It is crucial that the concrete surface is in a condition suitable for receiving
a coating, and this may mean that surface preparation is necessary. This
may involve removal of surface contaminants and features deriving
from the concrete itself (specifically laitance and efflorescence) which
will compromise adhesion of the coating [172]. It may also involve filling
blow holes at the surface and roughening the surface to enhance the bond.
Roughening can be achieved through the use of wire brushes, impact tools,
power grinding, sand/grit blasting or acid etching [175]. A primer coat may
also require applying prior to the main coat or coats.
 Bacterial Biodeterioration 141

For most formulations, application is usually best carried out on a dry

surface, but it is also important to ensure that there is as little moisture
beneath the surface as possible, since this tends to compromise the bond
between the concrete and the coating. Coatings can be applied using
brushes, rollers and sprays.
Where abrasion of the concrete is also a possible deterioration
mechanism, inorganic inclusions may be mixed with the organic coating to
provide additional protection. This may include particles of sand or fibres.
The cracking of concrete, resulting from loads in service or expansion and
shrinkage resulting from wetting and drying, has the potential to severely
compromise the effectiveness of coatings, since the coating may also crack,
leaving an unprotected route into the interior of the concrete. Where this is
likely to be an issue, coating formulation containing an elastomeric polymer
(such as an elastomeric polyurethane) or fibres may provide some additional
resilience [175]. One study examining the resistance of epoxy coatings
reinforced with glass fibre mat found that, based on the performance of
concrete specimens exposed to a 3% sulphuric acid solution, the presence
of the coating was estimated to potentially extend service life by up to 70
times [176]. Where discontinuities (‘holidays’) in the coverage were present,
however, performance was severely compromised.
It must be noted that surface coatings have a finite lifetime. Once
a coating no longer performs its intended task it must be replaced. The
longevity of coatings depends on the material used and the aggressive
nature of the environment in which they are used, not just in terms of the
aggressive substances that the coating is being used to provide protection
against, but other factors such as UV exposure and the presence of oxidizing
materials.
Guidance on concrete coatings has suggested that surface coatings can,
if applied correctly, perform appropriately for periods of more than 15 years
[172]. However, this may be optimistic for more aggressive environments,
including many of those in which bacterial activity is present. A study
has evaluated the durability of elastomeric coatings applied to surfaces
of concrete—in which corrosion of steel had already initiated—to prevent
the ingress of moisture and, thus, further corrosion [177]. The coatings
were found to be effective for between 2 to 5 years. Whilst it should be
stressed that this study did not examine bacterial attack or acid attack, these
timescales still have relevance. One of the key mechanisms affecting the
durability of coatings is the bond to the concrete substrate. The strength of
this bond has been observed to decline with time in epoxy coatings, with
the main mode of failure being localized debonding of the coating from the
concrete (blistering), which can occur within two years of application [178].
Where longer-lasting protection is essential, the use of more substantial
polymer liners is a more appropriate solution.
142 Biodeterioration of Concrete

Polymer modification

Polymer modification of concrete involves the introduction of a polymer

resin at the mixing stage which forms a matrix intermingled with that of
a conventional inorganic cement. A number of different polymer types
have been evaluated in terms of the level of acid resistance imparted to
concrete, with varied results. For this reason, the findings of studies in this
area are summarized in Table 3.10. The criterion used to judge success is,
in most cases, mass loss. Beneficial effects have been observed for most
polymer types, with the exception of acrylic polymers. Undesirable effects
are sometimes also observed with polymer modification, including loss of

Table 3.10 Effectiveness of polymer modification with different polymer matrices.

Study
[179] [180] [181] [67] [182] [74]
Styrene acrylic ester SB: Lactic/ SB:
improved acetic improved
acid:
improved
Acrylic polymer SB: SB: SB:
worsened worsened worsened
NB:
improved
Styrene-butadiene SB: SB:
improved improved
Vinyl co-polymer SB: SB:
improved improved
Polysiloxane SB:
improved
NB:
improved
Polycarboxylate SB:
improved
NB:
improved
Polymethylmethacrylate H2SO4:
improved
Polystyrene H2SO4:
improved
Polyacrylonitrile H2SO4:
improved
Polyvinyl acetate H2SO4:
improved
SB = sulphur bacteria; NB = nitrifying bacteria
 Bacterial Biodeterioration 143

strength, primarily due to retardation of cement hydration by the polymer

[183].
Other studies have been conducted in which polymer modification
has been conducted in combination with the use of other mix constituents
intended to enhance acid resistance. This makes it difficult to evaluate which
material is most important in imparting greater resistance. In one instance,
fly ash and polyester resins were used in combination [184] yielding greater
resistance to sulphuric, acetic, formic and lactic acid. In another study,
concrete mixes made with a ‘hybrid modified’ formulation was evaluated
[185]. ‘Hybrid’ in this instance means a cement matrix made from Portland
cement, fly ash, polyvinyl acetate latex, and sodium silicate (Na2SiO3). In
addition, a smaller quantity of sodium fluorosilicate (Na2SiF6) was added
as a hardener for the sodium silicate. Deterioration was measured in terms
of loss of compressive strength after exposure to a periodically refreshed
solution of 1% sulphuric acid. The hybrid concrete performed better than
the control mix, which contained the same quantity of fly ash, but without
the latex and silicate compounds.

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Chapter 4

Fungal Biodeterioration

4.1 Introduction
Fungi are eukaryotes: their cells contain nuclei and organelles both of which
are enclosed by membranes. This configuration of the cell is the same as
that for plants and animals.
Species of fungi can live in terrestrial or aquatic environments,
including seawater. Superficially fungi may seem very like plants—both
include unicellular and multicellular organisms, lack motility, and many
of the multicellular forms share some common morphological features.
However, in taxonomic terms, fungi occupy a separate kingdom to these
other organisms. There are a number of reasons for this. Firstly, unlike plants,
fungi obtain energy and carbon from organic compounds in the surrounding
environment. Secondly, the cell walls of fungi contain the protein chitin, as
opposed to plants, whose cell walls are cellulose.
Fungi can take the form of both single and multicellular organisms.
It is useful to discuss these forms separately, as the differences have some
significance to some of the topics covered later in this chapter.

4.1.1 Yeasts

Yeasts differ from other forms of fungi both structurally and in their means
of reproducing. Yeasts are unicellular, like bacteria, meaning that they are
composed of single discrete cells. Unlike bacteria, yeasts are non-motile, and
so only move when fluids in which they are present are mobile. Some yeast
are capable of attaching to surfaces, which occurs through the formation of
adhesive compounds, including sugars, at the cell surface [1].
Like bacteria, some yeasts reproduce asexually through fission.
However, it is more common for asexual reproduction to occur via budding.
Budding involves the formation of a small bud on the side of the yeast cell.
Once formed, the nucleus of the cell also splits to form two identical nuclei
(mitosis), with one half migrating into the bud. The bud then separates
154 Biodeterioration of Concrete

from the parent cell. Yeasts can also reproduce sexually, usually when the
organism faces a shortage of nutrients or other stress.

4.1.2 Multicellular fungi

Multicellular fungi differ from yeast in that they grow in the form of a system
of tubes which are capable of growth and branch formation (see Figure 4.1).
These tubes are referred to as hyphae, with a grouping of hyphae referred
to as the mycelium. The hyphae are effectively cells, although fungi vary in
how these hyphae are structured—in some cases individual hyphae will
be structured as a single cell, whilst others are divided up into multiple
cells by walls known as septa. However, the septa are normally perforated,
allowing fluid and organelles to move between the partitions.
Beyond this definition of multicellular fungi, there exists a great deal of
structural variation, from relatively simple moulds and rusts, to complex
structures including mushrooms and puffballs. Thus, further discussion of
the physical form that such fungi take will be made as required in discussing
those which affect concrete.

4.1.3 Lichen

Lichen are composite organisms containing more than one species, where
what may be a symbiotic relationship exists. However, the mechanisms by

septum - - - --- - - - - -

septum - - - --- - - - - -

Figure 4.1 A schematic diagram of a fungal hypha.

 Fungal Biodeterioration 155

which lichen may affect a concrete substrate on which they are growing are
largely the result of the fungal component of this co-existence.
The organisms that constitute a lichen are fungi (the mycobiont) and
photosynthesising organisms (the photobiont)—either algae (Chapter 5) or
cyanobacteria (Chapter 3). Usually, there are two species involved, but in
some cases three may be present [2].
Lichen adopt many different growth types, including crustose (forming
relatively flat structures whose underside is fully attached to the substrate
on which they grow), foliose (leaf-like) and fruticose (hair-like, strap-shaped
or shrub-like) [3]. Of specific interest with regards to concrete durability
is the endolithic growth form where the lichen exists entirely beneath a
mineral surface [4].
The photobionts are embedded within the structure of the mycobiont,
either evenly distributed through the body of the lichen (the thallus), or
present as discrete layers.
The photobionts in lichens undergo photosynthesis and produce
organic compounds for the mycobiont to use as a source of energy and
carbon. It has been queried whether the relationship is truly symbiotic, on
the grounds that there does not appear to be much benefit to the photobiont.
Indeed, it has been suggested that the relationship is potentially parasitic in
nature [2]. However, it is thought that residing within the fungus provides
the photobiont with a degree of protection from extreme conditions that it
would otherwise not have [6].

4.2 Fungal Metabolism

Fungi are heterotrophs, meaning that they obtain carbon and energy from
organic compounds. The compounds that can be used by fungi must be
relatively small molecules to allow them to be absorbed through the cell or
hyphal wall. However, fungi are capable of breaking down large molecules
into smaller ones through the use of enzymes. Enzymes are either released
into the surrounding environment, or are present on the surface of the
hyphae.
Many fungi feed on plant matter, which contains a significant
proportion of cellulose and hemicellulose. These compounds are polymers
whose monomeric units are sugars. Enzymes are used to cleave the bonds
between the sugar molecules, and break down the polymers into smaller
and smaller fragments until molecules such as glucose and fructose remain,
which can be absorbed.
Fungi are either obligate aerobes (they require oxygen for the production
of energy) or facultative anaerobes (not requiring oxygen, but with the
means of using oxygen where it is available).
The anaerobic process will often yield ethanol, as in the ethanol
fermentation carried out by yeasts in the production of wine and beer.
156 Biodeterioration of Concrete

However, a range of other metabolites may be formed including citric,

oxalic, gluconic, succinic, formic and malic acids [5].
The aerobic process of metabolising organic molecules leads to the
formation of carbon dioxide and water:
C6H12O6 + 6O2 → 6CO2 + 6H2O
In the above example, D-glucose is the organic compound involved.

4.3 Growth of Fungal Organisms and Communities

4.3.1 Growth

When nutrients are plentiful, fungi undergo exponential growth of the

mycelium, which is referred to as the trophophase [6]. However, where a
nutrient becomes scarce, secondary metabolites are formed whose purpose is,
at least partly, to initiate processes which offer the possibility of resolving
the nutrient shortage in a number of different ways. These metabolites
can include organic acids—many similar to those formed by anaerobic
processes—antibiotics and enzymes. At this point the trophophase ends
and the idiophase begins. During the idiophase development of biomass
slows or stops and the manner in which the hyphae grow changes to one
in which the formation of new branches becomes dominant.
A wide range of organic acids are produced by fungi as secondary
metabolites, including oxalic, citric, gluconic and malic acid. Those with
greatest significance to concrete durability are citric and oxalic acid. There
are a number of theories as to why these acids are formed.
In the case of oxalic acid, it is believed that the main reason for its
production is to increase rates at which plant matter is broken down [7].
Oxalic acid production is normally stimulated by poor nutrient availability
with respect to carbon [8]. The source of nitrogen may also be important,
with at least one study finding oxalic acid production to be much higher
when nitrogen was present as nitrate ions rather than ammonium ions
[9]. Higher concentrations of calcium also tend to promote oxalic acid
production [10].
Oxalic acid assists in the breakdown of plant matter for a number
of reasons. Firstly, cellulose begins to break down at low pHs. Secondly,
many of the enzymes employed by fungi to break down cellulose are most
effective at low pHs. Thirdly, it would appear that oxalic acid plays a role in
promoting the Fenton reaction, in which Fe(III) is reduced to Fe(II), leading
to the production of hydroxyl radicals which are employed in attacking
cellulose through oxidative degradation.
When oxalic acid comes into contact with calcium ions, calcium oxalate
is precipitated. The formation of this compound by fungi can take two
forms. Firstly oxalic acid and calcium come together within the walls of
 Fungal Biodeterioration 157

the hyphae to form crystals of calcium oxalate which begin to protrude

through the walls into the external environment as they grow [11]. However,
oxalic acid may also be released in exudates produced at the surfaces of
the hyphae. In this case the acid molecules encounter calcium ions in the
external environment and crystals are precipitated. Where calcium oxalate
is produced from exudate, it has been proposed that this may act as a means
of disrupting calcium pectate, found in cell walls of plants, allowing access
to cell interiors [12].
Much of the understanding of citric acid formation by fungi arises from
the industrial exploitation of Aspergillus niger as a means of manufacturing
this compound. However, similar factors influence fungi in nature. Citric
acid formation is promoted when certain nutrients become scarce. Of
particular importance are nitrogen [13], phosphorus [14] and the metals iron,
manganese and zinc [15]. Citric acid is also produced in larger quantities
under pH conditions of less than 2 [16] and (within a bioreactor) at higher
dissolved oxygen concentrations [17].
It has been proposed that the drop in pH associated with citric acid
production creates a hostile environment for other organisms, meaning that
acid producing fungi may gain an advantage over other bacteria and fungi.
Many of the acids formed are also effective at forming strong complexes
with metal ions, allowing them to solubilize nutrients that would otherwise
be relatively insoluble, and break down mineral matrices to release trace
quantities of nutrients held within [7].
As discussed earlier, lichen consist of photobionts (which undergo
photosynthesis) and mycobionts which are heterotrophic fungi which obtain
energy and carbon from compounds formed by the photobionts. Where the
photobionts are algae, the compounds formed are the polyols, erythritol,
ribitol and sorbitol, depending on the genus involved [6]. Where they are
cyanobacteria, the compound is glucose. The process of photosynthesis
is discussed in further detail in Chapter 5, and the manner in which the
mycobiont in lichen processes these organic compounds is identical to
that for other fungi. Lichen commonly produce oxalic acid as a secondary
metabolite, but may also produce a range of other organic acids referred
to collectively as lichenic acids. These are discussed later in this chapter.
Fungi require water, which is required to achieve the turgor pressures
needed for hyphae to grow. In many cases fungi may be in contact with
plentiful supplies of water, but where they are exposed to the atmosphere,
approaches to retain water are adopted. Fungi living on surfaces frequently
produce extracellular polymeric substances (EPS), to form biofilms similar to
those of bacteria. These biofilms have good water retention capabilities [18].
In the case of fungi living in seawater, where the process of osmosis
would otherwise cause the movement of water out of the hyphae and
into the salt-rich water in the surrounding environment, this is done by
synthesising high concentrations of water-soluble compounds known as
158 Biodeterioration of Concrete

polyols such that the solute concentration in the hyphae is higher than that
in seawater [6]. Moulds which grow on salty or sugary foodstuffs, such as
Aspergillus eurotium and various species of Penicillium produce glycerol, or
absorb solutes from the surrounding environment to achieve the same effect.
Under desiccating conditions, lichens also produce polyols, which
replace water molecules associated with macromolecules in their cells. This
allows them to survive long periods of desiccation in a seemingly lifeless
state, which they recover from on re-hydration [6].
Fungi, being heterotrophic do not undergo photosynthesis. Whilst they
utilise light as a means of triggering processes such as spore production
and dispersal, exposure to UV radiation is usually detrimental. For this
reason, many fungi exist with much or all of their mycelium in the dark.
For fungi growing on exposed mineral surfaces, one solution is to produce
pigments such as melanins, carotenoids and mycosporines, which provide
protection from sunlight [18]. As we will see later, another option on at least
some mineral surfaces is to send hyphae beneath the surface.

4.3.2 Reproduction

During the idiophase, once the hyphae begin to encounter the edge of the
substrate on which they are growing the process of reproduction may begin.
This usually takes the form of the formation of spore forming organs, which
can take many different forms.
Fungi are capable of both asexual and sexual reproduction, with both
types of reproduction usually playing a role in their life-cycle. Asexual
reproduction can occur through mycelial fragmentation where a mycelium
splits into two separate mycelia which grow separately from each other—
vegetative growth. More commonly, asexual reproduction occurs through
spore formation. This involves the development of spore forming organs by
the fungus. Hyphae in these organs undergo mitosis, and one of the nuclei is
encapsulated within a spore which grows from the hyphae before detaching.
The spore is released into the wider environment and may be dispersed by
the wind, by flowing water, or on the surface or in the digestive system of
animals. Some spores are motile. Once the spore is in a location conducive
to growth it will begin to form hyphae and grow into a mycelium.
Sexual reproduction occurs when hyphae from two separate mycelia
undergo meiosis to form haploid cells (cells containing a set of unpaired
chromosomes) which become fused together. The process of fusion can take
many forms, but the product is a zygotic diploid cell (containing paired
chromosomes). This cell then undergoes meiosis to form spores, which
are released. The important aspect of the spore formation process from
the perspective of concrete durability is that spores can be travel widely,
meaning that any concrete surface in contact with air or water is potentially
capable of acquiring fungal occupants.
 Fungal Biodeterioration 159

The fungal component of lichen can reproduce asexually or sexually in

the same manner as other fungi. Where asexual reproduction is vegetative,
both divided parts will contain photobionts. However, where reproduction
occurs via spores, there is no transfer of photobiont to the new organisms.
Thus, the mycobiont must normally reacquire photobionts, a process which,
in most cases, appears to rely on chance confluence [19].

4.3.3 Fungal communities

Because enzymes are employed by fungi outside the organism, there is

limited control over their formation of absorbable nutrients. This can often
lead to the production of excess quantities. Sometimes, these nutrients
are exploited by other fungi or bacteria, leading to the development of
complex communities of microorganisms [20]. Conversely, some fungi
release antibiotic compounds which inhibit growth and reproduction of
other fungal species and other micro-organisms. Bacteria can also produce
similar compounds, with the purpose of suppressing fungal activity.

4.4 Concrete as a Habitat for Fungal Life

In discussing the type of habitat a concrete surface provides to fungi, there
exists some overlap between the requirements of fungus and those of bacteria.
Where this is the case, the reader is referred back to Chapter 3. However,
there are sufficient differences in these requirements to further discuss
them. Moreover, research has been carried out into fungal colonization of
concrete the results of which are useful in understanding how damage is
done to concrete and also how this damage might be prevented. The three
key aspects which require consideration remain the same—the issue of pH,
the abundance or scarcity of the various nutrients required by fungi, and
the physical habitat that a concrete surface provides fungi.

4.4.1 pH

Most fungi grow at optimal rates under slightly acidic conditions. This
is illustrated in Figure 4.2 which shows the magnitude of fungal growth
in soil samples from a strip of land over which a pH gradient exists. The
optimum conditions for growth in this case are at a pH of around 5. The
optimum pH for growth, however, is dependent on the substrate in which
the fungus is growing.
Some fungus display optimal growth rates under alkaline conditions.
Some coprinus species (Coprinus radiatus, Coprinus micaceus and Coprinus
ephemerus) and carbonicolous (‘coal-inhabiting’) fungi have been found
to display optimal growth rates under pH conditions close to 8 [22, 23].
160 Biodeterioration of Concrete

_j
0 30

.,..,,
0::
w
~ 25
0
~
!
I
~
I
w I I
I I

..
20
~
1~ ·~I
z- "Ol
0~
I
w
1--
:.c 15
I
I I e
\
~ ~ I \
oo.

..
I \
a. fl ' -t
a:: 10
0
()
z : ' •-----
w : 1 ...

~
()
<(
4 6 10

pH
Figure 4.2 Growth of fungi (measured as acetate incorporation into
ergosterol) in soil samples taken from a strip of soil in which a pH
gradient exists [21].

Figure 4.3 shows quantities of fungal biomass developed by a range of

ammonia fungi, plus some non-ammonia fungi over a wide pH range. Many
of the early-phase ammonia fungi display optimum growth between pH 7
and 8. Some of the species still display relatively large growth rates at pH
9, with indications that growth may still be observed at still higher values.
However, it should be stressed that, none of the fungal genera discussed
above or in Figure 4.3 have been observed to colonise concrete surfaces.
Indeed, it is likely that in most cases, as is the case for bacteria
(Chapter 3), carbonation of concrete is necessary before fungal colonization
can occur. This has been demonstrated in a study in which concrete
specimens were inoculated with three isolated fungal strains (Alternaria
alternata, a species from the Exophiala genus, and Coniosporium uncinatum)
and incubated at 26ºC [25]. The specimens were made from the same
white Portland cement paste, but one group had been ‘weathered’ through
exposure to a carbonating atmosphere, whilst another was carbonated
and then leached by de-ionised water. After 4 weeks, the non-weathered
specimens had only small patches of fungal growth, whilst the weathered
specimens showed considerably more advanced growth, with the
carbonated and leached specimens showing the highest coverage by fungi
(Figure 4.4).
Lichen are subdivided in terms of the pH of the substrate on which they
are capable of growing. Silicicolous fungi require acidic substrates, whilst
calcicolous grow on neutral to alkaline substrates [4].
 Fungal Biodeterioration 161

Ammonia fungi,
early stage species Ambfyosporium botrytis 202
§ 400 --o- Ambfyosporium botrytis 220
'g --.--
......
Peziza urinophifa 165
E -----v--- Pseudombrophifa petrakii 236
.s= Tephrocybe tesquorum 190
~ - ·- D - · - Tephrocybe tesquorum 212
0 300
0, - -+-- Coprinus echinosporus 179
-----<)---- Coprinus echinosporus 215
0 -~- Coprinus phlyctidosporus NA0554
E
0
--10.-- Coprinus phlyctidosporus NA0559
::!: 200
Ol
I--
I
<..9
sUi 100

~
0

o~~~~~~==~~~============~
Ammonia fungi,
late stage species Laccaria bico/or 252
§ 200 --o- Hebefoma vinosophylfum 110

...
:0
<lJ
--.-- Hebe/oma vinosophylfum 135
E -----v--- Hebeloma radicosoides 9
.s= Hebefoma radicosoides SL610603
~
e
0)
15o

0
E
0
::!: 100
Ol
r..:
I
~
w
s 50
>-
c::
0

Col/ybia dryophila 86
§ 200 - -o - Marasmius pulcherripes 77
:0 - -.-- Lyophylfum semita/e AK9301
<lJ
E -- ---v- - - Amanita rubescens SL401801
.s= • Sui/Ius luteus SL710802
~ 150
Ol
0
E
0
::!: 100
Ol
r..:
I
(9
Ui
s 50

~
0

2 4 5 6 7 8 9 10 11 12 13 14

pH

Figure 4.3 Biomass developed by a range of ammonia and non-ammonia

fungi in growth media with adjusted pH, incubated in the dark at 23°C
for 14 days [24].
162 Biodeterioration of Concrete

0 weeks

...
1 . 0
:.:" • It :
,. ~

Carbonated
and leached

Non-
weathered
control

lcrn

Figure 4.4 Effect of weathering (carbonation and leaching) on the development of Conosporium
unicinatum on white Portland cement paste prisms [25].

4.4.2 Nutrients

The heterotrophic nature of fungi and the normal absence of organic

compounds in concrete, means that these substances must usually come
from external sources. However, one study has identified that mould-release
agents used on the surface of concrete formwork can act as a source of
energy and carbon for fungi [37].
Fungi also require sources of nitrogen, phosphorous, potassium,
magnesium and sulphur. The presence of the first two nutrients in concrete
is normally significantly limited, whilst potassium and magnesium may be
present in slightly higher quantities. Sulphur is typically more abundant,
since it is present as gypsum in Portland cement. This aspect is discussed
in more detail in Section 3.4.3 of Chapter 3.
Regardless of availability, fungi possess the means of extracting these
nutrients. Phosphorous is normally present in soil and rocks as insoluble
minerals such as apatite. It has already been seen that one of the most
common secondary metabolites of fungi is oxalic acid, and this is capable
of releasing phosphate through the formation of the highly insoluble
compound calcium oxalate [26]:
Ca5(PO4)3OH + 5H2C2O4  5CaC2O4 + 3H2PO4 + H2O.
In the same way, sulphur can be solubilized in the form of sulphate [27]:
 Fungal Biodeterioration 163

CaSO4.2H2O + H2C2O4  CaC2O4 + H2SO4 + 2H2O.

In the above example, gypsum is the source of sulphate, but this
could also be a sulphate-containing cement hydration product such as
monosulphate or gypsum.
Nutrients required in very small quantities by fungi include calcium,
iron, copper, manganese, zinc and molybdenum. Whether these elements are
present in concrete is more dependent on individual sources of constituent
materials, but it is conceivable that all may be present in the relatively low
concentrations required by fungi. It is worth noting that low concentrations
of zinc, iron and particularly manganese appear to be stimuli for the
production of larger quantities of citric acid (see Section 4.2) [6]. All of these
metals form strong complexes with both oxalic acid and citric acid as shown
in Tables 4.1 and 4.2 for Zn, Cu, Mn and Mo, and in Chapter 2 for Fe and
Ca. Thus, fungi also have the means of releasing these micronutrients from
soil, rock and, indeed, concrete.
However, this does not provide the whole picture—both citrate and
oxalate salts of zinc, copper and manganese will be formed. Of these, the
citrate salts are the more soluble (‘slightly soluble’, meaning 1–10 g/l),
whilst the oxalate salts are considerably less soluble, although both sets of
compounds are much more soluble than the oxides, which are likely to be
the form present in soil and rock [31]. High concentrations of these metals
are likely to be toxic to fungi, and so it would appear that the release of
secondary metabolites provides a mechanism for controlling availability
to within ranges which are beneficial for the organism [26]. Compounds
formed by the reaction of molybdenum and oxalic and citric acids appear
to be more soluble [32], although this element is much less abundant in the
Earth’s crust, and so harmful concentrations are unlikely to arise.
The issue of calcium is a still more interesting one. Unlike the other
micronutrients, citric acid production is generally increased when calcium
levels increase in solution [33]. Calcium will react with citric acid to produce
calcium citrate. In calcareous rocks and concrete, the precipitation of calcium
citrate within pores will lead to cracking and fragmentation. This process
will be discussed in more detail later in this chapter, since it has serious
implications for concrete durability. However, the fact that citric acid
production is stimulated by the presence of calcium—and hence proximity
to calcium-bearing minerals—suggests that this is another means by which
fungi attempt to maintain levels of nutrients, since a cracked mineral surface
is likely to yield more nutrients than one which is intact.
As well as producing oxalic acid, lichen produce a number of relatively
large organic acid molecules which are members of the lichenic acids
(Table 4.3). These acids are highly effective at chelating metal ions such as
Al, Fe, Ca and Mg, and are likely to be produced as a means of breaking
down minerals to release nutrients [35].
164 Biodeterioration of Concrete

Table 4.1 Stability constants of complexes formed between citrate ions and fungal micronutrient
elements.

Complex Reaction Stability Reference

Constant

Zn Zn2+ + C6H5O73– ⇌ Zn(C6H5O7)– 6.21

Zn2+ + 2C6H5O73– ⇌ Zn(C6H5O7)24– 7.40

Zn + C6H5O7 + H ⇌ Zn(C6H6O7)
2+ 3– +
10.20

Zn2+ + C6H5O73– + 2H+ ⇌ Zn(C6H7O7)+ 12.84

[28]
Cu Cu2+ + C6H5O73– ⇌ Cu(C6H5O7)– 7.57

Cu2+ + 2C6H5O73– ⇌ Cu(C6H5O7)24– 8.90

Cu2+ + C6H5O73– + H+ ⇌ Cu(C6H6O7) 10.87

Cu2+ + C6H5O73– + 2H+ ⇌ Cu(C6H7O7)+ 13.23

2Cu2+ + 2C6H5O73– ⇌ 2Cu(C6H5O7)22– 16.20

Mn Mn + C6H5O7 ⇌ Mn(C6H5O7)
2+ 3– –
4.28

Mn2+ + C6H5O73– + H+ ⇌ Mn(C6H6O7) 9.60

Mo(VI) MoO42– + C6H5O73– + H+ ⇌ MoO4(C6H6O7)4– 8.35

MoO4 + C6H5O7 + 2H ⇌ MoO4(C6H7O7)

2– 3– + 3–
15.00

MoO42– + C6H5O73– + 3H+ ⇌ MoO4(C6H8O7)2– 19.62

MoO42– + C6H5O73–+ 4H+ ⇌ MoO3(OH)(C6H8O7)– 21.12

2MoO4 + 2C6H5O7 + 4H ⇌ Mo2O8(C6H7O )

2– 3– +
7 2
6–
31.02

2MoO42– + 2C6H5O73– + 5H+ ⇌ Mo2O8(C6H8O7)(C6H7O7)5– 35.86

2MoO42– + 2C6H5O73– + 6H+ ⇌ Mo2O8(C6H8O7)24– 40.08 [29]

MoO42– + 2C6H5O73– + 4H+ ⇌ MoO4(C6H7O7)24– 25.34

MoO4 + 2C6H5O7 + 5H ⇌ MoO4(C6H8O7)(C6H7O7)

2– 3– + 3–
29.54

MoO42– + 2C6H5O73– + 6H+ ⇌ MoO4(C6H8O7)22– 33.34

2MoO42– + C6H5O73– + 3H+ ⇌ Mo2O8(C6H8O7)4– 21.73

2MoO4 + C6H5O7 + 4H ⇌ Mo2O7(OH)(C6H8O7)

2– 3– + 3–
26.90

2MoO42– + C6H5O73– + 5H+ ⇌ Mo2O7(OH)2(C6H8O7)2– 31.53

4MoO42– + 2C6H5O73– + 9H+ ⇌ Mo4O13(OH)3(C6H8O7)25– 60.76

4MoO4 + 2C6H5O
2–
7
3–
+ 10H ⇌ Mo4O12(OH)4(C6H8O7)2
+ 4–
64.69

4MoO42– + 2C6H5O73– + 11H+ ⇌ Mo4O11(OH)5(C6H8O7)23– 77.45

 Fungal Biodeterioration 165

Table 4.2 Stability constants of complexes formed between oxalate ions and fungal micronutrient
elements.

Complex Reaction Stability Reference

Constant

Zn Zn2+ + C2O42– ⇌ Zn(C2O4) 3.88

Zn2+ + 2C2O42– ⇌ Zn(C2O4)22– 6.40

Zn + C2O4 + H ⇌ Zn(C2HO4)
2+ 2– + +
5.54

Zn2+ + 2C2O42– + 2H+ ⇌ Zn(C2HO4)+ 10.76

[28]
Cu Cu2+ + C2O42– ⇌ Cu(C2O4) 4.84

Cu2+ + 2C2O42– ⇌ Cu(C2O4)22– 9.21

Cu2+ + C2O42– + H+ ⇌ Cu(C2HO4)+ 6.31

Mn Mn2+ + C2O42– ⇌ Mn(C2O4) 9.98

Mn2+ + 2C2O42– ⇌ Mn(C2O4)22– 16.57

Mn2+ + 3C2O42– ⇌ Mn(C2O4)34– 18.42

Mo MoO42– + C2O42– + 2H+ ⇌ MoO4(C2H2O4)2– 13.62

[30]
2MoO 4
2–
+ 2C2O4 + 5H ⇌ Mo2O7(OH)(C2H2O4)2
2– + 3–
31.20

2MoO42– + 2C2O42– + 6H+ ⇌ Mo2O6(OH)2(C2H2O4)22– 34.08

4.4.3 Concrete as a physical habitat for fungi

The attachment of fungi to a surface, such as concrete, is usually assisted by

the production of EPS to act as an adhesive. There is some disagreement in
the literature with regards to the extent to which the roughness of a concrete
surface encourages colonization by fungi. One study found that greater
roughness was conducive to the growth of Cladosporium sphaerospermum
[36]. Another—examining the growth of mixed communities of fungi—
found little difference between surfaces of different roughness, although
only three surface finishes were examined, two of which were similar [37].
As for the case of bacteria (Chapter 3), there is a strong correlation
between the rate of colonization of a concrete surface and the water/cement
ratio of the material [37]. This is shown in Figure 4.5, which plots coverage
of mortar tiles by fungi versus this ratio. It is proposed by the authors of
the study from which these results come that this is possibly the result of
a higher surface area, higher moisture retention at the surface, or higher
nutrient availability. The study was also the one which found no relationship
between surface roughness and growth, which would appear to rule out
166 Biodeterioration of Concrete

Table 4.3 A selection of lichenic acids and some species which produce them [34].

Lichen Species

Parmelia stenophylla

Umbillicaria arctica

Parmelia furfuracea
Parmelia conspersa
Cladonia sylvatica
Baeomyces roseus

Caloplaca elegans
Cladonia furcata

Evernia vulpina
Acid
Baeomycic acid ͻ
O OH
O CH3
HO O
O
CH3 OH
H3C OH

Atranorin ͻ
O OH
O CH3
HO O CH3
O
CH3 O
H3C OH

Lecanoric acid ͻ
CH3
OH OH
O
HO O
O OH
CH3

Olivetoric acid ͻ
HO

OH
O
O
O
OH

H3C OH
H3C O

Gyrophoric acid ͻ
OH CH3 OH
HO O O O

O O OH
CH3 OH CH3

Table 4.3 contd. ...

 Fungal Biodeterioration 167

...Table 4.3 contd.

Salazinic acid ͻ ͻ
O
OH OH
O
O
O

CH3
HO O
O

HO

Fumarprotocetraric acid ͻ ͻ
O
O O CH3
O O
HO

HO
O OH

O CH3 O
OH

Lobaric acid
O O
O OH

H3C OH
O
O
O
CH3

CH3

Physodic acid ͻ
O
O
O OH

OH
O
O
HO
H3C

CH3

Vulpinic acid ͻ

OH

O
O

O O

CH3

Ursolic acid ͻ
H3C

H3C

H3C O

OH
CH3
CH3
HO
H3C CH3

Table 4.3 contd. ...

168 Biodeterioration of Concrete

...Table 4.3 contd.

Parietin ͻ
H3C
O

CH3
HO O
O
HO

Usnic acid ͻ ͻ ͻ
H3C
O
O
OH
H3C
H3C O

HO O

O CH3

100.----------------------------------------------.

-•- LaGrange
- -o- Savannah
--T- Atlanta
80
-v- Gainsville
(3
z
::>
_,
LL.
60
>-
co
L.LJ
<.?
<{
0:: 40
L.LJ
>
0
u
20

0 ~----.------------.------------,-----------.-----~

0.3 0.4 0.5 0.6

WATER I CEMENT RATIO

Figure 4.5 Fungal coverage of mortar tile specimens inoculated with fungal
isolates derived from concrete surfaces from various locations in Georgia,
USA. After inoculation, tiles were incubated in the dark with a source of
nutrition (potato dextrose broth) for 7 days [37].

the first of these. Given that nutrients are also in relatively short supply
in concrete itself, it is most probably the greater capacity of high water/
cement ratio to retain moisture that is most important.
Once attached, hyphae begin to grow over the concrete surface. The
hyphae are typically coated in a layer of EPS and exudates of secondary
metabolites, thus forming a biofilm (see Figure 4.6). At locations beneath
 Fungal Biodeterioration 169

the biofilm, cracking and spalling of the concrete is sometimes observed.

A scanning electron microscope image of a hypha is shown in Figure 4.7.
Hyphae typically have diameters between 4 and 6 μm [40], although
many of the fungi found growing on mineral surfaces have far narrower
hypha—around 1 μm [41]. Nonetheless, this means that their penetration
into the pores of pristine concrete is likely to be limited. Concrete can be
damaged by fungi through both biochemical and biomechanical processes.
Deterioration by biochemical means involves both the dissolution of the
concrete by acidolysis, but also potentially the mechanical damage of the
cement matrix through the precipitation of expansive salts (these processes
are discussed in more detail in subsequent sections).
Biomechanical mechanisms principally involve the penetration of
hyphae into mineral substrates. The mechanism by which this occurs is

Calcium
oxalate
crystals

Figure 4.6 Interaction of a hypha growing on the surface of concrete [38].

Figure 4.7 Cryo scanning electron microscope image of an Aspergillus niger

hypha growing on a quartz surface [39].
170 Biodeterioration of Concrete

through the widening of narrow openings through pressures exerted by the

hyphae. These pressures are achieved through the development of turgor
pressure of fluids within the hyphae through osmotic processes. Exerted
pressures deriving from hyphae of fungi (albeit species which grow on
plants) have been found to be around 0.05 MPa [42]. This is by no means
enough to fracture concrete, which typically has tensile strengths two
orders of magnitude higher. However, it is important to remember that the
macro-scale properties of concrete will not be uniformly found throughout
the material on a micro-scale. Moreover, the presence of melanin in the cell
walls imparts rigidity which allows a much greater pressure to be exerted.
One study, which used optical measurement techniques to determine force
exerted by hyphae tips of a melanin-forming fungus recorded forces which
were estimated to equate to an exerted pressure of 5.4 MPa [43]. It should be
noted that the species investigated in this study (Colletotrichum graminicola)
grows on plants, although most of the species known to inhabit concrete
surfaces also produce melanin.
It has also been proposed that EPS exuded by fungi into fissures between
mineral grains may act to physically damage mineral surfaces through
swelling resulting from the absorption of water [44].
Regardless, it is likely in the case of fungi which reside on mineral
surfaces that prior damage will have been done to the concrete via
biochemical attack. One study which measured chemical and physical
changes observed in Portland cement paste exposed to the culture fluid
in which either Aspergillus niger or a sterile fungal strain were growing,
used an experimental configuration which achieved this without physical
contact between the fungus and the cement [45]. This approach ruled out
any biophysical mechanisms of deterioration. In the case of Aspergillus
niger, oxalic and gluconic acid were formed, whilst gluconic and malic acids
were produced by the sterile fungal strain. The effect of exposure to the
culture fluid leached calcium and increased porosity, leading to a decrease
in flexural strength.
It should be noted that the presence of glucose in the culture would also
have aided in the leaching of calcium, and the absence of a control exposed
to the culture without fungi, means that the extent to which the organic acids
play a role is not entirely clear. Indeed, the use of glucose in contact with
cement and concrete in investigations of fungal deterioration is a common
feature of such studies, and the damaging effect of this compound on its
own must be borne in mind when interpreting results. Nonetheless, the
development of porosity in this way is likely to lead to both more accessible
routes into the cement matrix, and also a weaker matrix which offers more
opportunity to be further broken by biomechanical means.
Certainly, hyphae do penetrate the cement matrix (see Figure 4.8)
and there is strong evidence that the tips of hyphae can, under the right
conditions, sense the route forward that offers the least resistance [39].
 Fungal Biodeterioration 171

Figure 4.8 Fungal hyphae growing beneath the surface of a Portland

cement paste [25].

4.5 Fungi on Concrete

Many studies have been conducted to characterise fungal species found on
the surfaces of buildings, and it is likely that most of these species would
be capable of growing on concrete. However, subsequent discussion will
be limited to fungi which have been confirmed as being capable of growing
on concrete surfaces. Table 4.4 lists fungi which have been successfully
grown on concrete or cement paste surfaces in the laboratory, or identified
on concrete in the field.
Another study has conducted more general characterisation of the
biomass growing on the external walls of buildings in Europe and Latin
America [46]. Whilst specific genera were not identified, fungi were found
to make up the second most frequently encountered organism on buildings
in Latin America (after cyanobacteria), but was the least common in Europe.
The influence of the type of substrate was also examined: composite
materials (in this context cement, mortar concrete and brick) tended to
contain fungal life less frequently than they did algae or cyanobacteria,
which was also the case for stone buildings. Painted surfaces were most
frequently occupied by fungi.
Characterisation of fungi growing on the original concrete shield
around the damaged nuclear reactor in Chernobyl in Ukraine has also
been conducted [47]. These are listed separately from the other studies in
Table 4.5. This is partly because it is not clear whether all samples were
172 Biodeterioration of Concrete

Table 4.4 Fungi identified growing on concrete surfaces, or successfully grown in the laboratory.

Genera Species Fungi Division/ Location References

Class
Alternaria – Ascomycota/ Georgia, USA/São [48, 59]
Dothideomycetes Paulo, Brazil
alternata Laboratory [25, 38, 54]
Aspergillus – Ascomycota/ São Paulo, Brazil [59]
Eurotiomycetes
glaucus UK [49]
candidus Moscow, Russia [53]
flavus Moscow, Russia [53]
flavipes Laboratory [38, 54]
fumigatus Moscow, Russia [53]
niger Laboratory/ [50, 45, 38,
Moscow, Russia 53, 54, 56,
57]
ochraceus Moscow, Russia [53]

repens Laboratory [38, 54]

terreus Moscow, Russia [53]
versicolor Laboratory [38, 51, 53,
54]
Aureobasidium – Ascomycota/ São Paulo, Brazil [59]
Dothideomycetes

pullulans Moscow, Russia [53]

Chaetophoma – Ascomycota/– São Paulo, Brazil [59]
Cladosporium – São Paulo, Brazil/ [57a, 59]
Rio Grande, Brazil/
Pirassununga,
Brazil
cladosporiodes Georgia, USA [48, 38, 53,
54]
sphaerospermum Laboratory/Daegu, [36, 55, 56]
South Korea
Coniosporium uncinatum Ascomycota/ Laboratory [25]
Eurotiomycetes
Curvularia – Ascomycota/ São Paulo, Brazil [59]
Euascomycetes

Table 4.4 contd. ...

 Fungal Biodeterioration 173

...Table 4.4 contd.

Genera Species Fungi Division/ Location References

Class
Epicoccum nigrum Ascomycota/ Georgia, USA [48]
Dothideomycetes
Exophiala – Ascomycota/ Laboratory [25]
Chaetothyriomycetes
Fonsecaea – Ascomycota/ São Paulo, Brazil [59]
Eurotiomycetes
Fusarium – Ascomycota/ Laboratory [52]
Sordariomycetes
– Georgia, USA [48]
Geotrichum candidum Ascomycota/ Moscow, Russia [53]
Saccharomycetes
Helminthosporium – Ascomycota/ São Paulo, Brazil [59]
Dothideomycetes
Monilinia – Ascomycota/ São Paulo, Brazil [59]
Ascomycetes
Mucor – Zygomycota/ Georgia, USA [48]
Mucormycotina
Nigrospora – Ascomycota/ São Paulo, Brazil [59]
Saccharomycetes
Paecilomyces lilacinus Ascomycota/ Laboratory [38, 54]
Eurotiomycetes
varioti Moscow, Russia [53]
Penicillium – Ascomycota/ UK [49]
Eurotiomycetes
brevicompactum Moscow, Russia [53]
chrysogenum Laboratory [51]
expansum Moscow, Russia [53]
funiculosum Moscow, Russia [53]
islandicum Moscow, Russia [53]
oxalicum Georgia, USA [48]
purpurogenum Moscow, Russia [53]
spinulosum Moscow, Russia [53]
vermiculatum Moscow, Russia [53]
Pestalotia/ – Ascomycota/ São Paulo, Brazil [59]
Pestalopsis Sordariomycetes

Table 4.4 contd. ...

174 Biodeterioration of Concrete

...Table 4.4 contd.

Genera Species Fungi Division/ Location References
Class

Pestalotiopsis maculans Ascomycota/ Georgia, USA [48]

Sordariomycetes
Phoma – Ascomycota/ São Paulo, Brazil [59]
Dothideomycetes
Rhizopus oryzae Zygomycota/ Moscow, Russia [53]
Mucormycotina
nigricans Moscow, Russia [53]
Scytalidium – Ascomycota/ São Paulo, Brazil [58]
Leotiomycetes
Trichoderma – Ascomycota/ São Paulo, Brazil [59]
Pezizomycotina
asperellum Georgia, USA [48]
viride Moscow, Russia [53]
Trichothecium roseum Ascomycota/ Moscow, Russia [53]
Pezizomycotina

Table 4.5 Fungi identified growing on the concrete shield around the damaged reactor at
Chernobyl [47].

Genera Species Genera Species Genera Species

Acremonium strictum Chaetomium globosum Mucor plumbeus
Alternaria alternata Chrysosporium pannorum Paecilomyces variotii*
Aspergillus flavus* Cladosporium cladosporioides Penicillium chrysogenum

fresnii* herbarum citrinum*

fumigatos sphaerospermum hirsutum

niger other species hordei

ochraceus* Doratomyces stemonitis ingelheimense

ustus* Fusarium merismoides Phialophora melinii*

Aureobasidium pullulans oxysporum Stachybotrys chartarum

versicolor solani Sydowia polyspora

Beauveria bassiana* Geotrichum candidum* Ulocladium botrytis

Botyritis cinerea other species

* Only found in locations where severe contamination with radioactive material had occurred
(radiation levels = 40–220 mR/h).
 Fungal Biodeterioration 175

all taken from concrete surfaces, and also because the researchers found
different species in different proportions where levels of radiation were
higher, presenting the possibility that the findings are not representative
of a more conventional environment. Indeed, it would appear that some
melanin-forming fungi are capable of radiotrophism—obtaining energy
from ionizing radiation [60].
Nonetheless, many of the fungi identified from the Chernobyl site are
also seen in Table 4.4.
In addition to the genera and species listed in Tables 4.4 and 4.5, a
number of sterile mycelia growing on concrete surfaces have been identified
in characterization studies [45, 47, 51]. Sterile mycelia ‘mycelia sterilia’ are
fungi that do not produce spores, meaning that characterising them in
taxonomic terms is problematic.
Another fungus worthy of mention is Serpula lacrymans, or ‘dry rot’.
This is a fungus which lives and feeds on wood and can potentially cause
severe damage to timber and timber-frame structures. However, it appears
to benefit considerably from growth in close proximity to sources of calcium
and iron. These sources can potentially include mortar, calcium silicate
bricks, mineral insulation wools and concrete [61]. Where such a source
exists, the fungus will grow over the surface of the calcium source, with
possible growth of hyphae beneath the surface of the material [62]. The
calcium is taken up by the hyphae, translocated through the mycelium and
subsequently used in the formation of calcium oxalate through reaction
with oxalic acid. As discussed previously, the acid is formed with the main
objective of assisting in the breakdown of cellulose in the timber. However,
it would appear that calcium oxalate formation is also used as a means of
buffering the pH within a range which is optimal for the organism [63].
The white rot fungus Resinicium bicolor has also been found to use calcium
translocation in a similar manner [64].
Whilst no evidence can be found in the literature of damage to concrete
by Serpula lacrymans, the fact that calcium is removed from its source means
that it is likely that damage of some magnitude occurs.
Table 4.6 lists species of lichen identified growing on concrete and
mortar by researchers.

4.6 Chemical Deterioration from Fungal Activity

In the discussion of both the metabolism of fungi and nutrients available
in concrete earlier in this chapter, it has become clear that oxalic and citric
acid play an important role in the chemical interaction of concrete and
fungi. Table 4.7 shows the results of a study examining the organic acids
produced by filamentous fungi, having isolated the results which per-
tain to species found in Tables 4.4 and 4.5. The table identifies instances
in which each acid was produced by the various strains of each species
 Table 4.6 Lichen found growing on concrete and mortar surfaces.

Genera Species Growth Form Reference Genera Species Growth Form Reference
Acarospora cervina Crustose [67] erysibe Crustose [65, 66]
murorum Crustose [66] turicensis Crustose [68]
subcastanea Crustose [66] Lecanora albescens Crustose [65, 66, 69]
Aspicilia Contorta ssp. Crustose [68] dispersa Crustose [66, 67, 70]
hoffmanniana
Caloplaca sp. – [70]
176 Biodeterioration of Concrete

aurantia Crustose [68] muralis Crustose [66, 70]

cinnabarina Crustose [66, 71] pruinosa Crustose [68]
citrina Crustose [65, 66, 69, 71] umbrina Crustose [66]
Lecidea sp. – [70]
erythrantha Crustose [65, 66] Lepraria sp. Crustose [68]
erythrocarpa Crustose [68] lesdainii Crustose [68]
flavescens Crustose [66] nivalis Crustose [68]
holocarpa Crustose [65, 66, 67] Leproplaca xantholyta Crustose [68]
lactea Crustose [68] Opegrapha calcarea Crustose [68]
teicholyta Crustose [66, 67] Phaeophyscia chloantha Foliose [66]
variabilis Crustose [68] Physcia undulate Foliose [66]
 velana Crustose [68] dubia Foliose [67]
Candelaria concolor Foliose [66] Protoblastenia sp. – [70]
Pseudosagedia linearis Crustose [68]
Candelariella aurella Crustose [66, 67, 70] Punctelia constantimontium Foliose [66]
vitellina Crustose [67] subpraesignis Foliose [66]
Catapyrenium squamulosum Squamulose [68] Pyxine berteroana Foliose [66]
Catillaria lenticularis Crustose [66] Rinodina bischoffii Crustose [68, 70]
Clauzadea immersa Crustose [68] Sarcogyne orbicularis Crustose [66]
Collema auriforme Foliose [68] regularis Crustose [68]
crispum Foliose [68] Squamarina concrescens Squamulose [68]
tenax Foliose [68] Staurothele frustulenta Crustose [65, 66]
Dirina massiliensis Crustose [68] monosporoides Crustose [66]
Dirinaria picta Foliose [66] Teloschistes chrysophthalmus Fruticose [66]
Endocarpon pusillum Squamulose [68] Toninia aromatica Crustose [68]
Fulgensia sp. Crustose [68] Verrucaria hochstetteri Crustose [68]
Heterodermia speciosa Foliose [66, 71] macrostoma Crustose [68]
Hyperphyscia coralloides Foliose [66] muralis Crustose [68]
syncolla Foliose [66, 69] nigrescens Crustose [67]

Table 4.6 contd. ...

Fungal Biodeterioration 177
...Table 4.6 contd.

Genera Species Growth Form Reference Genera Species Growth Form Reference
variabilis Foliose [66] viridula Crustose [68]
Xanthoria sp. – [70]
viridissima Foliose [66] candelaria Foliose [66]
Lecania sp. Crustose [66] fallax Foliose [66, 69]
cuprea Crustose [68] parietina Foliose [65, 66]
178 Biodeterioration of Concrete

erysibe Crustose [65, 66] erysibe Crustose [65, 66]

turicensis Crustose [68] turicensis Crustose [68]
 Fungal Biodeterioration 179

Table 4.7 Instances of organic acid production produced by various strains of fungi of the
genera Aspergillus [5].

Acid

Isobutyric

Propionic
Gluconic
Ascorbic

Succinic
Fumaric

Itaconic

Tartaric
Butyric

Formic

Oxalic
Acetic

Lactic

Malic
Citric
Species
Aspergillus flavus 1 1 2
(2 strains)
Aspergillus flavipes 1 1 1 1 1 1
(1 strain)
Aspergillus niger 1 4 7 11 1 8 3 1 10 14 1 9 9
(14 strains)
Aspergillus terreus 1 1 1 1
(1 strain)

studied. It should be noted that more strains of Aspergillus niger were ex-
amined in the study than any of the other species. After oxalic and citric
acids, malic, succinic, tartaric and gluconic and butyric acids are the most
commonly encountered. The action of butyric acid on concrete has already
been examined in Chapter 3. However, the remaining compounds are dis-
cussed below.
4.6.1 Oxalic acid
The action of oxalic acid on concrete is somewhat unusual, in that it can
often have a protective effect. The reason for this can be seen in the solubility
diagrams for oxalic acid in Chapter 2: in the case of Ca, Al and Fe, a solid
phase is precipitated which persists to very low pH values. These solid
phases are calcium oxalate monohydrate (Ca(C2O4).H2O—whewellite),
aluminium trioxalate tetrahydrate (Al2(C2O4)3.4H2O) and iron (III) trioxalate
pentahydrate (Fe2(C2O4)3.5H2O).
We have seen in Chapter 3 that in the case of sulphuric acid attack the
precipitation of salts resulting from reactions between acids and cement
can be problematic if the molar volume of the precipitate is significantly
greater than that of the hydration products it replaces. Whilst the molar
volume of the iron and aluminium salts is not known, the molar volume of
whewellite is 63.8 cm3/mol. Whilst this is larger than the molar volume of
the portlandite that it will replace (32.9 cm3/mol) it is less than the molar
volume of gypsum (74.5 cm3/mol). Thus, rather than cause expansion
cracking in the manner of gypsum, a protective outer layer forms which
prevent further ingress of acid. Thus, mass loss from cement paste and
concrete specimens stored in solutions of oxalic acid is virtually zero [72].
180 Biodeterioration of Concrete

The protective effect obtained by bringing oxalic acid in contact with a

concrete surface is well-known, and there are a number of surface treatments
available which are based on oxalic acid. This poses an important question:
why can fungi cause damage to concrete seemingly, at least in part, through
the release of an acid that would normally protect it.
There are, in fact, two forms of calcium oxalate which are commonly
encountered in nature: whewellite and weddellite (Ca(C 2O4).2H2O). In
experiments where cement paste or concrete are exposed to relatively strong
solutions of oxalic acid, whewellite is formed exclusively [73]. However,
weddellite is encountered in other circumstances. Weddellite has a higher
molar volume than whewellite: 79.2 cm3/mol. This is also higher than that
of gypsum, and it is therefore possible that its precipitation may have the
potential to cause damage to concrete.
As discussed earlier, the formation of calcium oxalate by fungi occurs
either in the hyphal walls or outside the hyphae. When produced within
the organism, it would appear that lichen can control whether whewellite
or weddellite are formed, with most species producing weddellite [74]. This
ability to control calcium oxalate precipitation is likely to be possessed by
fungi more generally. However, outside of the organism it must be presumed
that fungi have much less control over calcium oxalate precipitation. Instead,
the form precipitated is dependent on the ratio of calcium to oxalate, with
a lower ratio favouring whewellite [68]. Presumably as a result of this,
silicicolous lichens growing on calcareous surfaces tend to form weddellite,
whereas they produce whewellite on siliceous surfaces [75].
Therefore, it is conceivable that damage can be done to concrete through
the release of oxalic acid as long as the concentrations are sufficiently low
that weddelite is formed. In support of this, crystals of calcium oxalate
formed on concrete showing clear signs of expansion and deterioration
under the action of Aspergillus niger were found to be composed of a mixture
of the two calcium oxalate forms [38].
It should also be stressed that the protective effect of whewellite
formation is observed when the entire surface of cement paste and concrete
are exposed to a solution in which the concentration of oxalic acid is
uniform. It is possible that the introduction of oxalic acid to localised
points—as would be the case where fungi occupy the surface—may have
a damaging effect. For instance, a small droplet of oxalic acid solution
coming into contact with the surface of a large crystal of portlandite might
create sufficient localized stress in the crystal lattice as a result of whewellite
formation to cause it to fracture.

4.6.2 Citric acid

The nature of the interaction of citric acid with concrete is a process with
the potential to cause significant damage. The reason for this has little to do
 Fungal Biodeterioration 181

with its strength as an acid, which is relatively low (see Chapter 2). Instead,
the main process of deterioration is the precipitation of calcium citrate
tetrahydrate (Ca3(C6H5O7)2.4H2O). There are, in fact, two different forms
of this salt—the mineral earlandite [76] and a form which is obtained by
precipitation through combining solutions of calcium and citric acid [77].
It is the second form which appears to form when citric acid comes into
contact with cement [78].
Earlandite has a molar volume of 285 cm3/mol, and it is likely that
the molar volume of the second form is similar. This is more than 8 times
greater than portlandite (see Chapter 2). The effect—at least at higher
concentrations—is an extremely rapid and progressive fragmentation at
the concrete surface [73]. Figure 4.9 shows the development of deteriorated
depth in Portland cement paste specimens with time for citric acid and
oxalic acid. Results for acetic acid are also shown to provide comparison
with an organic acid that largely causes deterioration by acidolyisis. The
rate of deterioration in the case of citric acid is substantially greater than
acetic acid.
Figure 4.10 shows mass loss from cement pastes of various types
at two different citric acid concentrations, whilst Figure 4.11 shows
micro-CT cross-sections through the specimens. In the case of the higher
concentration of citric acid, the greatest resistance is displayed by the
calcium sulphoaluminate cement, and the least by the PC/fly ash blend.
This is also reflected in the CT scans, where the dramatic disintegration

14
• Oxalic acid
0
12 I 0 Citric acid
I 'I' Acetic acid
E I
E I
I - 10 6
1- I
a.. I
w I
I
0 8
--------.,--
"'--
--
0 0
w I
1- I - ~- -
<( 6 I
0:: I /
/

0 I /
/~
ii 9/
w 4 /
1- I ,,/ 'I'
w I
0 I y/
2
9/
I/
0
0 100
- -
200 300
TIME, days

Figure 4.9 Development of degraded depth in cylindrical Portland cement

paste specimens exposed to 0.1 M solutions of oxalic, citric and acetic acids [72].
182 Biodeterioration of Concrete

100

90

C/)
C/)
Cll
E 80
Cll
>-
""0

4-- 70
0
~
0

(/)
(/) 60 e PC, 0.10M
<(
0 65% PC /35% FA, 0.1 0 M
2:
"' GSA, 0.10 M
\1 PC, 0.01 M
50
• 65% PC /35% FA, 0.01 M
o GSA, 0.01 M

40
0 20 40 60 80 100

TIME, days
Figure 4.10 Loss of mass of cement paste specimens (water/cement ratio = 0.5) made from
Portland cement (PC), a PC/FA blend, and a calcium sulphoaluminate cement exposed to
0.10 and 0.01 M solutions of citric acid [78] plus other data.

Figure 4.11 micro-CT scans showing cross-sections through cement paste specimens
exposed to a 0.1 M solution of citric acid for 14 days [78].

of the cement is evident, with the fly ash specimen having undergone the
greatest loss of material. The reason for this is most probably related to the
relative abilities of the materials to resist fragmentation. All of the cement
pastes had the same water/cement ratios, and so it is likely that the PC/
FA blend would have the lowest strength, thus making it most susceptible
to fragmentation. Since fragmentation will uncover the unaffected cement
within, allowing it to be attacked, deterioration will progress at a faster rate.
 Fungal Biodeterioration 183

At lower concentrations, different results are observed: the PC paste

lost the least amount of mass, with the CSA specimen performing poorly.
pH measurements of the exposure solutions indicate that the reason for the
high performance of PC at lower concentrations is the result of the acid in
the exposure solution being completely neutralized by the cement. This did
not occur in the case of the PC/FA and CSA pastes. In the case of FA, this
is simply because the paste contains less portlandite as a result of dilution
by the fly ash and consumption during the pozzolanic reaction. However,
in the case of CSA, neutralization had not yet occurred.
The process of neutralisation demonstrated using geochemical
modelling is shown in Figure 4.12 for typical PC, 65% PC/35% FA blend
and CSA compositions. It is evident that neutralization (indicated by a rapid
increase in pH) occurs rapidly for the PC paste, takes longer for the FA
blend and even longer for CSA. The neutralisation reaction where Portland
cement is involved follows the reaction:
Ca(OH)2 + 2H+ → Ca2+ + 2H2O
Whereas calcium aluminate cements follow two reactions, the first of
which is:
Ca3Al2O9.6H2O + 6H+ → 3Ca2+ + 2Al(OH)3 + 6H2O
Thus, an outer layer of Al(OH)3 —gibbsite, albeit very poorly crystalline
—develops at the surface of the cement. As more acid enters the cement
matrix the pH drops further, at which point the Al(OH)2 starts to decompose:
2Al(OH)3 + 6H+ → 2Al3+ + 6H2O
Where a calcium sulphoaluminate cement is involved the first reaction
will be somewhat different, since it will involve aluminate hydration
products such as ettringite:
Ca6Al2(SO4)3(OH)12∙26H2O + 6H+ → 6Ca2+ + 2Al(OH)3 + 3SO42– + 32H2O
but the second reaction remains the same. The nature of the calcium
aluminate cement reactions means that the material has a higher acid
neutralization capacity than PC. However, the second reaction will only
occur to any significant degree if the pH of the solution in contact with
the Al(OH)3 phase falls below around 5—this is because gibbsite remains
effectively insoluble above this point, as seen in the solubility diagrams in
Chapter 2.
The two-stage reaction of calcium aluminate cements means that
the second neutralisation event is delayed. This delay can potentially be
indefinite: where a weak acid or very dilute acid solution is present, the
Al(OH)3 layer at the surface may never dissolve, because the pH remains
too high. Regardless, the delay gives the acidic species an opportunity
to diffuse further into the material, causing further deterioration. This is
184 Biodeterioration of Concrete

14 .-----------------------------------------------------,

12

---
- - - - "'7: .

..--~-
10 /
/ .. ··
I
I
8 I
I
I I
0..

------- ---
~I
6
/ . ..-:. ~ ···
,,,.•""" .
4
ll /
PC
2 65% PC I 35% FA
GSA

0+------,,------,------,-------,------,------~

0 50 100 150 200 250 300

TIME, days
Figure 4.12 pH of 0.01 M citric acid solutions in contact with volumes of hardened
cement paste with typical compositions for Portland cement (PC), a 65% PC/35%
fly ash blend, and a calcium sulphoaluminate cement, calculated using geochemical
modelling techniques. Solid:liquid ratio = 1:100.

what is occurring for the CSA cement paste exposed to the dilute solution
in Figure 4.12.
The citrate ion is capable of forming relatively strong complexes with
calcium, aluminium and iron. It is also capable of forming complexes with
silicon (see Chapter 2). Within the context of concrete durability, this is of less
concern, since fragmentation will typically remove material indiscriminately
at a greater rate.

4.6.3 Tartaric acid

Tartaric acid is a relatively weak acid, but one which is capable of forming
insoluble calcium tartrate on contact with cement. The influence of calcium
tartrate precipitation on the integrity of cement is a mixture of positives
and negatives.
Calcium tartrate is an expansive product, with a molar volume of 144
cm3/mol. This effect is seen in Figure 4.13 which shows micro-CT scans
obtained from a series of cement paste specimens exposed to a 0.1 M
solution. The expansive nature of the product is evident from the images
obtained from PC and PC/FA pastes, with a thick product having formed
within the silica-gel layer of the specimens, which has been considerably
disrupted, leading to partial delamination from the surface.
 Fungal Biodeterioration 185

Figure 4.13 micro-CT scans showing cross-sections through cement paste specimens
exposed to a 0.1 M solution of tartaric acid for 90 days [78].

However, a feature of this tartrate ‘crust’ is that, unlike calcium citrate,

it does not fall away from the concrete, but remains attached, leading to
an accumulation of material. This would appear to have something of a
protective influence towards the concrete beneath it. Indeed, in the days
when winemaking was commonly conducted against unlined concrete,
the surface would be allowed to develop a crust of tartrate to limit further
deterioration [79].
The effect of cement type on resistance to tartaric acid is also evident
from Figure 4.13, with the PC/FA blend performing best, and calcium
sulphoaluminate cement performing least well. The reason for this is
possibly, in part, the absence of a tartrate layer.
Figure 4.14 plots the depth of the acid-deteriorated layer with time for
a Portland cement paste exposed to a 0.10 M tartaric acid—without renewal
of the solution. Results from a number of other acids including acetic acid
are also included [73]. Figure 4.15 shows mass loss from the same set of
experiments.

4.6.4 Malic acid

Malic acid also produces insoluble salts—calcium malate trihydrate and

dihydrate (molar volumes 128.0 and 115.0 cm3/g respectively). It is likely
that calcium malate trihydrate is formed, since it is the least soluble of
the two. Precipitation has a partly protective effect in the same manner as
tartaric acid. However, comparing acetic acid and malic acid deterioration
depth and mass loss results in Figures 4.14 and 4.15 it is evident that mass
loss from Portland cement pastes is closer to that of acetic acid, whilst the
deteriorated depth is much less, suggesting that fragmentation of the outer
acid-deteriorated layer has occurred to some degree.
Figure 4.16 shows the results of geochemical modelling of malic acid
attack on Portland cement. According to the model, only calcium malate
186 Biodeterioration of Concrete

14

• Tartaric acid
12 0 Malic acid
E "' Acetic acid
E v Succinic acid
::C 10
I-
D...
UJ
0 8
0
UJ
I-
<( 6
0::
0
0::
UJ 4
I-
UJ
0
2

0
0 50 100 150 200 250 300 350
TIME, days
Figure 4.14 Development of degraded depth in cylindrical Portland cement
paste specimens exposed to 0.1 M solutions of tartaric, malic, acetic and succinic
acids [72, 73].

60

• Tartaric acid 'V

----
0 Malic acid
50
"'
'V
Acetic acid
Succinic acid
................ / \1
/

~ 40 /.g 'V
0

(/)- ~ _ _ :!!
(/) V'9

--.-
v/
0_.J 30
_________
"
/ _()
(/) ,q ......
(/)
<( /~
~ 20 r.:~·~_g--0

. .
'S/ .v
~ly r:P
v/j ____ ---- --------.
..
10
!o
,_
0
~- _,. .;-.~· --
0 100 200 300 400

TIME, days
Figure 4.15 Mass loss from cylindrical Portland cement paste specimens exposed
to 0.1 M solutions of tartaric, malic, acetic and succinic acids [72, 73].
 Fungal Biodeterioration 187

10°.-------------------------------------------------,

!-------------------=======l
§1l
<:::
0 10-1
E
6
w
~
:~:: c:=c:-~~~~~=~i I".
0 10-4 11 e
(f) ~·.

(f) 10-5 II . /'- _____________________ _

0 10-6
:\ f.
II I .... ......... .... . . .
\'f"-
...J

~ 10-' I:/
0
I- 10-a I
10-9 +----.----.----,----,----.----.----.----.----,,---~
~·-·-·-·-·-·- · - · -·-·-·-·- · - 12
I
I 10
0.03
I pH
8 I
c.

0
"'
Q)
__ ___ . / 6
E
~
0.02 - ·- ·- 4
I- Calcium malate trihydrate
2
i=
z
<(
~
a (Ca/Si=O.B)
0.01

Portland ite

0 2 3 4 5 6 7 8 9 10

DEPTH FROM SURFACE, mm

Figure 4.16 Concentrations of dissolved elements (top) and quantities of solid phases obtained
from geochemical modelling of malic acid attack of hydrated Portland cement. Model
conditions: acid concentration = 0.1 mol/l; volume of acid solution = 4 l; mass of cement 80
g; diffusion coefficient = 5 × 10–13 m2/s.

trihydrate is precipitated as a very narrow band at the interface between

the decalcified and partially decalcified zones.

4.6.5 Gluconic acid

There is a lack of experimental data relating to the deterioration of

cement and concrete exposed to solutions of gluconic acid. However,
some indication of its likely effect can be inferred from the data relating
to the acid and its salts in Chapter 2. It is a somewhat stronger acid than
the other compounds discussed in this section. Moreover, whilst it forms
188 Biodeterioration of Concrete

a salt with calcium (calcium gluconate monohydrate), it is more soluble

and only likely to be formed where very high concentrations of the acid
are present. For this reason, it is likely that deterioration will be primarily
through acidolysis, which will be slightly more aggressive than for the
other fungi-derived acids.
Figure 4.17 shows the results of geochemical modelling of attack by
gluconic acid. Along with the absence of any salt precipitate, it is worth
noting that ferrihydrite is absent, in contrast to the case of malic acid. This
reflects the strong complexes that gluconic acid is capable of forming with
iron (III) ions.

10°
-=:: 10-1
0
E 10-2
0 - -·- ·
-----
-:-- ·· - ··- ·
L.U 10-3
> -'\..

" ' , ________________________ _

....J
10-4

· · §a·
0 "--
.
(/)
(/) 10-5
0 10-6
....J
<( ····· ····· ··· ···· · Si
I- 10-7
0 ------ AI
I- 10-8 - -- - Fe
10-9
/
.~·-·- · - · - · - · - · -·-- · -·-·-·-·-·-· 12
I
10
0.03 I
I pH 8 I
c_

C/l I
Q) 6
0 I
E ·-·-·-'
4
>-- 0.02
I-
CSH (Ca/Si=O.B) 2
i= Gibbsite
z
<(
~ Ettringite (AI)
a
0.01
CSH (Ca/Si=1.1)

Portlandite

0 2 3 4 5 6 7 8 9 10

DEPTH FROM SURFACE, mm

Figure 4.17 Concentrations of dissolved elements (top) and quantities of solid phases obtained
from geochemical modelling of gluconic acid attack of hydrated Portland cement. Model
conditions: acid concentration = 0.1 mol/l; volume of acid solution = 4 l; mass of cement 80
g; diffusion coefficient = 5 × 10–13 m2/s.
 Fungal Biodeterioration 189

4.6.6 Succinic acid

Succinic would appear to attack Portland cement at a rate which is

comparable to acetic acid [67a]. This is not surprising, since it is only slightly
weaker than this acid. Succinic acid reacts with calcium in cement paste to
precipitate calcium succinate mono- and trihydrate. Unlike tartaric acid the
formation of these compounds does not appear to have a protective effect,
although it has been suggested that its formation is not detrimental [73].
However, by comparing Figures 4.14 and 4.15 it is evident that there is a
discrepancy between the deteriorated depth results (which are comparable
to acetic acid) and the mass loss results. This indicates that fragmentation of
the outer acid-affected layer is most probably occurring, leading to greater
rates of mass loss compared to acetic acid. Figure 4.18 shows the results of
geochemical modelling of succinic acid attack on a PC paste.

10°

"'
0 1Q-1 Ca
E 10-2
Si
6 AI
LlJ 10-3 Fe
~ 1Q-4
0
(/)
(/) 10-5
15 1Q-6
...J

~ 1Q-7
0
1- 10"'
1o-•
, ·- ·-·-·-·-·-·-·-·-·-·-·-·- 12
(

0.03 I 10
I pH
8 ~
I
({)
Q) ) 6
0
E
0.02
- -- ·- 4
~ Calcium succinate trihydrate
2
i=
z
<(
::>
0 C3AH 6
0.01

0 2 3 4 5 6 7 8 9 10

DEPTH FROM SURFACE, mm

Figure 4.18 Concentrations of dissolved elements (top) and quantities of solid
phases obtained from geochemical modelling of succinic acid attack of hydrated
Portland cement. Model conditions: acid concentration = 0.1 mol/l; volume of acid
solution = 4 l; mass of cement 80 g; diffusion coefficient = 5 × 10–13 m2/s.
190 Biodeterioration of Concrete

4.6.7 A broader consideration of deterioration from fungi-derived acids

Viewing the impact of acids commonly produced by fungi on concrete as a

whole, it is evident that salt precipitation is a common feature, but also that
the effect of precipitation is highly varied. In some instances the formation
of salts leads to significant fragmentation of cement, whereas in other cases
the effect is wholly or partly protective.
It is reasonable to assume that the molar volume of the salts plays an
important role in influencing the effect that each acid has. The molar volume
increases in the sequence oxalic < malic < succinic < tartaric < citric. The
molar volume of calcium gluconate monohydrate is currently unknown.
Whilst the calcium salts on the extreme end of this sequence correlate with
the magnitude of damage observed, the other members do not.
Another factor which most probably plays a role in determining the
nature of attack is the solubility of the salt. There are two reasons for this.
Firstly, a lower solubility will cause a larger quantity of salt to be precipitated
for a given acid concentration. Secondly, a lower solubility salt will be
precipitated at closer proximity to the unaltered cement. This is significant,
because if the salt has a high molar volume and is precipitated close to the
intact cement, more damage will be done in comparison to a salt precipitated
in a region of wholly decalcified cement.
Another interesting correlation between the magnitude of deterioration
and the characteristics of the acids is shown in Figure 4.19. This plots
logarithms of the first acid dissociation constants (pKa) of the acids discussed
above (minus gluconic acid) against the deteriorated depth observed in
PC pastes at 100 days exposure to 0.1 M solutions. The reason for this
correlation is not immediately obvious, and may be co-incidental. Indeed,
it might be expected that an inverse correlation might be expected, since
pKa is a measure of acidic strength. Nonetheless, this relationship requires
further investigation.
The ability of calcium aluminate cement to resist attack from the fungi-
derived acids discussed is even less well understood. It has been seen that
very different results are obtained when a calcium aluminate cement is
exposed to tartaric and citric acid solutions, with the former displaying
reduced resistance compared to PC, and the latter displaying superior
resistance. Calcium aluminate cements exposed to oxalic acid develop the
same type of protective oxalate layer. However, the behaviour of gluconic,
succinic and malic acid is currently uncertain. Moreover, it is imprudent to
attempt to estimate performance due to the erratic behaviour observed. For
instance, geochemical modelling predicts that a protective calcium tartrate
crust should be formed when tartaric acid comes into contact with calcium
aluminate cement. Instead, a zeolite-like phase is formed, presumably as a
result of the complexation of aluminium [78].
 Fungal Biodeterioration 191

14
E
E ,.ecitric
(/)-
12 /
>-
<{
/
/
0 /
0
0
10 /
/
1- /
<{ /
8 /
I
1- /
a.. /
L.LJ /
0 6 /
0 Succinice /
L.LJ /
1- /
<{ 4 /
0::: /
0 /
0::: 2 /
L.LJ / eMalic
1- /
L.LJ
0 Oxalic _/ ;eTartaric
0
3 4 5 6 7

Figure 4.19 Deteriorated depth at 100 days of Portland cement pastes exposed to
0.1 M solutions of various organic acids produced by fungi versus the logarithm
of each compound’s first acid dissociation constant pKa1. Deteriorated depth data
from [72, 73].

4.7 Damage to Concrete from Fungi

This section will examine instances in the literature where fungi has been
found to cause damage to hardened cement or concrete either in the
laboratory or in the field. Instances where physical damage was observed
are examined first, followed by a discussion of discolouration of concrete
surfaces by fungi.

4.7.1 Physical damage

In reviewing research in the literature involving physical damage to concrete

as a result of fungal activity, it is evident that Aspergillus and Fusarium
species feature more frequently than other species. Whilst it can be safely
concluded that these species are capable of causing deterioration of concrete,
it is probably not sound to unquestionably conclude that they are the most
common fungi which cause damage. The reason for this is that the discovery
that such species damage concrete is likely to have stimulated further study
using these species. However, it should be noted that studies where samples
have been taken from deteriorating concrete in the field have all identified
the two species as being present.
192 Biodeterioration of Concrete

One of the earliest studies examining the influence of fungal activity

on concrete durability looked at the effects of two species: Aspergillus niger
and a sterile fungus [45]. Two experimental configurations were set up.
In the first of these, the fungal strains were grown in a growth medium
fluid which was continuously pumped onto the surface of cement paste
samples, such that the fungi came into contact with the cement and grew
on its surface. In the second, a similar process was followed, but a filter was
placed between the fungal culture, meaning that only the culture fluid and
any metabolites produced by the fungus came into contact with the cement.
The Aspergillus niger strain used in the experiments was found to
produce gluconic and oxalic acid. The sterile fungus produced gluconic
and malic acid. Where the cement came into contact with the organisms,
there was substantial removal of calcium in the case of the sterile fungus,
but no more than the control in the case of Aspergillus niger. One possible
reason for this is evident from powder X-ray diffraction traces obtained from
the cement paste specimens after exposure—calcium oxalate is present in
the paste in contact with Aspergillus niger (although not commented on by
the researchers). Thus, calcium was being retained in the cement paste as
insoluble calcium oxalate.
Despite this retention of calcium, the porosity of cement specimens
increased considerably in both cases, as illustrated in Figure 4.20, which
shows the difference in pore size distributions relative to the control.

14 .-------------------------------------------------~

-Control
12 r::::::l Mycelia sterila
- Aspergillus niger

0 +-------v
• _______ I I-Y------------cr--------y-.---------------J
>0.9 0 9-0 06 0 06-0 009 <0 009

PORE SIZE RANGE, flm

Figure 4.20 Changes in porosity in Portland cement pastes resulting from
exposure to the culture fluid used to grow two fungi: a sterile fungi (Mycelia
sterila) and Aspergillus niger [45].
 Fungal Biodeterioration 193

The main change observed is an increase in the volume of porosity with

diameters in the range 0.06–0.9 μm. This is attributed to the dissolution of
crystals of Portlandite (Ca(OH)2). There is also an increase in the volume
of larger pores, which is attributed to the cracking of shrinkage-prone
decalcified CSH gel at the cement surface after drying.
In addition there was a substantial loss in flexural strength resulting
from contact with the fungi (Figure 4.21), although it should be noted that
strength was not measured in control specimens and was still likely to be
considerable due to the presence of glucose, which forms complexes with
calcium and accelerates leaching.
Where the experiments were conducted such that the cement paste
specimens only came into contact with the filtered culture fluid, similar
results were obtained, indicating that the deterioration process was largely
chemical. However, deterioration was greater in the case where Aspergillus
niger grew on the cement itself.
Another laboratory experiment in which substantial damage was done
to concrete by a fungal strain examined the growth of a Fusarium species on
a section of concrete pipe sprayed with a culture medium acidified to pH 7
using hydrochloric acid (HCl) [52]. A considerably greater loss of calcium
was estimated to be occurring from the pipe inoculated with Fusarium
compared to the control, although this was based on measuring the quantity
of HCl neutralised—an approach which employs assumptions which may
not always be true. Nonetheless, the loss of mass from the concrete was

500
___._ Aspergillus niger
0 · · Mycelia sterila

400

z
6 300
<(
0
--'
l1J
0:::
::J 200
--'
<(
L.L.

100

0
0 2 4 6 8 10 12

TIME , months

Figure 4.21 Loss in flexural strength (expressed as the failure load under
three-point loading) resulting from growth of Aspergillus niger and a sterile
fungus (Mycelia sterila) [45].
194 Biodeterioration of Concrete

24% at 147 days exposure, after removal of loosely attached precipitates

at the surface.
Experiments have been conducted examining the interaction of concrete
chips in an agar growth medium inoculated with various fungal strains:
Alternia alternata, Aspergillus niger, Aspergillus versicolor, Cladosporium
cladosporioides, Aspergillus repens, Aspergillus flavipes and Paecilomyces
lilacinus [38]. In almost all cases, discolouration of the cement occurred—the
exception being in the case of Aspergillus versicolor. Additionally, a number
of concrete specimens underwent expansion and cracking: Aspergillus
niger, Aspergillus versicolor, Cladosporium cladosporioides and Aspergillus
flavipes. Visual inspection indicated that Aspergillus niger caused the most
substantial deterioration. SEM examination and powder X-ray diffraction
analysis found crystals of calcium oxalate (in the form of both whewellite
and weddelite) encrusting the concrete surface.
The researchers also conducted infrared spectroscopy and chemical
analysis on droplets of exudate formed on Aspergillus niger hyphae growing
on a concrete chip. They concluded that the exudate contained molecules
with carboxylate groups which had formed complexes with calcium and
silicon from the concrete.
A similar study examined the growth of Alternaria alternata, Aspergillus
flavipes, Aspergillus niger, Aspergillus versicolor, Aspergillus repens, Cladosporium
cladosporioides, Paecilomyces lilacinus on cement paste specimens [54]. In
many cases discolouration, swelling and sometimes cracking were observed.
The most substantial damage was observed in the case of Aspergillus niger,
although deterioration was also observed in the case of Alternaria alternata
and Cladosporium cladosporioides. Calcium oxalate formation (in the form
of both whewellite and weddellite) was observed in the cases of all three
of these fungi, plus Aspergillus versicolor. Cracking was observed for both
Aspergillus niger and Alternaria alternata.
An evaluation of fungal inhabitation of concrete surfaces in two
buildings in Moscow used for food processing—a large bakery plant and
a meat processing plant—found a wide range of species (included in Table
4.4) and substantial deterioration [53]. This took the form of cracking and
spalling of the concrete surface. Non-destructive testing of the colonised
surfaces (probably using a rebound hammer, but not explicitly stated) found
that the strength of the concrete had declined by up to 43% in the case of the
bakery and 25% in the case of the meat processing plant. Another interesting
feature of the concrete studied was that colonised surfaces had carbonated
to a greater depth: 5–10 mm, compared to 2–3 mm in unaffected surfaces.
This was attributed by the researchers as being the result of CO2 production
as a result of respiration of the fungus. This is wholly conceivable, but the
presumably higher porosity of the deteriorated concrete is also likely to
have played a role.
 Fungal Biodeterioration 195

The most deteriorated area investigated was the basement of the bakery
where there was a high quantity of airborne flour dust, which presumably
promoted growth as a source of carbon and energy. Aspergillus niger was
prevalent in this environment.
Deterioration of concrete by fungi was also identified in a UK study. In
this case the fungi were growing below the waterline in a partially flooded
cellar [49]. The fungi involved were sampled and isolated and identified as
being Aspergillus glaucus and a Penicillium species. The isolated fungi were
then placed in flasks with growth medium and small concrete specimens. A
drop in pH of around 1.5 after 40 days was observed in the flask containing
Aspergillus glaucus, which corresponded to a loss in mass from the concrete
specimen of around 6%. Calcium was found to be the main element leached
from the specimen during mass loss. The Penicillium fungus did not produce
a drop in pH and mass loss was of a lesser magnitude.
The influence of two lichen species (Acarospora cervina and Candelariella
vitelline) growing on asbestos cement roofs has been evaluated [65]. In the
case of Acarospora cervina, the roof material on which it was growing was
found to contain traces of whewellite, indicating oxalic acid production.
This phase was not identified in the roof on which Candelariella vitelline
was growing, which is known to produce pulvinic acid derivatives instead.
Damage to the roofs was not quantified, but it was proposed that the lichen
were altering the nature of the asbestos fibres, rendering them amorphous
and less toxic. The study was conducted from the perspective of the health
hazards associated with asbestos, and so this effect, plus the physical
coverage of the roof by lichen which was presumed to prevent dispersal of
asbestos fibres into the wider environment, was considered a favourable one.
One notable feature of the results of experiments investigating fungal
deterioration of concrete is that, despite the presence of fungi known for
the production of both oxalic and citric acid (such as Aspergillus niger), only
calcium oxalate has been observed when the concrete is analysed using
powder X-ray diffraction, and only in some cases. From the group of studies
where this technique was used to analyse concrete or cement surfaces
affected by fungi or lichen, only three positively identify calcium oxalate
[38, 67, 54]. In all other cases it is absent [45, 68], although it is incorrectly
identified in one instance.
The reason for this is not clear. It is certainly true that calcium oxalate is
the less soluble of the two compounds, meaning that, where the two acids
are present, calcium oxalate will precipitate in preference to calcium citrate.
Thus, the absence of the citrate salt is conceivable. However, the absence
of calcium oxalate is harder to explain. In some instances, the absence is
simply because oxalic acid is not produced by the fungi. It is also possible
that calcium oxalate is being dissolved by other acids. However, the salt
remains insoluble even to very low pH, meaning that, if this were the case,
196 Biodeterioration of Concrete

the acid involved would need to be one which forms very strong complexes
with calcium.
Oxalic acid is also decomposed to hydrogen peroxide and carbon
dioxide when illuminated by sunlight in the presence of oxygen. Whilst
direct decomposition of calcium oxalate does not appear to occur, dissolution
of oxalate ions into water at a concrete surface would allow the gradual
removal of oxalate. The reaction is significantly catalyzed by the presence
of dissolved iron (III). It has been determined that under UV intensities of
0.65 m Einstein/m2s, 1 μM dissolved iron (III) was able to decompose oxalic
acid at a rate of 10 nM/s [80].
In at least one case, calcium oxalate was found to be absent at the fungi-
inhabited surface of a cement paste specimen, despite analytical evidence
that oxalic acid was being produced. It had previously been thought that
once calcium oxalate was precipitated, fungi were subsequently unable
to utilise the oxalate in any manner [81]. Recently, however, experiments
conducted into calcium oxalate formation by a range of fungi found
that, for some species, there was a subsequent disappearance of oxalate
crystals [82]. It was proposed that the fungi were releasing enzymes such
as decarboxylase or oxalate oxidase which were breaking down the oxalate
to produce hydrogen peroxide (H2O2) which could be used in the oxidative
decomposition of cellulose.
The fungi species found to be capable of this process were Pleurotus
tuberregium, Polyporus ciliates, Agaricus blazei, Pleurotus eryngii, Pleurotus
citrinopileatus, Pycnoporus cinnabarinus and Trametes suaveolens. It should
be stressed that none of these species have been identified growing on
concrete surfaces.

4.7.2 Discolouration of surfaces

In many cases, discolouration of concrete surfaces by fungi, rather than

physical damage, may be the more important issue. In the case of fungi,
discolouration occurs when the fungi produce pigments, the most common
of which are the melanin compounds (see also Section 4.4.3). These are
typically brown or black in colour. The majority of the species listed in
Table 4.4 are known to be capable of producing melanins or melanin-like
compounds.
The accretion of living and dead fungal cells, biofilm materials and
pigments can lead to the formation of a black crust at the concrete surface.
Moreover, biofilms have been likened to fly-paper, since their adhesive
characteristics lead them to accumulate wind-borne dust and pollution
particles [83].
In the case of lichen, whilst melanin may be produced by these
organisms under certain circumstances [84], pigmentation will usually
 Fungal Biodeterioration 197

come from chlorophyll—used for photosynthesis by the photobiont, and

imparting a green colour—and certain lichenic acids (see Section 4.4.2)
which can impart a range of colours, including shades of yellow, brown,
orange and red.
Whilst a discussion of aesthetic matters is beyond the scope of this
book, it is probably reasonable to say that colour changes imparted to the
surface of buildings and other components of the built environment by the
colonisation of a surface by lichen (Figure 4.22) is often considerably less
disagreeable to the eye in comparison to the dark crusts produced by many
fungi. As is hopefully evident from Section 4.7.1, there would appear to be
a lack of reported evidence of damage to concrete from lichen. Moreover,
there is some evidence that lichen layers, in some instances, act to physically

Figure 4.22 Lichen growth on a concrete surface.

198 Biodeterioration of Concrete

protect surfaces [67, 85]. With this in mind, the decision of whether lichen
needs to be removed on aesthetic grounds needs to be made on the basis of
the nature of the building, its surroundings and, potentially, the opinions
of its users.
Nonetheless, there are functional reasons why both fungi and lichen
may need to be removed. One of these is that, in wet conditions, colonised
surfaces may present a slip hazard. Moreover, where concrete surfaces are
adjacent to timber components in a structure, removal may be prudent to
avoid colonisation and deterioration of the wood by fungi.

4.7.3 Corrosion of steel

Fungi have been found to be capable of inducing corrosion in steel

tendons in post-tensioned concrete. The technique utilizes tendons held
within polymer sheaths which are placed into tension after construction
of a structural element is complete. To allow the tendon to move freely
during tensioning, a lubricating grease is present within the sheath. A
study investigating the possibility of fungal corrosion isolated species of
Fusarium, Penicillium and Hormoconis from grease in cables taken from an
existing structure [87].
The isolated organisms were then used to inoculate sheathed tendons,
which were then sealed and held at a temperature of 23°C, alongside control
specimens which had only had distilled water introduced into them. The
inoculated specimens displayed localized corrosion. Most of the controls
displayed no corrosion, with the exception of a sheath that later tested
positive for Fusarium.
Infra-red spectrometry was employed to examine chemical changes in
the grease, and identified the development of organic acids with carboxylate
and hydroxy groups, leading the researchers to conclude that corrosion
was induced by fungi oxidizing the lubricant (a metal soap hydrocarbon
grease) as a source of energy.

4.8 Limiting Fungal Deterioration

Measures for limiting fungal deterioration can be achieved in three ways.
Firstly, the composition of the concrete can be formulated such as to either
limit the extent to which the concrete surface permits colonization, or such
that resistance to attack is enhanced. Secondly, the concrete surface may be
treated to resist colonization. Thirdly, where the surface is already colonized,
cleaning and maintenance may be conducted. These three approaches are
discussed below. The approach taken throughout is to discuss established
techniques before discussing more developmental approaches.
 Fungal Biodeterioration 199

4.8.1 Concrete composition

It is uncertain as to whether cement type plays any role in encouraging or

discouraging colonisation and growth by fungi. A study which examined
the development of fungi on mortar surfaces examined the influence of
including ground granulated blastfurnace slag (up to 50%), fly ash (to 25%),
silica fume (to 15%) and metakaolin (8%) [37]. Little difference was found
in the rates of growth.
Another study found that higher levels of GGBS in combination with PC
in cement pastes did limit fungal growth (in this case a mixture of Aspergillus
niger, Chaetomium globosum, Penicillium funiculosum and Gliocladium virens)
[80]. Levels of GGBS up to 80% by mass were used, and levels of 65% and
over appeared to limit growth entirely after a period of 9 months in contact
with a growth medium. Leach tests were carried out on the slag and no
organic or inorganic substances likely to have a detrimental influence on
fungal growth were detected. The researchers proposed that the high pH
of the GGBS was responsible, but this seems unlikely, since PC will have
a higher pH.
Further analysis of the data from this study presents another possibility:
the pastes with high slag contents contained lower levels of gypsum. This
correlation is shown in Figure 4.23. Given the importance of sulphur as a

3
.$
·c: I
:::J
I

.
i::' I
jg I
I
:0 I
Cii
I' 2 ., I

I
~
0 I
I

0::: I
(j I
I
(5 I
z I
::J
LL •• I
I• •
LL I
0 I

..
LlJ I
LlJ I
0::: I
(j I
LlJ
0 0 /
0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0

GYPSUM CONTENT, % by mass

Figure 4.23 Correlation between the gypsum content of PC/GGBS blend

cement pastes and the degree of fungal growth. Fungal growth is quantified
using a scale employed by the Slovakian standard test for measuring resistance
to mould growth on construction materials. 0 = no growth; 1 = negligible
growth; 2 = 25% coverage [88].
200 Biodeterioration of Concrete

nutrient (see Section 4.4.2) the limited sulphate levels in the cement may
be the reason for the absence of fungi.
Regardless of its influence of on fungal growth rates, cement type will
also influence rates of deterioration. There is incomplete understanding
of this area with respect to the acids typically produced by fungi, and
performance is dependent on the type of acids. However, it would appear
that, in most cases PC or calcium aluminate cements offer a slight advantage.
In the case of tartaric acid, PC/fly ash blends, and possibly PC slag blends
may be more beneficial. A feature of many of the acids formed by fungi is
that they form an insoluble and often expansive salt with calcium. Moreover,
biophysical processes operate by generating stresses within concrete which
lead to fracture. There is, therefore, a very strong argument, where fungal
attack is a possibility, for employing concrete with a low water—cement
ratio, and a hence high strength to resist the stresses associated with
expansion. This has the secondary benefit of limiting rates of colonisation
and growth through reduced water absorption (see Section 4.4.3).
The mechanical aspects of attack from fungi means that the choice of
aggregate used will have a lesser influence on performance in comparison
to modes of deterioration such as bacterial attack, where acidolysis can
be checked by the neutralising capacity of calcareous aggregate. Indeed,
calcareous aggregate will presumably also form expansive salts in contact
with fungi-derived acids.
Another approach to limiting fungal growth employs titanium dioxide
(TiO2) powder. The use of TiO2 in concrete exposed to the atmosphere as a
means of maintaining a clean surface and reducing atmospheric pollution
has been a developing technology over the past 20 years. TiO2 acts as a
photocatalyst: when illuminated with sunlight the surface of the compound
reacts with oxygen and water in the atmosphere to produce oxidising and
reducing species which are subsequently able to react with and break down
various airborne pollutants including oxides of nitrogen (NOx), sulphur
dioxide (SO2) and various volatile organic compounds (VOCs) [89]. The
anatase form of TiO2 is most effective. These reactions also act on non-
volatile organic substances which have become attached to the concrete
surface, ultimately leading to their degradation to an extent which allows
them to be easily washed away from the surface by rainwater.
The effectiveness of this so-called ‘self-cleaning’ characteristic of
concrete containing TiO2 with regards to preventing the colonisation and
growth of fungi on concrete found the approach to be successful [37]. A series
of mortar tiles were prepared containing a small quantity (unspecified)
of anatase nanoparticles in the cement. The tiles were inoculated, along
with mortar tiles made with the same cement without TiO2, with various
fungal strains and incubated for a week in an environment where a nutrient
solution was sprinkled periodically onto their surface. The tiles were lit
for 6 hours with near-UV radiation followed by 6 hours of darkness. After
 Fungal Biodeterioration 201

incubation, the tiles with TiO2 were found to be considerably more resistant
to fungal colonisation than the controls.
Whilst TiO2 particles in concrete appear to be effective against fungal
colonization, long-term performance is currently not well-understood.
Certainly it appears that where levels of surface contamination by dirt are
high, the material does not perform as well [90]. Moreover, where surfaces
are horizontal, and hence the self-cleaning process is not assisted by gravity,
accumulation of dirt—and presumably fungus—is still possible.
It is possible to add fungicidal admixtures at the mixing stage of concrete
production. There are a wide variety of substances with fungicidal effects.
These include compounds containing copper, zinc or boron, disinfectants
(such as sodium hypochlorite), quaternary ammonium compounds and
various organic compounds which are toxic to fungi. However, when
used as an admixture it is also necessary that a fungicide does not have a
detrimental effect on concrete properties.
Much of the guidance regarding anti-fungal admixtures identifies
polyhalogenated phenols and copper compounds as suitable candidates,
but this is somewhat outdated [91].
Research into various potential formulations for copper-based antifungal
admixtures examined a wide range of soluble candidate compounds [92].
The study measured growth of Aspergilus niger and Penicillium viridis on
the surfaces of concrete specimens made from concrete which had various
concentrations of the compounds added. Of the compounds evaluated,
copper arsenite and copper acetoarsenite were found to be most effective,
although additions of at least 5% by mass of cement were required. The
use of copper based admixtures, however, has side-effects. The presence
of copper will normally retard the hydration reactions of Portland cement
[93], and it was found that the compressive strength of the concrete mix
containing 10% copper acetoarsenite was around 80% less than the control.
Setting time was, in fact, accelerated. Additionally, copper-based admixtures
will normally impart an—often dramatic—colour change to concrete.
The main concern with such admixtures however, is the presence of
arsenic which is hazardous to human health and a possible carcinogen.
As a result, these compounds are now restricted in many countries. Other
copper-based compounds may potentially suitable, but the retardation of
cement hydration is far from ideal.
Polyhalogenated phenols have also been demonstrated as being
effective against mould growth when used as a concrete admixture [94].
Again, there are concerns regarding the human health and environmental
hazards presented by many of these compounds, and many substances are
now strictly regulated. Indeed, the main reason for exercising caution before
deciding upon the use of biocidal admixtures in concrete is the potential
for releasing ecotoxic substances into the wider environment.
202 Biodeterioration of Concrete

The search for less environmentally harmful antifungal admixtures has

been challenged by the need to identify substances that do not detrimentally
influence the strength development of Portland cement. For instance, one
study found that both sodium hypochlorite and a mixture of chlorhexidine
and cetrimide reduced concrete strength considerably and increased
porosity [95].
The use of two other organic fungicidal agents as admixtures has been
investigated more recently [96]. These were nitrofuran and an isothiazoline/
carbamate mixture. The latter of these displayed an ability to limit growth
of Aspergillus niger when included as a constituent in mortar. Moreover,
additions of this formulation had negligible influence over both compressive
and flexural strength development, even up to the largest dosage (5% by
mass of the cement).
In the place of these compounds, a more recent development in
fungicidal admixture technology has been the introduction of formulations
which employ a so-called ‘electro-physical’ mechanism [97]. The
compounds involved are silane compounds which, like water-repellent
admixtures, attach to the surface of hydration products in the cement
matrix. Joined to the silane group is a polymer chain component which
possesses a positively charged group which electrostatically attracts,
and then ruptures, the outer cell membranes of micro-organisms. The
precise nature of this charged group is unclear, although patents exist for
similar products containing molecules comprising silane groups joined to
quaternary ammonium groups [98]. This approach clearly has benefits:
by anchoring the molecules to surfaces, the admixture should, in theory,
last indefinitely. Additionally, anchoring presumably limits the release of
biocide into the wider environment.
Patents also exists for cement containing the mineral colemanite
(Ca[B3O4(OH)3].H2O) [99]. This mineral is known to have anti-fungal
characteristics as a result of its boron content [100], although its effectiveness
when used as an admixture in concrete is undocumented.
Many concrete admixtures also contain fungicidal/bactericidal
additives to aid in preserving the admixture itself prior to use. The extent
to which such substances contribute towards protecting hardened concrete
during service is unknown, but is presumably small.
Another approach under development is the possible use of fungicidal
micro-capsules [101]. These consist of granules of between 300 and 1000
μm. The outside of the capsule consists of a thin polyurethane membrane.
Inside is a pellet consisting of a mixture of a fungicide of relatively low
human toxicity (D-limonene, a naturally-occurring biocide found in the skin
of citrus fruits) and particles of zeolites. The zeolite particles are present
to provide structure and mechanical strength to the pellet. The micro-
capsules are included in concrete and mortar as a partial replacement of
the aggregate.
 Fungal Biodeterioration 203

The principle behind the micro-capsules is that, as time passes, the

membrane will become ruptured leading to a slow release of the biocide.
The presence of the capsules reduced strength (10% at a level of 15% by
mass of cement) and increased drying shrinkage. However, it was effective
in limiting fungal growth on concrete surfaces on which fungi grew
substantially in the absence of the capsules.
A similar approach has been adopted where the delivery mechanism
is through polymer fibres impregnated with biocides [102, 103, 104]. The
biocide is introduced during the fibre extrusion process. In the case of the
impregnation of polypropylene fibres with a commercial biocide based
on 2,4-dichlorophenol, inclusion of fibres at a level of 0.025% by mass of
concrete was found to limit mould growth in laboratory experiments [102].

4.8.2 Protection after construction

After construction, paints may also be used to place a protective barrier

between the concrete and the damaging effects of fungal activity. It should
be noted that the presence of a coating of paint will not necessarily prevent
the establishment and growth of fungus, although it does affect the types
of species involved to some extent [48]. Silicate paints, however, do possess
the potential to limit fungal colonisation [105]. This is for two reasons.
Firstly, the paint tends to be alkaline, which presents a hostile environment
for fungi (see Section 4.4.1). Secondly, silicate paints allow the concrete to
‘breathe’—water vapour is not prevented from leaving the concrete pores,
thus limiting the extent to which accumulation of moisture beneath paint
surfaces might otherwise encourage fungal growth.
Treatment with hydrophobic impregnants, such as silane compounds, is
a well-established approach for increasing the water repellency of concrete
surfaces. These compounds have also been used in the preservation of stone
to prevent fungal colonisation [106]. The main reason for their effectiveness
is their ability to limit the extent to which water can accumulate at pores
in the concrete, which is a key requirement for fungi (see Section 4.4.3). It
is also likely that the presence of silanes at the surface prevents the ingress
of acids. Whilst there is little by way of scientific evidence demonstrating
whether this approach is appropriate for controlling fungal colonisation of
concrete structures, it seems reasonable to assume that it would be. Indeed,
the presence of fungi and algae is proposed as evidence—during visual
inspection of concrete structures—that a surface has been inadequately
treated with silanes [107].
Some concerns have been raised with regards to the longevity of such
treatments [108]. However, when used correctly, a service life of around
15 years can be expected for most commercially-available products [109].
It should, however, be stressed that silane treatments limit the extent to
which water can penetrate pores, and cannot entirely prevent it. Sufficient
204 Biodeterioration of Concrete

pressures are able to overcome the water repellency. These pressures

need not be high: a wind-blown raindrop travelling at a reasonably high
horizontal velocity might quite easily generate sufficient pressure to enter
a concrete pore despite treatment with a silane or similar compound [109].
A traditional approach used on masonry, but not commonly encountered
in the field of concrete construction, is the use of strips of copper flashing on
structures [110]. The strips are typically set into mortar joints. As rainwater
runs over the metal strip a small quantity of copper is dissolved, which
has a similar effect to a copper biocide treatment. Whilst this approach is
apparently highly effective, it will usually leads to a green discolouration
of the masonry surface.

4.8.3 Cleaning and maintenance

The growth of fungus and lichen on concrete surfaces can, of course, be

checked by periodic cleaning. This can be achieved through brushing with
wire brushes or similar abrasive methods, scraping, and the use of high-
power water jets. This approach may not be appropriate where abrasion of
the concrete surface is to be avoided, such as where a high quality surface
finish must be preserved. The case of the cleaning of asbestos cement roofs
has been identified as another instance where abrasion of the surface must
not occur, since it has the potential to release hazardous asbestos fibres
[111]. In such cases, the use of fungicidal treatments may be a possibility.
A study examining the possibility of treating lichen-colonized asbestos
cement roofs evaluated a wide range of anti-fungal treatments in terms
of both their ability to remove lichen and their persistence in preventing
re-colonization of the surfaces [112]. The results of this evaluation are
shown in Figure 4.24. The most effective treatments were copper and
boron compounds, plus a commercial formulation based on methyl
1-(butylcarbomoyl)-2-benzimidazolecarbamate and 5,6-dihydro-2-methyl-
N-phenyl-1,4-oxathiin-3-carboxamide. Many other treatments had very
little influence on fungal growth, and potassium permanganate actually
encouraged growth. Given the role of both potassium and manganese as
nutrients, this is not perhaps surprising.
One proposed approach to controlling fungal colonisation under
development is the use of other organisms which are capable of inhibiting
fungal growth. One study has evaluated the effectiveness of the bacteria
Bacillus aryabhattai for this purpose, and found it capable of inhibiting
growth of Cladosporium sphaerospermum [55].
The same research group has subsequently broadened its search for
antifungal bacteria. This investigation concluded that Bacillus aryabhattai
(and another bacteria—Bacillus thuringiensis) was less effective at
preventing fungal growth on cement pastes than Arthrobacter nicotianae,
Bacillus thuringiensis and Stenotrophomonas maltophilia [56]. Growth of
 Fungal Biodeterioration 205

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Figure 4.24 Lichen coverage of asbestos cement roof panels after treatment with potentially
fungicidal additives. Panels initially had c 70% lichen coverage. After treatment, panels were
left outside for a period of 83 months prior to evaluation of coverage [112].

both Aspergillus niger and Cladosporium sphaerospermum species were

investigated. Further research has also identified the effectiveness of
Paenibacillus polymyxa E681 against Aspergillus niger [57]. In all of these
cases, the bacteria were selected not only on their ability to control fungal
growth, but also in their ability to form calcium carbonate minerals to
repair cracks in concrete. The research represents a relatively early stage
in understanding, and it is not yet clear how these bacteria would be best
206 Biodeterioration of Concrete

deployed in the built environment. However, research into technologies

which exploit calcite forming bacteria for concrete repair have devised a
number of possible means by which such bacteria can be introduced into
fresh concrete with the possibility of subsequent release where and when
damage occurs.

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Chapter 5

Plants and Biodeterioration

5.1 Introduction
The kingdom of plants incorporates a significant variety of organisms in
terms of both size and form. Their two common features are the use of
photosynthesis as a means of obtaining energy and the material that makes
up their cell walls: cellulose.
The interaction of plants with concrete structures in manners which can
be problematic are both chemical and physical. However, most physical
damage primarily derives from larger plants. For this reason, these divisions
of the plant kingdom are discussed below. Whilst usually not harmful to
concrete, the development of moss and similar plants can sometimes play
a role in the establishment of other plants on concrete surfaces, and the
division of plants in which mosses are placed (Bryophyta) is also covered.

5.1.1 Algae
The term algae covers a wide range of organisms, whose similarity and
distinction from other organisms is not easy to define. One definition is that
they obtain energy by photosynthesis using chlorophyll, and that they have
reproductive cells that are not protected by layers of sterile cells [1]. Algae
can be both unicellular and multicellular, with some of the multicellular
species, such as seaweeds, being able to grow to a considerable size. Both
unicellular and multicellular algal forms are most probably present in the
photograph in Figure 5.1. Many of the unicellular species are motile.
When algae are discussed in this chapter, it will only be in the context
of eukaryotic algae—organisms whose cells contain a nucleus and other
organelles contained within membranes. Cyanobacteria are prokaryotic
organisms—lacking a nucleus bound within a membrane—which are often
also included within the grouping of algae. Cyanobacteria are covered in
Chapter 3.
 Plants and Biodeterioration 213

Figure 5.1 Algae in the form of an algal biofilm and seaweed at the
waterline of a concrete bridge.

Plants and most algae undergo oxygenic photosynthesis—they are

photoautotrophs utilising sunlight as a source of energy, carbon dioxide
as a source of carbon, and water as the electron donor:
photons + 2H2O + CO2  [CH2O] + O2 + H2O
where [CH 2O] is a carbohydrate—not necessarily with this specific
chemical formula—such as glucose. In fact, algae can be both autotrophic
or heterotrophic, with a number being capable of obtaining energy through
both mechanisms (mixotrophic).
The carbohydrate acts as a means of storing energy, which can be
accessed by processes of cellular respiration where carbohydrate is
oxidised. However carbohydrates may also be used as the building blocks
for cell walls or precursor molecules in the biosynthesis of other molecules
needed by the organism. In the case of algae, cell membranes are made
from fibrillar components embedded in an amorphous matrix [1]. The
fibrillar components are most commonly made from cellulose (polymerised
glucose), but can sometimes be polymers of mannose (mannan) or xylose
(xylan). The amorphous matrix is also composed of polysaccharides in the
form of alginic acid, fucoidin, agar and carrageenan.
Algae can be categorised in terms of the substances used for
photosynthesis. Green algae use Chlorophyll a and b, whilst red algae use
chlorophyll a and c plus phycobilins. Brown algae use chlorophyll a and
c, and fucoxanthin.
214 Biodeterioration of Concrete

Because of the presence of chlorophyll, the presence of algal biofilms

is usually fairly evident, as a result of their colour. However, it should be
noted that cyanobacteria (see Chapter 3) also contain similar pigments, and
are often encountered alongside algae.
The larger, multicellular algae more closely resemble higher plants,
in that they possess comparable structures. The structural features that
make up multicellular algae such as seaweeds are shown in Figure 5.2.
These features include laminae, which are flat structures approximately
resembling leaves. Seaweeds may also possess air bladders—which are
used to keep the plant afloat—and a stem known as a stipe. It should be
stressed that significant variation exists between different genera and species
with regards to the shape, location and prominence of these features. For
instance, many seaweeds do not possess a stipe, whilst air bladders can
also be located as features of the laminae.
Seaweeds do not, however, possess roots: they are anchored to the
substrate on which they grow by a structure known as a ‘holdfast’ which
may take many forms depending on the nature of the substrate. Where algae
are growing in granular media it may take the form of a bulb, whereas in
substrates made of finer particles, it may more closely resemble a rootlike
structure. On solid surfaces, the holdfast takes the form of a flat structure
which is attached to the surface using adhesive substances secreted by the
organism.

Air bladder

Figure 5.2 Structural features of seaweed.

 Plants and Biodeterioration 215

5.1.2 Vascular plants

Vascular plants are multicellular organisms which possess two tissues

which allow the distribution of substances to all of their cells. The first of
these is the xylem which transports water and nutrients from the roots to
the rest of the plant. The second is the phloem which is used to transport
the products of photosynthesis from photosynthesising cells to other cells,
including those in the roots. The majority of photosynthesis is conducted in
cells within the leaves which normally possess a large surface area which
is able to maximise the number of cells illuminated by sunlight.
An extremely generic schematic diagram of some of the key features of
a vascular plant is provided in Figure 5.3. However, the description above
avoids one extremely important aspect of the vascular plants, which is the
sheer diversity of scale and form that exists within the vascular plants.
The cell walls of plants is composed largely of cellulose. The xylem also
contains lignin, which imparts rigidity to varying degrees.

5.1.3 Bryophyta

Bryophytes are a division of plants which differ from the higher plants
in that they lack a vascular system—they lack a xylem and phloem for

Air bladder
Air bladder

Air bladder

Figure 5.3 Schematic diagram of the common features

of a vascular plant.
216 Biodeterioration of Concrete

the distribution of water. Of most significance—within the context of this

chapter—are the mosses (Bryophyta)—Figure 5.4.
Mosses are found in both terrestrial and aquatic environments. They
are multicellular and consist of a short stem with simple leaves, and are
anchored to their substrate by thread-like rhizoids. Rhizoids appear to be
purely employed for this purpose and, unlike the roots of vascular plants,
are unable to absorb water and nutrients. However, they can, in some cases,
penetrate porosity in stone or concrete.

5.2 Plant Growth and Reproduction

The growth of vascular plants occurs through a process of cell division,
taking the form of the development and growth of shoots and leaves, the
radial growth and thickening of roots and the stem/trunk.

Figure 5.4 Moss on a concrete wall.

 Plants and Biodeterioration 217

Bryophyte growth takes the form of progressive, lengthening of the

stem, the growth of new leaves, further development of rhizoids and the
development of sporophytes on female plants (discussed below).
Growth in the case of algae is very much dependent on the type and size
of the organism. Whilst unicellular algae effectively ‘grow’ by reproduction,
such organisms will often form highly organised structures with algae
of the same species. For instance, non-motile unicellular algae may form
filamentous structures-long strings of cells–or palmelloid structures–
groupings of cells embedded in a mucilage. Similarly, motile algae may
form small, mobile colonies. The multicellular algae grow in a manner
more like that of vascular plants, although the absence of roots means that
growth is limited to the other parts of the organism.
Reproduction in plants can occur via a number of processes, with a
diversity of variations in how these processes are achieved. Given the focus
of this book, it is of limited value to discuss these processes in too much
detail. However, the reproductive processes of algae, vascular plants and
bryophytes are outlined below, since there are aspects which will help in
understanding how concrete surfaces can become colonised by plants.

5.2.1 Algal reproduction

The means by which algae reproduce are diverse and complex. However,
they can be grouped into three general forms: vegetative, asexual and
sexual. Vegetative reproduction involves either cell division or division of
multicellular algae through various mechanisms. Asexual reproduction
employs spores, or similar entities, which grow into a plant genetically
identical to the parent plant. Sexual reproduction involves either isogamy
(where algal cells from different individuals form similar motile gametes
which fuse together) or heterogamy (where gametes with very different
size and/or characteristics are produced).

5.2.2 Higher plant reproduction

Plants can reproduce sexually and asexually. Sexual reproduction involves

the transfer of male gametes in the form of pollen (in the case of flowering
plants) to female ovules, also located within the flowers. This transfer is
often achieved through another organism that acts as a pollinator. The
pollinator is most commonly an insect, but other animals can also be
employed. Fertilization leads to the formation of seeds within fruit. The
manner in which seeds are dispersed is of some relevance to the topic under
discussion, with wind and animal dispersion being two common means
by which this is achieved. Seeds will germinate when they experience
appropriate temperatures and levels of moisture and oxygen.
218 Biodeterioration of Concrete

In the case of the ferns, reproduction occurs through a slightly different

mechanism. The fern (in its ‘sporophyte’ form) develops leaves known
as sporangium which produce spores. These spores are released and
dispersed by processes such as the action of wind, animals, gravity and
the movement of water. Once a spore encounters suitable conditions, it
germinates to form a small plant—a ‘gametophyte’—which possesses
two organs: the antheridia and the archegonia. The antheridia produces
motile sperm, whilst the archegonia contains female egg cells. The sperm
can swim away from the antheridia if water is present, for instance, as a
result of rainfall. Once a sperm fertilizes an egg cell, a zygote forms which
grows into a sporophyte (essentially what would normally be considered
the ‘fern’ stage of the lifecycle).

5.2.2 Bryophyte reproduction

Mosses reproduce via spores. When a spore germinates it develops into

either a male or female gametophyte. The male form develops antheridia,
whilst the female plant develops archegonia, not unlike those of the ferns.
In suitable conditions, motile sperm from the antheridia on a male plant
to the egg-containing archegonia on a female plant, leading to fertilization.
This leads to the growth of a sporophyte, which is an entity completely
separate to the gametophyte on which it forms, but partly dependent on
the gametophyte for nutrients. The sporophyte develops from within the
antheridia, growing upwards from the top of the female gametophyte. A
capsule is located at the end of the sporophyte which eventually ripens
and releases spores which are usually dispersed by the wind. Once suitable
conditions are encountered by the spore, it germinates, firstly developing
as a protonema, which will ultimately grow to form a gametophyte.
In all of the above descriptions of the means by which plants reproduce,
the fundamental theme that is of significance with regards to the appearance
and growth of plants on concrete surfaces is that seed and spore formation
plays a frequent role, and that many of these are dispersed by the wind or
animals in such a way that the appearance of plants in relatively inaccessible
locations on buildings is perfectly possible. This aspect of the life-cycle of
plants will be explored further in the next section, where concrete as an
environment for plant life is examined.

5.3 Concrete as an Environment for Plant Life

The growth of plants on concrete is a process which normal requires a
number of conditions to converge. These include the availability of water,
the chemical nature of the concrete surface, and the availability of nutrients.
All of these aspects are discussed in this section. However, before they can
 Plants and Biodeterioration 219

be discussed, it is important to consider the processes which allow plants

to reach—sometimes very remote—locations on a concrete structure.

5.3.1 Steps towards plant colonisation of concrete surfaces

The manner in which concrete surfaces are colonised by plants varies

depending on nature of the organism. It is again useful, therefore, to divide
the discussion in terms of whether algae, vascular plants or bryophytes
are involved.

Algae

For algae to be brought into contact with a concrete surface, there must
be some movement of algae-bearing water past the surface. The rate of
flow plays an important role in the rate at which algae become attached to
this surface. A study which examined rates of attachment of a number of
species of algae delivered to different surfaces by flowing water travelling
at different rates found that slower rates favoured colonisation [2].
The same study also found that attachment rates were dependent on the
nature of the surface. However, the findings are somewhat counterintuitive,
with algae often attaching with greater efficiency to hydrophobic surfaces
with low free surface energies.
It has been proposed that the attachment of algae is the result of
extracellular polymeric substances (EPS) produced by these organisms. This
is supported by research which examined the nature of the polysaccharides
produced by a number of species of algae isolated from building façades [3].
The polymers were categorised in terms of whether they were released into
the environment surrounding the cell (released polysaccharides, RPS), or
were retained as the outer envelope of the cells (capsular polysaccharides,
CPS). The polymers were found to be anionic in nature and, in some
cases, reduced the surface tension of water. The kinetics of CPS adsorption
onto a glass surface and silicon wafers coated with a hydrophobic agent
were characterised. It was found that, whilst the rates of adsorption were
similar initially, the hydrophobic surface ultimately adsorbed more of the
polysaccharides.
In the case of the larger multicellular algae, colonisation is likely to be
initiated by a spore becoming attached to the surface.

Vascular plants

It has been proposed that higher plants may become established on concrete
surfaces through two mechanisms [4].
The first mechanism occurs on both horizontal and vertical surfaces and
involves seeds of plants becoming lodged in cracks or holes in the surface.
220 Biodeterioration of Concrete

The second mechanism requires the establishment of moss (or

potentially other Bryophyta members) on a horizontal concrete surface (see
Figure 5.5). The moss has the effect of trapping dust and other particulates
carried by the wind leading to a situation where a sufficient quantity of
substrate is formed to support the establishment of higher plants with roots.
Only certain plants are suited to the colonisation of a concrete surface.
In their review of the damage that plants are capable of doing to historic

Figure 5.5 Moss on a horizontal surface providing a substrate for vascular

plant growth.

stone buildings, Lisci et al. [4] draw up a list of characteristics likely to

favour a given plant in colonising such surfaces:
Suitable seeds: only smaller seeds are likely to be able to become lodged in
small cracks and holes. However, if the surface in question is vertical or
above ground level, there must also be a means of the seed reaching such
locations. Thus, plants whose seeds are adapted to be carried by the wind,
or are present in fruits which are eaten by birds are clearly more likely to
be found on such surfaces.
Drought resistance: seeds which are able to germinate in the presence of only
small quantities of water, and plants which are able to grow under similar
conditions are likely to be less affected by the fluctuations in moisture which
are likely to be encountered on a stone or concrete surface.
 Plants and Biodeterioration 221

Appropriate means of reproduction: plants which reach sexual maturity rapidly

are more likely to grow the population of plants occupying a surface.
Additionally, if plants possess a means of vegetative production, this will
also assist in this.
Suitable rooting mechanisms: plants which are deep rooting and whose roots
are particularly suited to growth through rock and similar materials are
likely to have an advantage on stone and concrete surfaces.
The same researchers devised a list of European plants which were
known to populate historic stone buildings, along with their propensity
to cause damage. Table 5.1 contains plants from this list which were also
identified as being harmful to stone structures, plus some other species
from other sources. The criteria for selecting harmful species was formation
of woody stems and roots likely to cause damage to building as they
grew in a radial direction. Added to this table are seed sizes and dispersal
characteristics, whether the plant is calcicole or calcifuge, and its preferred
pH conditions. Table 5.2 lists plants likely to be damaging from a list of
plants found growing on historic structures in tropical regions compiled
by Kumar and Kumar [7].
It should be stressed that, as a result of the nature of the main sources
of the identified species in Tables 5.1 and 5.2, it is likely that the list is
somewhat biased towards Mediterranean and Indian species. However, the
list is nonetheless useful, in that it allows the consideration of what types
of plants are most likely to grow on a concrete structure.
The first consideration is what types of seed are most likely to become
lodged in a concrete surface. Given that concrete surfaces are typically
relatively smooth, it is likely that smaller seeds will have a greater chance
of locating themselves in a cavity such as a blowhole or crack. It is unlikely
that many substantial cracks will exist shortly after construction, and so
other forms of deterioration may need to occur prior to the establishment
of vascular plants by this route. The thousand seed weight provides some
indication of the relative size of the seeds in the Tables. The density of seeds
from different genera and species can differ to quite a large extent, and so
attempting to translate thousand seed weight into linear dimensions is not
advisable. However, it is safe to conclude that the seeds with the lower
thousand seed weights are more likely to locate themselves in the types of
crack or recess that might be present on a concrete surface.
Additionally, whilst it is evident from the table that some plants can
grow in relatively alkaline conditions, the extremely high pH of relatively
young concrete is likely to be excessive. This is discussed further in Section
5.3.2. It is also worth noting that the vast majority of the European plants
in Table 5.1 are calcicole in nature, meaning that they favour substrates in
which calcium is present (see Section 5.3.3).
 Table 5.1 Vascular plants likely to be harmful inhabitants of European historic structures and walls [4, 5, 6].

Order Family Genus Species Common Dispersal Thousand Calcicole/ Optimum References
Name Seed Calcifuge pH
Weight, g

Apiales Araliaceae Hedera helix Ivy animal 69.30 Calcicole 6.0–7.5 [8, 12]

Asterales Compositae Dittrichia viscosa Woody 0.38 Adaptable – [8, 17]

Fleabane ecotype

Brassicales Capparidaceae Capparis spinosa Caper animal 9.00 Calcicole 7.5–8.0 [8, 10, 16]
222 Biodeterioration of Concrete

Brassicaceae Erysimum cheiri Wallflower wind 1.40 Calcicole 5.5–7.5, but [8, 11, 12]
can tolerate
higher pH

Matthiola incana Stock wind 1.62 Calcicole 6.0–7.5 [8, 12]

Caryophyllales Polygonaceae Fallopia japonica Japanese wind/animal 2.065 – 3.0–8.5 [8, 15, 18]
Knotweed

Celastrales Celastraceae Euonymus europaeus Spindle- animal 40.60 Calcicole 6.0–8.0 [8]
tree

Dipsacales Caprifoliaceae Sambucus nigra Elderberry animal 26.00 Calcicole 5.5–6.5 [8, 12]

Valerianaceae Centranthus ruber Red wind 1.77 Calcicole 5.0–8.0 [8]

Valerian

Fabales Leguminosae Robinia pseudoacacia False Gravity/root 18.00–20.00 Calcicole 4.6–8.2 [8, 13, 14]
acacia/ suckers
Black
locust
Lamiales Oleaceae Syringa vulgaris Lilac 5.60 Calcicole 6.0–7.5 [8, 12]

Fraxinus excelsior Ash wind/gravity 62.6 Calcicole – [8]

Buddlejaceae Buddleja – Buddleja/ 0.01–1.62 Calcicole 4.0–6.0 [12]

Buddleia

Pinales Taxaceae Taxus baccata Yew animal 60.5 Calcicole 5.5–7.5

Polypodiales Dryopteridaceae Dryopteris filix-mas Male fern – – –

Ranunculales Ranunculaceae Clematis vitalba Old man’s wind 2.10 Calcicole 5.5–8.0 [8, 15]
beard

Rosales Ulmaceae Celtis australis Nettle-tree animal 203.70 Calcicole 6.0–7.8 [8, 13]

Ulmus minor Elm wind 5.14 Calcicole 5.5–8.5 [8, 13]

Moraceae Ficus carica Fig animal 1.30 No 5.5–8.5 [8, 9, 13]

preference

Rosaceae Rubus – Bramble animal 0.028–10.40 Calcicole 5.0–6.0 [8, 12]

Rhamnaceae Rhamnus alaternus Buckthorn animal 20.27 Calcicole – [8]

Uricaceae Parietaria judaica Pellitory of wind/water/ 0.247 Calcicole – [8]

the wall animal

Sapindales Simaroubaceae Ailanthus altissima Tree of wind 29.40 Calcicole 5.5–8.5 [8, 13]
Heaven

Sapindaceae Acer pseudoplatanus Sycamore wind 97.00 – – [8]

Plants and Biodeterioration 223
Table 5.2 Vascular plants identified as being harmful inhabitants of historic structures in tropical regions [7]. Thousand seed weight data from [8].

Order Family Genus Species Common Name Dispersal Thousand Seed Weight, g

Brassicales Capparaceae Capparis flavicans – – [genus range = 3.54–220.00]

zeylanica Ceylon caper –

Fabales Fabaceae Vachellia nilotica Gum Arabic tree – 119.00

Albizia lebbeck Lebbek tree – 123.00
Senna occidentalis Senna coffee – 17.00
224 Biodeterioration of Concrete

Dalbergia sissoo North Indian rosewood – 41.80

Gentianales Apocynaceae Calotropis procera Apple of Sodom wind 9.57

Lamiales Lamiaceae Leucas biflora Two-flowered leucas – [genus range = 0.27–2.60]

Lamiales Orobanchaceae Lindenbergia indica Nettle-Leaved Lindenbergia – 0.041

Malpighiales Euphorbiaceae Croton bonplandianus Jungle tulsi – 7.77

Myrtales Lythraceae Woodfordia fruticosa Thathirippoovu – 0.28

Rosales Moraceae Ficus benghalensis Indian banyan – 1190.00

religiosa Bodhi tree – 0.54
rumphii – – –

Ulmaceae Holoptelea integrifolia Indian elm wind 34.00

Rhamnaceae Zizyphus jujuba Jujube –

Sapindales Meliaceae Azadirachta indica Neem – 241.00

 Plants and Biodeterioration 225

5.3.2 pH

As for the other microorganisms discussed in previous chapters, algae are

generally sensitive to pH, with acidic and alkaline environments limiting
growth. This is shown in Figure 5.6 which shows the growth rate of
populations of the freshwater algae Cryptomonas.
The sensitivity to pH varies between genera and species, and there are
a number of acidophilic algae. Alkaliphilic species, whilst seemingly less
common, are also encountered [20], but the species typically encountered
growing on concrete do not appear to have this characteristic.
Normally, maximum rates of algal growth tend to be observed around
a pH of 8, meaning that relatively young concrete does not represent a
friendly environment. However, carbonation and other processes which
reduce the pH of the cement matrix mean that this situation is usually a
temporary one. Figure 5.7 shows the progressive growth of populations
of Klebsormidium flaccidum on mortar surfaces in both carbonated and
uncarbonated conditions [21]. Complete occupancy of both types of mortar
eventually occurs, but this occurs much faster for the carbonated surface.
The experiment involved periodic trickling of a suspension of algae over
the mortar surfaces alongside periodic illumination with lamps to provide a
means of photosynthesis. Thus, the eventual occupancy of the uncarbonated
surface was probably the result of both leaching of alkaline species from
the mortar and carbonation as a result of exposure to the air between these
periods of exposure to algae.

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I- 0.0
s
0
~
0 -0 .1
LLJ
>
~ -0 .2
5
LLJ
~
•
-0 .3

•
-0.4 2;---------:4;----~6~----:--8-----.----
10 12

pH
Figure 5.6 Population growth rate of a species of Cryptomonas versus pH [19].
226 Biodeterioration of Concrete

100
~
0
I
~
::R
0 80 1 I
.•
I
u.i I I
u ell
<(

•
LL rp

''
0::: 60 I
:J I
(/) I
LL I
0 (])
I
UJ I
(!)
<(
0:::
40
(]!
t (
UJ I I
> I

I
(/)
0 20 I
u rp
I
I •
0
Uncarbonated
Carbonated

0
0 20 40 60 -- 80 100 120
TIME, days

Figure 5.7 Coverage by Klebsormidium flaccidum versus time for carbonated and
uncarbonated mortar surfaces [21].

Mosses typically prefer more acidic conditions than most plants.

Figure 5.8 shows the length of growth of the protonema (the early initial
structures formed by moss after germination—see Section 5.2) of the moss
Calymperes erosum. An optimum growth rate occurs around a pH of 4.5.
However, there are a number of alkaliphilic species which can tolerate a
somewhat higher pH. For instance, Scorpidium cossonii has been shown to
favour environments with a pH of 8.30 and tolerating conditions greater
than pH 9 [22]. Other research has reported moss growing on stone debris
from walls with a pH of around 10 [6].
The preferred pH conditions of vascular plants is highly species-
dependent. However, examining the preferred pH conditions of plants
identified as being harmful to buildings in Table 5.1, it is evident that the
midpoint of the range is typically located close to pH neutral conditions.
Some caution should be employed in interpreting these preferred pH
ranges, since plants are normally able to grow in more alkaline conditions
to the upper limit, albeit at a limited rate. Thus, if the shape of the plot
of shoot and root mass development by buddleia plants versus pH
(Figure 5.9) is examined, it is evident that growth is still likely beyond a pH
of 8.0. Nonetheless, as for the other examples discussed above, mature—and,
hence, carbonated concrete—is likely to better support growth.
 Plants and Biodeterioration 227

140
E
::t
<{ 120
~
w /----"
z
0 100 ' I ~
I- I \
0 I \
a: I
I \
\
11. 80
>-
a: I
I \
\
<:<: I \
~ 60 I ~\
0::
11. I '
•

.--
LL.
40 II ' '-
0 II '
J:
I- __../ .
(.9
20
z
w
....1

0
2 4 6 8
pH

Figure 5.8 Length of primary protonema grown from germinated Calymperes

erosum spores after 11 days under different pH conditions [23].

22

20

18 I
•
0
Shoots
Roots
I
/
/
........ -·--- ' '
~"
16
/ /
/
'
0)

~---
14 /
/ '•
J: /
(.9 12 /
Lij /
s 10 /
/

>-
•
/
a: 8
0
6
_ _ _ _ _ _ Q_ _ _ _ _ _

2 0' __..-
----- -o-- -o

0+-------.------.-------,------,-------,-----~
4.0 4.5 5.0 5.5 6.0 6.5 7.0

pH

Figure 5.9 Dry mass of the shoots and roots versus pH for Buddleia davidii plants
60 days after being transplanted to a pine bark medium. pH differences were
achieved through amendments of dolomitic lime [24].
228 Biodeterioration of Concrete

5.3.3 Nutrients

The essential macronutrient elements for plants are carbon, hydrogen,

oxygen, phosphorus, nitrogen, sulphur, potassium, calcium and magnesium
[25]. Hydrogen is obtained from water, which will be discussed in a later
section.
In the higher plants and many algae, carbon is obtained from carbon
dioxide, in which case, the shortage of sources of carbon that many
organisms living on concrete encounter will not be an issue. Carbon dioxide
is taken in by plants through the leaves, and through the cell membranes
by algae.
All of the remaining nutrients must be taken up by the roots of vascular
plants.
Being located underground, roots cannot undergo photosynthesis.
Thus, the energy for root growth is obtained from the sugars produced by
the leaves via cellular respiration. This requires oxygen, resulting in the
production of carbon dioxide. For plants growing on a concrete surface, the
availability of oxygen to roots is not an issue, unless the surface becomes
submerged by water. However, where roots of trees are growing beneath
concrete pavements, the lack of oxygen can have the effect of directing
the growth of roots upwards, which can present problems. This issue is
discussed later in this chapter.
Nitrogen, on the other hand, is likely to be in shorter supply, since this
usually needs to be in the form of nitrate or ammonium ions for higher
plants. There is very little nitrogen in any form in concrete, and so it must
come from an external source. However, plants may also obtain the element
from nitrogen gas, through the process of nitrogen fixation: the conversion
of nitrogen gas to ammonia.
Nitrogen fixation employs microorganisms as a means of converting
atmospheric nitrogen. These organisms may either be free and living in
close proximity to the plant roots, or for some plants, symbionts held
within root nodules or sometimes other structures within the plant. The
micro-organisms involved are most commonly bacteria, but can also be
cyanobacteria. Ammonia may subsequently be oxidised to nitrite, and then
nitrate, by nitrifying bacteria (See Chapter 3).
Algae must also obtain nitrogen in the form of nitrates of ammonia.
However, it is notable that populations of algae often develop co-existing
with those of cyanobacteria [26] whose ability to fix nitrogen is presumably
beneficial.
The availability of sulphur, potassium and magnesium have been
dealt with in some detail in Chapter 3. However, it is worth revisiting the
availability of calcium in concrete due to its abundance, and also because
of its importance as a nutrient to a number of plants.
 Plants and Biodeterioration 229

The higher plants can broadly be categorised as either calcicoles or

calcifuges. Calcifuges grow in soils with a low calcium concentration and, in
some cases, may grow less well if this concentration is increased. Calcicoles,
on the other hand, grow in soils with high calcium concentrations and
display symptoms of calcium deficiency when this element is scarce [27].
Table 5.1 also identifies whether each plant listed is calcicole or calcifuge,
with the vast majority falling into the former category. Thus, it can be
expected that most of the plant species growing on concrete structures will
be calcicole in nature.
One of the elements present in concrete which has the potential to make
concrete a less hospitable environment to plants is aluminium. Aluminium-
whilst a potentially beneficial nutrient in small quantities—is toxic to plants
[28]. Concentrations sufficient to cause detrimental effects are not normally
encountered under pH conditions greater than 5, making this a lesser threat
to plants growing on concrete. However, it is worth noting that one defence
that plants seemingly employ to combat aluminium toxicity is the exudation
of organic acids [29]. These acids include citric, malic, oxalic, tartaric and
fumaric acid [30], but others may also be exuded. It is proposed that these
acids form complexes with aluminium in solution, rendering it less available
for uptake by the plant and, hence, less toxic.
However, whilst increased aluminium concentrations (and sometimes
concentrations of other metals) stimulate the release of these acids, this is
also stimulated by the absence of phosphorus. This indicates that the acids
most probably have two roles: limiting aluminium toxicity and increasing
phosphate availability [30]. It has been proposed that the release of these
acids solubilises phosphate through the formation of stronger complexes
than phosphate with aluminium, iron and calcium and adsorbing on
charged surfaces that would otherwise attract phosphate.
Thus, whilst exudation of organic acids as a result of high aluminium
concentrations is an unlikely requirement in most cases for plants growing
on concrete, the typically low concentrations of phosphate present in cement
and aggregate might be sufficient to stimulate this. Given the damaging
effect that citric acid can have on concrete (see Chapter 4), release of acid
from roots has the potential to contribute towards deterioration.
Calcicole plants typically produce larger quantities of organic acid. A
series of experiments which examined both the nature and concentrations
of different organic acids produced from the roots of two calcicole and two
calcifuge species found that acetic, lactic, malic, succinic, oxalic and citric
acid were produced by the two calcicole species (Gypsophilia fastigata and
Sanguisorba minor) [31]. Whilst the calcifuge species (Deschampsia flexuosia
and Viscaria vulgaris) did exude most of these acids to some extent, the
quantities were much less. Gypsophilia fastigata also produced significant
quantities of tartaric acid. Further experiments revealed that oxalic acid
230 Biodeterioration of Concrete

had the effect of solubilising phosphate, whilst citric acid solubilised iron
and manganese from the soils used.
Heterotrophic algae have also been found to produce organic acids.
These include formic, pyruvic, lactic and succinic acids. One study
examining acid formation by three types of green algae (Chlorella vulgaris,
Scenedesmus basilensis and a species of Chlamydomonas) [32]. Under aerobic
conditions with glucose as the source of energy, pyruvic acid was produced
in the largest quantities by Chlorella and Scenedesmus. Chlamydomonas
produced formic acid in larger concentrations under aerobic conditions,
albeit in much smaller concentrations overall. Under anaerobic conditions,
all three algal cultures produced lactic acid. These acids are most likely
formed as products of metabolising glucose, rather than as a means of
adjusting the surrounding conditions, as in the case of plant roots. However,
it should be noted that even under autotrophic conditions (i.e., in the absence
of glucose) the acids were all produced by the mixotrophic Chlorella vulgaris,
although in much smaller concentrations.
One possible example of exudation of organic acids by algae being
used functionally is in the case of cryptoendolithic algae which appear to
live in layers just beneath rock surfaces as a means of limiting the harmful
effects of extreme environmental conditions such as high or low ambient
temperatures and low humidity [33]. It has been proposed that these
recesses—and possible subsequent biodegradation of the stone—have been
formed through the exudation of acids [34]. This strategy demands that the
rock is sufficiently optically clear to permit sunlight to pass through the
surface to allow the algae to photosynthesize, with sandstones proving ideal
[33]. Whether typical concrete formulations contain suitable aggregates to
permit this is uncertain.
One nutrient required by a specific group of algae is silicon. The
group in question is the diatoms, which are unicellular algae found in
both freshwater and marine environments. The silicon is used by diatoms
to form a porous, symmetrical cell wall made of hydrated silica known as
a frustule. There is obviously a quantity of silicon present in cement and
aggregate. Generally the Si in the cement matrix will be present as CSH
gel, and will be more soluble than that present in the aggregate. However,
the solubility of Si in CSH is still low. Whilst, as will be seen in Section 5.4,
diatoms may be found in close proximity to concrete structures, diatoms do
not attach to surfaces, but instead remain suspended in water. Moreover,
there is no evidence that diatoms produce substances to render Si more
available. Thus, concrete does not provide a habitat for diatoms, and is
unlikely to be affected by their proximity.
It is generally believed that the rhizoids of moss solely perform the
function of anchors. Rhizoids appear to be capable of penetrating rock,
mortar and concrete, but seemingly only if the porosity is sufficient to allow
 Plants and Biodeterioration 231

this [35]. Many bryophyta—especially of the Sphagnum genus—produce

quantities of secondary metabolites including phenolic compounds and
carbohydrates [36]. Only some of these substances are released by the moss
and many of the compounds are present as soluble constituents in the cells
or as components of the cell walls.
The phenolic compounds appear to play a number of roles. Firstly, they
protect the moss from being eaten by herbivores, through an unpleasant
taste, toxicity and an ability to ‘mask’ cellulose for digestion. Additionally,
it is believed that the compounds can inhibit the growth of vascular plants.
Finally their presence tends to acidify the soil, to produce conditions which
are optimal for the moss.
The carbohydrates, which include uronic acids, appear to be able to
undergo cation exchange of protons with nutrient ions including calcium
and magnesium. Additionally, cation exchange leads to the release of
protons which further acidify the substrate.
It would appear that a lower calcium concentration in the substrate
promotes the development of a higher cation exchange capacity in the
cell walls [37]. Sphagnum mosses are also known as peat mosses, and are
encountered in peat bogs. However, other moss species also form similar
compounds, albeit often in lesser quantities.

5.3.4 Concrete as a habitat for plants

Algae

The availability of moisture is a fundamental requirement of algae, and as

a result it might be expected that water/cement ratio is likely to play an
important role in defining how hospitable a concrete surface is. This has
already been seen in Chapter 3 (Figure 3.3) for mixed communities of algae
and cyanobacteria. The only study in which algae were the sole occupier, the
experimental programme utilised mortars with relatively high ratios and
used test conditions which were unlikely to allow the mortar surface to dry
to any great extent [21]. As a result, rates of growth were almost identical.
However, it is reasonable to assume that a higher water/cement ratio will
produce a surface which is more amenable to algal growth.
The same study investigated the influence of surface roughness and
found a rougher surface allowed faster rates of colonisation by algae.

Vascular plants

Water is also of key importance to vascular plants, and so the presence of

moisture within the concrete surrounding a plant is conducive to survival
and growth. Whilst it is likely that concrete with a high water/cement
ratio would provide a larger reservoir of water to plants—as for other
232 Biodeterioration of Concrete

organisms—this effect has not been investigated. However, it is likely that

other factors will also play a role in providing a moist environment. In
particular, the establishment of bryophytes (see 5.3.1) is likely to provide
the means for the retention of water close to the surface.

5.4 Plants on Concrete

Whilst a significant amount of research has been conducted into the
identification of algae occupying concrete surfaces, data on bryophytes is
limited to a small number of studies, whilst coverage of vascular plants is
non-existent. In the case of vascular plants, there is at least the data provided
in Table 5.1, for other substrates comparable in some regards to concrete.
This section focusses, therefore, on algae and bryophytes.

5.4.1 Algae

Table 5.3 lists the species identified by studies examining algal growth
on concrete and mortar surfaces either in the field or under laboratory
conditions.

5.4.2 Bryophytes

There have been a limited number of studies on the characterisation of

bryophyte species growing on concrete surfaces. This limitation is further
compounded by the fact that these studies are largely limited to a relatively
narrow geographical area. Nonetheless, many of these studies have studied
extensive sites and have been extremely thorough in the characterisation
process. Thus, whilst the list of species provided in Table 5.4 is a long one,
it must be recognised that the full list of species that have the potential to
grow on concrete is considerably larger.

5.5 Damage from Root Growth

The main process which leads to damage to structures by vascular plants is
the radial growth of roots and woody stems in confined spaces within which
a plant is growing. This type of physical damage is, of course, not unique
to concrete structures, and reports of plant damage to concrete structures is
not particularly well reported in academic literature, although its occurrence
is without question. For this reason, aspects of the discussion below have
been extended to include sources involving construction materials other
than concrete, where the findings are still relevant.
The pressures exerted by roots as they grow can act both radially
(outwards) and longitudinally (at the root tip). The magnitude of these
 Plants and Biodeterioration 233

Table 5.3 Algae identified growing on concrete surfaces, or successfully grown in the laboratory.

Genus Species Class/Order/Family Habitat Location References

Aglaothamnion – Florideophyceae/ Marine Finland [46]
Ceramiales/
Callithamniaceae

Apatococcus lobatus Chlorophyta/ Freshwater France [26]

Trebouxiophyceae/
Chlorellales
Ascophylum nodosum Phaeophyceae/ Marine Fylde coast, [39]
Fucales/Fucaceae UK
Bracteacoccus – Chlorophyta/ Freshwater France [26]
Chlorophyceae/
Chlorococcales
Ceratium – Dinophyceae/ Marine [46]
Gonyaulacales/
Ceratiaceae
Chlorella ellipsoidea Chlorophyta/ Freshwater France/ [26, 43]
Trebouxiophyceae/ Laboratory
homosphaera Chlorellales France [26]
minutissima Boersch, [3, 26]
France/
France
vulgaris Laboratory [41]
mirabilis Laboratory/ [42, 26]
France
Chlorosarcinopsis eremi Chlorophyta/ Freshwater France [26]
Chlorophyceae/
minor Chlorosarcinales France [26]

Choricystis minor Chlorophyta/ Freshwater France [26]

Chlorophyceae/
chodatii Chlamydomonadales France [26]

Coccobotrys verrucariae Chlorophyta/ Freshwater France [26]

Chlorophyceae/
Chaetophorales
Coccomyxa olivacea Chlorophyta/ Freshwater France [26]
Chlorophyceae/
Chlamydomonadales
Desmococcus olivaceus Chlorophyta/ Freshwater France [26]
Trebouxiophyceae/
Prasiolales
Fucus spiralis Heterokontophyta/ Marine Fylde coast, [39]
Phaeophyceae/ UK
Fucales
Geminella terricola Chlorophyta/ Freshwater France [26]
Trebouxiophyceae/
Chlorellales
Halidrys siliquosa Ochrophyta/ Marine Fylde coast, [39]
Phaeophyceae/ UK
Fucales

Table 5.3 contd. ...

234 Biodeterioration of Concrete

...Table 5.3 contd.

Genus Species Class/Order/Family Habitat Location References
Hormotila mucigena Chlorophyta/ Freshwater France [26]
Chlorophyceae/
Chlorococcales
Keratococcus bicaudatus Chlorophyta/ Freshwater France [26]
Trebouxiophyceae/
Chlorellales
Klebsormidium – Charophyta/ Freshwater Dorset, [44]
Klebsormidiophyceae/ UK/Porto
Klebsormidiales Alegre,
Brazil

flaccidum Archamps, [3, 21, 42]

France/
Nantes,
France/
Laboratory
pseudostichococcus France [26]
subtile Toulouse, [40]
France
Microspora – Chlorophyta/ Freshwater Toulouse, [40]
Microsporales/ France
Chlorophyceae
Palmellopsis gelatinosa Chlorophyta/ Freshwater France [26]
Chlorophyceae/
Chlamydomonadales
Pleurochrysis carterae Prymnesiophyceae/ Freshwater Laboratory [47]
Isochrysidales/
Pleurochrysidae
Pleurococcus – Chlorophyta/ Freshwater Dorset, UK/ [44]
Chlorophyceae/ Carmona,
Chaetophorales Spain/
Valladolid,
Mexico/
Porto Alegre,
Brazil
Polysiphonia – Florideophyceae/ Marine Finland [46]
Ceramiales/
Rhodomelaceae
Sphacelaria – Phaeophyceae/ Marine Finland [46]
Sphacelariales/
Sphacelariaceae
Stichococcus – Chlorophyta/ Freshwater Dorset, UK/ [44]
Trebouxiophyceae/ Valladolid,
Microthamniales Mexico/
Porto Alegre,
Brazil
bacillaris Archamps, [3, 26, 42,
France/ 43]
Laboratory/
France

Table 5.3 contd. ...

 Plants and Biodeterioration 235

...Table 5.3 contd.

Genus Species Class/Order/Family Habitat Location References
Trebouxia – Chlorophyta/ Freshwater France [26]
Trebouxiophyceae/
Microthamniales
Trentepohlia – Chlorophyta/ Freshwater Dorset, UK/ [40, 43, 44]
Ulvophyceae/ Carmona,
Trentepohliales Spain/
Valladolid,
Mexico/
Porto
Alegre,
Brazil/
Toulouse,
France/
Portugal
iolithus France [26, 48]
Ulva – Chlorophyta/ Marine Fylde coast, [39]
Ulvophyceae/Ulvales UK
fasciata Marine Puducherry, [38]
India
Porphyra umbilicalis Rhodophyta/ Marine Fylde coast, [39]
Rhodophyceae/ UK
Bangiales
Diatoms
Unidentified species – Marine Fylde coast, [39]
UK
Aulacoseira – Bacillariophyta/ Freshwater Argentina [45]
Coscinodiscophyceae/
Aulacoseirales

pressures is relatively small (between 0.7 and 2.5 MPa) [52]. However,
given that a root growing into a concrete crack will be exerting pressure
in a manner which will be resolved as a tensile stress in the concrete itself,
there still exists the potential for damage.
The tensile strength of concrete is notably low. The following equation
has been proposed for the estimation of tensile strength of concrete from
compressive cylinder strength [52]:
ft = 0.3fc 32
where ft and fc are the tensile strength and compressive cylinder strength
respectively. Thus, concrete with a cylinder strength of 30 MPa will have a
tensile strength of around 2.9 MPa. Furthermore, given that the tips of cracks
will also act as sources of stress concentration, the widening of fissures in
concrete is less of a challenge for plants than it might immediately seem.
The production of root exudates by plants is likely to exacerbate the
magnitude of damage that can be done by a plant, particularly where
the substances produced include compounds such as citric acid, whose
236 Biodeterioration of Concrete

Table 5.4 Bryophytes identified growing on concrete surfaces.

Genus Species Class/Order Location References

Abietinella abietina Bryopsida/ Belarus [51]

Hypnales/
Thuidiaceae
Amblystegium serpens Bryopsida/ Western [50, 51]
Hypnales/ Caucasus,
Amblystegiaceae Russia/Belarus
varium Western [50]
Caucasus, Russia
Anomodon viticulosus Bryopsida/ Western [49]
Hypnales/ Caucasus, Russia
Thuidiaceae
Atrichum undulatum Polytrichopsida/ Belarus [51]
Polytrichales/
Polytrichaceae
Barbula unguiculata Bryopsida/Pottiales/ Western [50, 51]
Pottiaceae Caucasus,
Russia/Belarus
Brachythecium albicans Bryopsida/ Belarus [51]
Hypnales/
campestre Brachytheciaceae Belarus [51]
mildeanum Belarus [51]
populeum Western [50]
Caucasus, Russia
rivulare Belarus [51]
rotaeanum Western [50]
Caucasus, Russia
rutabulum Western [50, 51]
Caucasus,
Russia/Belarus
salebrosum Belarus [51]
starkei Belarus [51]
Brachytheciastrum velutinum Bryopsida/ Belarus [51]
Hypnales/
Brachytheciaceae
Bryoerythrophyllum recurvirostrum Bryopsida/Pottiales/ Belarus [51]
Pottiaceae

Table 5.4 contd. ...

 Plants and Biodeterioration 237

...Table 5.4 contd.

Genus Species Class/Order Location References
Bryum algovicum Bryopsida/Bryales/ Belarus [51]
Bryaceae
argentium Western [50, 51]
Caucasus, Russia
/Belarus
bimum Western [50]
Caucasus, Russia
caespiticum Belarus [51]
capillare Belarus [51]
creberrimum Belarus [51]
funckii Lithuania [49]
klinggraeffii Belarus [51]
moravicum Belarus [51]
schleicheri Belarus [51]
warneum Belarus [51]
Callicladium haldanianum Bryopsida/ Belarus [51]
Hypnales/
Hypnaceae
Calliergonella cuspidata Bryopsida/ Belarus [51]
Hypnales/
Hypnaceae
Campyliadelphus chrysolphyllum Bryopsida/ Belarus [51]
Hypnales/
Amblystegiaceae
Campylidium sommerfeltii Bryopsida / Belarus [51]
Hypnales /
Amblystegiaceae
Campylium stellatum Bryopsida/ Belarus [51]
Hypnales/
Amblystegiaceae
Ceratodon purpureus Bryopsida/ Belarus [51]
Dicranales/
Ditrichaceae
Chiloscyphus latifolius Jungermanniopsida/ Belarus [51]
Jungermanniales/
polyanthus Lophocoleaceae Belarus [51]

Climacium dendroides Bryopsida/ Belarus [51]

Hypnales/
Climaciaceae

Table 5.4 contd. ...

238 Biodeterioration of Concrete

...Table 5.4 contd.

Genus Species Class/Order Location References

Cololejeunea rossetiana Jungermanniopsida/ Western [50]
Porellales/ Caucasus, Russia
Lejeuneaceae
Conocephalum conicum Marchantiopsida/ Belarus [51]
Marchantiales/
Conocephalaceae
Ctenidium molluscum Bryidae/Hypnales/ Western [50]
Hylocomiaceae Caucasus, Russia
Dicranella cerviculata Bryopsida/ Belarus [51]
Dicranales/
heteromalla Dicranellaceae Belarus [51]

Dicranum flagellare Bryopsida/ Belarus [51]

Dicranales/
scoparium Dicranaceae Belarus [51]

Didymon rigidulus Bryopsida/Pottiales/ Belarus [51]

Pottiaceae
Dreponacladus polycarpus Bryopsida/ Belarus [51]
Hypnales/
Amblystegiaceae
Encalypta streptocapra Bryopsida/ Belarus [51]
Encalyptales/
Encalyptaceae
Eurhynchium angustirete Bryopsida/ Belarus [51]
Hypnales/
crassinervium Brachytheciaceae Western [50]
Caucasus, Russia
Fissidens adianthoides Bryopsida/ Belarus [51]
Dicranales/
dubius Fissidentaceae Lithuania [49]

Fontinalis - Bryopsida/ Argentina [45]

Hypnales/
Fontinalaceae
Funaria hygrometrica Bryopsida/Funariales/ Belarus [51]
Funariaceae
Grimmia muehlenbeckii Bryopsida/ Belarus [51]
Grimmiales/
pulvinata Grimmiaceae Western [50]
Caucasus, Russia
Gymnostomum aeruginosum Bryopsida/Pottiales/ Western [50]
Pottiaceae Caucasus, Russia
Hedwigia ciliata Bryopsida/ Belarus [51]
Leucodontales/
Hedwigiaceae

Table 5.4 contd. ...

 Plants and Biodeterioration 239

...Table 5.4 contd.

Genus Species Class/Order Location References
Homalia trichomanoides Bryopsida/ Belarus [51]
Hypnales/
Neckeraceae
Homalothecium lutescens Bryopsida/ Belarus [51]
Hypnales/
sericeum Brachytheciaceae Western [50]
Caucasus, Russia
Homomallium incurvatum Bryopsida/ Lithuania [49]
Hypnales/Hypnaceae
Hygroamblystegium juratzkanum Bryopsida/ Belarus [51]
Hypnales/
varium Amblystegiaceae Belarus [51]

Hygrohypnum luridum Bryopsida/Hypnales/ Western [50]

Campyliaceae Caucasus, Russia
Hylocomium splendens Bryopsida/Hypnales/ Belarus [51]
Hylocomiaceae
Hypnum cupressiforme Bryopsida/ Belarus [51]
Hypnales/Hypnaceae
Jungermannia atrovirens Jungermanniopsida/ Western [50]
Jungermanniales/ Caucasus, Russia
Jungermanniaceae
Leskea polycarpa Bryopsida/ Belarus [51]
Hypnales/Leskeaceae
Leptobryum pyriforme Bryopsida/Bryales/ Belarus [51]
Bryaceae
Leucodon sciuroides Bryopsida/ Belarus [51]
Leucodontales/
Leucodontaceae
Marchantia polymorpha Marchantiopsida/ Belarus [51]
Marchantiales/
Marchantiaceae
Mnium marginatum Bryopsida/Bryales/ Belarus [51]
Mniaceae
Orthotrichum anomalum Bryopsida/ Belarus [51]
Orthotrichales/
cupulatum Orthotrichaceae Belarus [51]
diaphanum Belarus [51]
gymnostomum Belarus [51]
obtusifolium Belarus [51]
pallens Belarus [51]
patens Belarus [51]
pumilum Belarus [51]
speciosum Belarus [51]

Table 5.4 contd. ...

240 Biodeterioration of Concrete

...Table 5.4 contd.

Genus Species Class/Order Location References
Oxyrrhynchium hians Bryopsida/ Western [50, 51]
Hypnales/ Caucasus,
Brachytheciaceae Russia/Belarus
Oxystegus tenuirostris Bryopsida/Pottiales/ Western [50]
Pottiaceae Caucasus, Russia
Platyhypnidium riparioides Bryopsida/ Western [50]
Hypnales/ Caucasus, Russia
Brachytheciaceae
Pohlia nutans Bryopsida/Bryales/ Belarus [51]
Mniaceae
Pseudoleskeella catenulata Bryopsida/ Lithuania [49]
Hypnales/
nervosa Pseudoleskeellaceae Belarus [51]

Plagiochila porelloides Jungermanniopsida/ Belarus [51]

Jungermanniales/
Plagiochilaceae
Plagiomnium affine Bryopsida/Bryales/ Belarus [51]
Mniaceae
cuspidatum Belarus [51]
elatum Belarus [51]
ellipticum Belarus [51]
rostratum Western [50]
Caucasus, Russia
undulatum Belarus [51]
Plagiothecium laetum Jungermanniopsida/ Belarus [51]
Jungermanniales/
Plagiochilaceae
Pleurozium schreberi Bryopsida/Hypnales/ Belarus [51]
Hylocomiaceae
Polytrichum formosum Polytrichopsida/ Belarus [51]
Polytrichales/
juniperinum Polytrichaceae Belarus [51]

Pylaisia polyantha Bryopsida/ Western [50, 51]

Hypnales/ Caucasus,
Pylaisiaceae Russia/Belarus
Racomitrium canescens Bryopsida/ Belarus [51]
Grimmiales/
Grimmiaceae
Rhynchostegiella teneriffae Bryopsida/ Western [50]
Hypnales/ Caucasus, Russia
Brachytheciaceae

Table 5.4 contd. ...

 Plants and Biodeterioration 241

...Table 5.4 contd.

Genus Species Class/Order Location References
Rhynchostegium confertum Bryopsida/ Western [50, 51]
Hypnales/ Caucasus,
Brachytheciaceae Russia/Belarus
Rhytidiadelphus squarrosus Bryopsida/ Belarus [51]
Hypnales/
triquetrus Hylocomiaceae Belarus [51]

Sanionia uncinata Bryopsida/Hypnales/ Belarus [51]

Scorpidiaceae
Sciuro-hypnum oedipodium Bryopsida/ Belarus [51]
Hypnales/
populeum Brachytheciaceae Belarus [51]

Serpoleskea subtilis Bryopsida/ Belarus [51]

Hypnales/
Amblystegiaceae
Stereodon fertilis Bryopsida/ Belarus [51]
Hypnales/
pallescens Hypnaceae Belarus [51]

Schistidium apocaprum Bryopsida/ Belarus [51]

Grimmiales/
crassipilum Grimmiaceae Belarus [51]
Syntrichia papillosa Bryopsida/Pottiales/ Lithuania [49]
Pottiaceae
ruralis Belarus [51]
virescens Belarus [51]
Thuidium assimile Bryopsida/Hypnales/ Belarus [51]
Thuidiaceae
Tortella tortuosa Bryopsida/Pottiales/ Belarus [51]
Pottiaceae
Tortula – Bryopsida/Pottiales/ Portugal [43]
Pottiaceae
mucronifolia Belarus [51]
muralis Belarus [51]
obtusifolia Lithuania [49]
Weissia contraversa Bryopsida/Pottiales/ Belarus [51]
Pottiaceae

damaging effects have already been discussed in Chapter 4. Research into

this aspect of deterioration currently appears to be non-existent. Most of
the other acids produced by plant roots have been dealt with in previous
chapters. However, formic and fumaric acid have not. Examination of
the nature of the interaction between these acids and calcium, iron and
242 Biodeterioration of Concrete

aluminium ions in Chapter 2 indicates that their effect will largely be that
of acidolysis.
A special case with regards to interaction of vascular plants with
concrete structures is that of climbing plants (Figure 5.10). There has been a
great deal of debate with regards to whether such plants are harmful to the
walls on which they grow, with particular attention to the ivy, Hedera helix.
One study of ivy growing on a specially constructed stone test wall found
that the plant was not particularly predisposed to enter holes and cracks
[54]. However, observation of ivy on historic structures has identified many
instances where the roots enter existing voids in walls leading to damage
[54, 55]. It has been proposed that this process is promoted by shortages of
water, and that events such as the cutting off of the stem of the plant at the
base may cause smaller rootlets to permeate such voids more aggressively
to obtain water [54].
The types of voids which are likely to be amenable to plants such as
ivy are likely to be far scarcer on a concrete structure, compared to a stone
wall, but might include joints, or cracks resulting from other forms of
deterioration. Where the incursion of roots into the fabric of a structure is
unlikely, there is some evidence that ivy provides protection to buildings,
principally in its ability to capture air-borne particulates before they become
attached to the building surface. It is argued that this has the effect of
keeping the surfaces themselves clean, which may also have the additional
benefit of limiting the delivery of nutrients to the surface which might
otherwise be used by micro-organisms in biodeterioration processes [56].

Figure 5.10 A climbing plant on a concrete wall.

 Plants and Biodeterioration 243

Other climbing plants attach themselves to vertical surfaces using

suckers. These suckers sometimes exude organic acids. For instance, the
Virginia creeper (Parthenocissus quinquefolia) exudes oxalic acid from its
suckers and other parts [57]. It has been suggested that exudation of acid
by suckers can potentially damage the underlying wall [83]. However,
doubts have been expressed, certainly with regards to whether oxalic acid
production will lead to deterioration [57].
Another issue related to root growth is the damage to concrete
pavements, foundations and pipes by tree roots. The growth of roots
is generally directed downwards and away from sunlight (negative
phototropism) [58]. However, the direction of growth is also influenced
by concentration gradients of water, nutrients and oxygen. Where areas
around trees are paved with concrete, or other surfacing materials, the
highly obscuration of sunlight is unlikely to suppress upward growth.
Moreover, impermeable paving materials tend to prevent the evaporation
of water from soil, leading to condensation of water at the soil–concrete
interface. Thus, root growth very close to this interface is not uncommon.
The problem is often made worse by the presence of highly compacted
soil beneath a pavement which resists root penetration and contains limit
concentrations of oxygen. Subsequent radial growth of the roots as the tree
matures can then lead to the generation of flexural stress directly underneath
the concrete, producing cracking and buckling.
Another mechanism by which tree roots can damage structures is
through the uptake of water from soils prone to shrinkage. Whether a soil
can undergo significant shrinkage is dependent on whether expansive clays
(such as montmorillonites) are present, the size of the clay particles (with
smaller particle sizes yielding greater shrinkage) and the extent to which
clay particles are aligned in a preferred orientation within the soil (with a
poor degree of alignment leading to larger quantities of shrinkage [58]).
Where trees are present in such soils, their uptake of water will lead
to accelerated drying during dry periods, which is liable to lead to an
amplification in the magnitude of shrinkage. This can present problems for
the foundations of nearby low-rise buildings [59] since movement of the soil
can generate stresses in foundations which can cause substantial damage.
The extent to which trees threaten foundations is partly dependent on
the water demand of the species of tree. Trees with particularly high water
demand include elm, oak, poplar, willow and cypress species [60].
It is usually convenient, for a number of reasons, for the movement of
water across distances (either as drinking water, sewage or drainage water)
to be conducted via pipes located underground. However, their need to
locate water means that the roots of plants will grow towards underground
pipes and through the mechanisms already discussed, invade pipes, usually
through joints. This has two detrimental effects. Firstly, once breached, the
244 Biodeterioration of Concrete

pipe will leak, with environmental and efficiency implications. Secondly, the
pipe will gradually become blocked by the invading root (see Figure 5.11).
A number of studies have been conducted into this phenomenon, with
many taking the form of surveys of water networks in specific geographical
locations. Such studies have been made possible through the use of mobile
cameras that can be run through pipes to inspect their interior. One such
study, conducted in Sweden, examined both the frequency of intrusion
and related each instance of intrusion to tree and shrub species growing in
close proximity to the pipes [62]. The species found to be most frequently
responsible for intrusions were Malus floribunda (Japanese flowering
crabapple) and Populus candensis (Canadian poplar). The two main types of
pipe encountered in the survey were concrete and PVC, and it is interesting
to note that the concrete pipes were considerably more resistant to intrusion
than the polymer pipes. The survey detected 0.661 root intrusions per pipe
joint in the case of PVC, whilst the frequency was only 0.080 in the case
of concrete. This may reflect the greater mass of the concrete pipes which
would be likely to present greater resistance to the growth pressures of roots.
A similar study conducted in Poland made a similar observation,
although in this case the two types of pipe encountered were concrete and
vitrified clay [61]. The frequency of intrusion was expressed in a slightly
different form (intrusions per 100 m). Concrete pipes proved more resistant,
with an intrusion rate of 3.24 intrusions per 100 m, compared to a value of
3.99 for vitrified clay pipes. Smaller diameter pipes are also more vulnerable
to intrusion [63].

Figure 5.11 Root intrusion in a pipe [61].

 Plants and Biodeterioration 245

A survey of pipes in Melbourne, Australia found that the two tree

species most likely to be involved in root intrusion were of the genera
Eucalyptus and Melaleuca [64].
Reports of the deterioration of concrete surfaces resulting from the
establishment of moss are scarce and based purely on observation. Where
this does appear to have occurred, it has been attributed to the penetration
of rhizoids into the material. A study of lichen and moss on cathedrals in
Spain found that bryophytes growing on the mortar of stonework appeared
to be responsible for its disintegration in this way [65]. It should be noted,
however, that penetration of rhizoids may have occurred through cracks
resulting from prior deterioration.

5.5 Deterioration from Algae

The issue of whether the attack of concrete by algae is a real phenomenon is
still to be decided. It is certainly true that the cement matrix—and possibly
the aggregate—of concrete contain nutrients for algae, and the uptake of
these nutrients will lead to the development of porosity in the material [66].
Moreover, the production by algae of organic acids likely to accelerate this
process is confirmed. However, what is theoretically possible is only partly
supported by scientific evidence.
Whilst a few of studies have examined the interaction of algae with
concrete, with the assumption that the interaction is one of biodeterioration,
this has typically not included quantitative measurements of loss of
mechanical properties or mass. One group of researchers have identified
chemical changes in concrete surfaces resulting from colonisation by marine
algae–probably Ulva fasciata [38, 67]. The results of energy dispersive X-ray
spectroscopy and powder X-ray diffraction analysis certainly indicate
a change in the composition of colonised concrete surfaces relative to
similar surfaces exposed to only water. Specifically, there appears to be an
increase in the quantity of calcium at the concrete surface at the expense
of other elements. Whether this is the result of removal of material or the
precipitation of calcium-rich material at the surface is not clear, although
the presence of higher concentrations of calcite at the colonised surfaces
points to the latter. The influence of these changes on concrete properties
was not investigated further.
Another study has employed microscopy as a means of observing the
manner in which marine algae interact with concrete armour deployed in
coastal protection applications [39]. The study revealed the presence of the
algae species Ascophylum nodosum, Fucus vesiculosus, Fucus spiralis, Porphyra
umbilicalis, Halidrys siliquosa and species of the genus Ulva. Microscopy
revealed the growth of algal filaments in the interface between fine aggregate
and the cement matrix, with the filaments also growing through the cement
matrix in places. The researchers propose that this process of growth may
246 Biodeterioration of Concrete

contribute towards the erosion of such materials, alongside the abiotic

processes acting on coastal protection structures in a coastal environment.
They also propose that the use of marine aggregate already carrying algae
may introduce these organisms into concrete, with the algae surviving the
initial high pH conditions in concrete as dormant forms such as spores. Both
hypotheses are credible, but evidence of either is very limited.
It has also been argued that the presence of algae may also protect
concrete surfaces from the mechanical action of the marine environment
which can cause deterioration [68].
Research has examined the interaction of marine algae with macro- and
micro-synthetic fibres used within similar concrete components. In this
case, deterioration was more conclusively demonstrated [69, 70]. The macro
fibres included were composed of both polyethylene and polypropylene,
whilst the microfibers were polyethylene. Colonising the concrete surface
was algae from the genus Ulva. Microscopy identified numerous instances
where the algal filaments had grown into the fibres leading to a loss of cross-
sectional area. It has been demonstrated that algae can use polyethylene
as a substrate, although whether they can utilise carbon in the polymer
is unclear [71]. The researchers detail a proposed mechanism by which
deterioration of fibres may ultimately lead to more substantial loss of mass
from the concrete as a whole.
Algae has also been blamed for accelerated corrosion of steel
reinforcement in concrete [66]. The proposed mechanism for this process
centres around the photosynthetic nature of algae. During periods of
daylight, algae will undergo photosynthesis, producing oxygen. Thus, the
presence of algae at the surface of a reinforced concrete structural element
may produce localised oxygen enrichment. Such enrichment has the
potential to create an ‘aeration cell’ in which two locations around a steel
reinforcement bar experience very different oxygen concentrations. This can
lead to an electrochemical cell being established with localised corrosion
producing pitting of the steel.
There is no question that microbially induced corrosion of exposed metal
surfaces is a real phenomenon [72]. However, whether it is a mechanism
that can lead to corrosion of steel embedded in reinforced concrete is
less certain. The same researchers who propose this mechanism have
conducted a preliminary investigation comparing the corrosion potential
of steel embedded in concrete cylinders (with a cover depth of 20 mm) and
submerged in a bioreactor illuminated with a light source operating under
a 12 hours on—12 hours off regime. The bioreactor contained a culture
medium in which the algae Pleurochrysis carterae had been cultured.
Two types of cement were evaluated—Portland cement and a
geopolymer cement made using fly ash. In the case of the Portland cement
concrete, the corrosion potential of the steel maintained a very slightly
negative value (indicative of a low potential for corrosion) throughout the 14
 Plants and Biodeterioration 247

day experimental period. Evaluation of cell numbers showed that the algae
were completely eradicated by 7 days, presumably as a result of the high pH
of the culture medium (around 10) originating from the Portland cement.
Where the geopolymer concrete was exposed to the culture medium in the
absence of algae, the corrosion potential rapidly dropped to a more negative
value, indicating a greater potential for corrosion. This was presumably
either due to the lower pH of the cement limiting passivation of the steel
surface or the higher porosity of the cement matrix, or a combination of
both. However, when algae were present the corrosion potential remained
at a level comparable to the Portland cement concrete until the end of the
14-day experiment, at which point there was an abrupt drop to a more
negative value. This drop approximately coincided with the extinction
of the algae population, leading the researchers to propose that the algae
might, in fact, be protecting the steel by maintaining a higher pH through
oxygen production.
Regardless of the precise nature of the processes occurring in these
experiments, there would appear to be no reported examples of such a
process occurring in the field. The main argument against such a process
is that, for it to be effective, there would need to be considerable diffusion
of oxygen into the concrete towards the steel surface. This is because algal
activity will be limited to a zone very close to the concrete surface, due to
the need for light. Because of the uncertainties surrounding this proposed
form of biodeterioration, it is not explored further in this chapter.
Nonetheless the presence of algae on concrete surfaces is often
prominently visible and the resulting stain can be detrimental in aesthetic
terms, particularly on lighter surfaces. Moreover, the presence of algae on
horizontal surfaces on which people walk can be slippery and consequently
hazardous.

5.6 Limiting Biodeterioration from Plants

It is hopefully evident that algal growth on concrete surfaces and the
establishment and growth of vascular plants on concrete structures are very
different processes. For this reason, it is necessary to distinguish between
the measures which can be used against algae and vascular plants.

5.6.1 Limiting algal growth

Approaches to controlling algal growth on concrete can be categorised in

terms of whether they involve control of environmental conditions, concrete
composition or treatments applied to the hardened concrete surface. These
different approaches are discussed separately below.
248 Biodeterioration of Concrete

Environmental factors

Aside from the nutrient needs of algae, the organisms require water and
sunlight. Thus, where either of these are absent, algae will be able to live.
Thus, surfaces which are permanently in darkness cannot be occupied.
Figure 5.12 shows the generation time (the time required for the population
to double) of a species of algae. At low levels of light intensity, very long
generation times are observed. However, as light intensity increases—for
this particular case—the generation time increases again beyond the
optimum intensity of around 60 W/m2. The intensity of sunlight incident
on the Earth’s surface will depend on the latitude and time of year (with
greater seasonal variation for latitudes further away from the equator).
However, intensities will typically be considerably higher than 60 W/
m2 at noon on a clear day (for instance around 500 W/m2 in spring and
autumn in the South of England). However, this is unlikely to compromise
algal growth too much, and many surfaces will seldom experience the full
intensity of sunlight where surfaces are horizontal, or where there is partial
shading and cloud cover.
Parts of a concrete structure which experience low levels of moisture
are unlikely to support algal growth. The environment surrounding a
structure will also define the availability of moisture. A survey involving
statistical analysis of algal colonisation of building façades across France
found that relative humidity was the most important factor in influencing
algal growth, with higher quantities of precipitation and closer proximity
of vegetation also encouraging growth [74].

60

50
•
I

VI
>-
"'
"0
40
ui
:2;
i=
z 30
0
i=
<(

.
a::
w 20
zw
___
--·-
--------
C)
I
10
~
··-------------
0+-----~----~----~----r-----r-----r---~
0 50 100 150 200 250 300 350

LIGHT INTENSITY, W/m 2

Figure 5.12 The generation time of the algae Botryococcus braunii

exposed to different intensities of light in fluid media held in flasks.
Light source not specified [73].
 Plants and Biodeterioration 249

Thus, interiors and exterior surfaces which receive shelter from rainfall
from other parts of a structure (such as cantilevered projections, awnings and
roof eaves) will potentially remain sufficiently dry to resist the establishment
of algae. Indeed, in designing a structure, there may exist possibilities for
incorporating features which will limit the wetting of concrete surfaces,
including the use of coping and capping, or projecting sills.
Algae were found to favour the North- and West-facing facades of
buildings in France [74]. This was attributed to the North side of buildings
receiving less sun, and consequently drying out less frequently. Favourable
colonisation of West-facing surfaces was considered the result of dominant
westerly winds bringing rain from the Atlantic Ocean. Such patterns
will clearly vary according to the geographic location of a building, and
knowledge of local conditions will at least allow identification of which
parts of a structure require measures to control growth.
Temperature ranges over which algae will grow vary between species.
However, for most species optimum growth rates occur somewhere
between 20ºC and 35ºC, although some algae continue to grow at higher
temperatures. This includes Chlorella vulgaris, which can grow on concrete
surfaces (Table 5.3) and which is capable of growth at temperatures as high
as 40ºC [75].
These ranges are clearly widely experienced as ambient temperatures,
meaning that the vast majority of concrete structures worldwide will
offer suitable environments for algae, at least at certain parts of the year.
Moreover, the control of external temperatures is not a practical option.
The amplitude of temperatures throughout the year has also been found
to play an important role, with smaller magnitudes of fluctuation limiting
evaporation rates and maintaining levels of moisture [74].

Material characteristics

As discussed in Section 5.3.4, the small volume of porosity resulting from

a low water/cement ratio will limit the extent to which the cement matrix
of concrete will act as a reservoir of moisture for algae. This is also likely to
limit the extent to which attachment of algae can occur, since a lower water/
cement ratio is also likely to yield a smoother surface. Cement type will
have little influence on rates of growth in the long term: initially Portland
cement will have a pH higher than that of combinations of Portland cement
and other materials, or calcium aluminate cements. However, the alkalinity
of the surface will gradually decline as the surface is leached by water or
undergoes carbonation.
Limiting algal growth through the use of TiO2 in the form of the
mineral anatase—in the same manner as for fungi in Chapter 4—has been
demonstrated. Research investigating the mineral’s effectiveness against
algal growth used the material as an addition in mortars which were
250 Biodeterioration of Concrete

subsequently inoculated with Stichococcus bacillaris and Chlorella ellipsoidea,

plus the cyanobacterium Gleocapsa dermochroa [43]. Mortar was also made
containing iron-doped anatase. The logic behind this approach was that
anatase doped in this manner had previously been found to possess a
higher photocatalytic efficiency [76]. The mortar composition was 12:4:4:1
sand:lime:anatase:Portland cement. Additionally, other mortar surfaces
were treated with two different biocides: alkyl-benzyl-dimethyl-ammonium
chloride/isopropyl alcohol (commercial name: Biotin T) and n.n-didecyl-
n-methyl-poly(oxyethyl) ammonium propionate/alkyl-propylene-
diamineguanidium acetate (Anios DDSH). Measurement of algal growth
over a 4 month period found that the anatase-bearing mortar was the most
resistant, followed by Biotin T (Figure 5.13). Interestingly, the doped anatase
was less effective.
A similar study also found that a commercial white cement containing
an unspecified quantity of TiO2 was wholly effective in preventing algal
growth (Chlorella vulgaris) over an experimental period of 16 weeks [77].
Similar laboratory-made materials consisting of reagent grade TiO2—at 5%
and 10% by mass—in white cement also containing GGBS were ineffective.
It was proposed that the mixes might have been more porous, encouraging
algal attachment and growth.

100

80
GROWTH RATIO, %

60

40

20

0
l

se

T
SH
se
tro

in
a

a
on

D
at

at

ot
D
An

an
C

Bi
s
io
ed

An
op
-d
Fe

 
Figure 5.13 Growth rate of algae and cyanobacteria on mortar surfaces,
Figure 5.13  Growth rate of algae and cyanobacteria on mortar surfaces, expressed as the quantity 
expressed as the quantity of chlorophyll a extracted as a percentage of the
of chlorophyll a extracted as a percentage of the control [42]. 
control [43].
 
Protection after construction 
A  study  examining  the  effectiveness  of  different  façade  coatings  measured  their  ability  to  support 
growth  of  three  different  species  of  algae  –  Klebsormidium  flaccidum,  Chlorella  mirabilis  and 
Stichococcus  bacillaris  [41].  The  coatings  investigated  were  three  mortars,  an  acrylic  water‐based 
organic coating and a similarly formulated paint. Neither the paint or the organic coating contained a 
biocide. The paint was considerably better at limiting growth in comparison to the other materials 
(Figure 5.14). Indeed, a study characterising and quantifying algal colonisation of a large number of 
building  façades  in  France  found  that  concrete  and  mortar  surfaces  were  more  amenable  to  the 
establishment  of  algal  populations  than  organic  surfaces  such  as  paint  [72],  since  the  isolation  of 
algae from the porosity of the concrete limits the availability of water. 
 
 Plants and Biodeterioration 251

Protection after construction

A study examining the effectiveness of different façade coatings measured

their ability to support growth of three different species of algae—
Klebsormidium flaccidum, Chlorella mirabilis and Stichococcus bacillaris [42]. The
coatings investigated were three mortars, an acrylic water-based organic
coating and a similarly formulated paint. Neither the paint or the organic
coating contained a biocide. The paint was considerably better at limiting
growth in comparison to the other materials (Figure 5.14). Indeed, a study
characterising and quantifying algal colonisation of a large number of
building façades in France found that concrete and mortar surfaces were
more amenable to the establishment of algal populations than organic
surfaces such as paint [74], since the isolation of algae from the porosity of
the concrete limits the availability of water.
The use of water repellents and biocides against algae growth (in this
case Chlorella vulgaris) on white architectural concrete and precast aerated
cellular concrete has been investigated [41]. In the case of the white concrete,
both biocides and water repellents were highly effective for the full test
period of 12 weeks. The treatments were less effective on aerated cellular
concrete, possibly as a result of this material’s more porous nature. It is
interesting to note that different water repellents had very different effects
on algal growth on aerated concrete, with some actually accelerating growth.
This is possibly explained by the enhanced ability for some algae to attach
themselves to hydrophobic surfaces (see Section 5.3.1). However, other

... ---- --...•

100
r:J--;---.

80
//,....,...........
,'

I
"" /

/
/
.,..,.

,-0
'2ft
? I
I I
/
,-
LLi
l
/
I ;:Y
<!J I I
I /
<( I I /
a::: 60 I I I /
w I I I
> I I

-·-
I I I
0 I I I 0
I
(.)
I I I
L.U I I
(.) 40 I I Paint
';/ I I
<( / - Q - Organic coating
LL I I I

-·-
I / One-coat mortar A
a::: /
I I - ~-

=> I l' I - 'J - One-coat mortar B

(f)
20
I
I /
~ I / Laboratory-made mortar
/

..
/
I/ / 0
/

__.... ____ ------·

1 / /

...
I/ ,~_,,
_,., ~;;::.-
1/
0
0 10 20 30 40

TIME, days

Figure 5.14 Growth of Klebsormidium flaccidum on different façade finishes [42].

252 Biodeterioration of Concrete

water repellents were more effective—specifically a silane-based product.

Biocides tended to prolong the period of time before algal growth started,
but growth occurred at a comparable rate to controls once this period
was over. The most effective strategy was found to be the combination of
water repellent and biocide—a chlorinated pyridine-based formulation
(Figure 5.15). Regardless of this, it is evident from the results of this study
that both approaches are likely to provide protection from algal growth for
a finite period. It should also be noted that the use of biocides on concrete
surfaces may be unacceptable in some applications—for instance, in coastal
or marine applications.
Whilst is has been seen that anatase additions to concrete and mortar
can limit algal growth, anatase-bearing coatings applied after construction
or precast manufacture of concrete could provide more economic protection,
with a greater permanency than biocides. One study has examined the
ability of TiO2 applied to the surface of autoclaved aerated concrete to
control algal growth (Chlorella vulgaris) [77]. The TiO2 was applied using
a vacuum impregnation technique. However, algae growth on this coated
surface appeared to be largely unaffected compared to the untreated
control surface. Indeed, a silane-based water repellent treatment proved
considerably more effective.
Nonetheless, more conventional coatings of TiO2 particles in a PMMA
binder (mass ratio 9:1) applied to a cement paste substrate have been shown
to be effective [78]. The use of a tungsten oxide (WO3) coatings was also
investigated, but proved ineffective. However, the approach was further

100

I
I - -· ~
I y-'V
•
I I I
I
80 I
I I
I ;f
I I
I I
I I
""u.i .o--0 I

•
0 I I
I fr ~o-/ I
60 I 'j1
I
(!)
<(
I
I
~j1 I
I
0:: I
LJ..I I /I -;:;
> I p I

-·-
0 40 I / I
u ;:r:-0' -;:;
I

f!1/"'-i,' '\. ~ I
vi ">~
I
I
I

_...,_ Water
- {)- repellent
Untreated

l ;f
,--,1/
20 Biocide
I / ; ,- \
\7
----
I Biocide then WR
~
- 'q -
II '--7
P1 I WR then biocide

0 -::: i::.: ~)· --

0 2 4 6 8 10 12 14 16

TIME, weeks

Figure 5.15 Development of algal coverage of autoclaved aerated concrete

with surface treatments of water repellent, biocide and combinations of
both where treatments were applied in different orders [41].
 Plants and Biodeterioration 253

refined through the use of both of these oxides coated with co-catalysts of
either platinum (Pt) or iridium (Ir) which proved to be highly effective at
limiting growth. It was demonstrated that the presence of these elements
was effective through a photocatalytic mechanism rather than one of
toxicity through limiting the light source used to wavelengths that did not
contribute significantly to the photocatalytic effect, which yielded growth at
a greater rate. Whether such an approach is practical for concrete structures
is questionable: both metals are extremely expensive.
The conclusion that must be drawn from these findings is that anatase
and the application of paint appear to offer the best means of achieving
concrete surfaces which limit algal growth. However, the use of water
repellent coatings, should not be entirely ruled out. In all cases, surface
coatings must be viewed as being non-permanent, and requiring re-
application later in a structure’s life.

Cleaning

Removal of algae from concrete surfaces can be approached in a number of

ways, but the main decision which needs to be made is whether a biocide can
or should be used. Biocides will kill the occupying algae, but normally the
dead cells will remain attached to the surface [79]. Removal can be achieved
through scraping or brushing, and in extreme cases abrasive blasting can
be used. This last approach, whilst highly effective at removing the residue
will also abrade the surface, which may not be desirable. Removal can also
be achieved through the use of high velocity water jets, which are highly
effective and less likely to damage the underlying concrete.
The duration of a biocide treatment is unlikely to be more than one
or two years [80]. Moreover, given that high velocity jetting will usually
remove algae regardless of whether a biocide has been used, there exists a
good argument for using this technique on its own.
The removal of stains resulting from algae using bacteria has been
investigated in the laboratory [81]. The experimental programme involved
concrete cubes which had been exposed to the elements for a period of
several years and consequently developed a covering of algae. The cubes
were placed in a volume of water such that only the upper face of each
cube was not in contact with water, to ensure that this surface remained
moist. The upper surface of the cubes was then sprinkled with droplets of a
solution consisting of water, bacteria of the genus Thiobacillus (see Chapter
3), powdered sulphur and ammonium sulphate. Sprinkling was conducted
four times a day for three days. After this period of treatment, the cubes
were dried and the treatment repeated a further two times.
The logic behind this approach is that the bacteria will utilise the sulphur
in the solution as a source of energy, producing sulphuric acid. This acid
will then react with the concrete surface, leading to a cleaning effect.
254 Biodeterioration of Concrete

The change in colour of the surface during this treatment was measured
using colorimetry. The treatment was found to yield a change in colour that
indicated a cleaning effect. This was not as effective as submerging the cubes
in the solution, or, indeed, submerging the cubes in a solution of sulphuric
acid. Petrographic microscopy of the surface indicated a layer of gypsum
has been formed (see Chapter 3). It is sound to assume that this layer would
eventually be weathered leaving a wholly clean surface behind. However,
whilst such a process would appear to be effective, the use of sulphuric
acid—regardless of its source—as a means of cleaning will clearly lead to
a loss of mass from the surface and might be deemed overly aggressive.

5.6.2 Limiting damage from vascular plants

As already discussed, the establishment of plants on a concrete surface

will usually either require the presence of cracks or open joints in which
seeds can become lodged, or a horizontal layer of moss in which particle
can become trapped. Thus, part of a strategy of preventing of plants from
becoming established should be in limiting the occurrence of such features.
Cracks can arise in concrete for a wide range of reasons at various
points in the life of a structure. Causes of cracking can include the thermal
contraction of concrete shortly after placement, drying of concrete in the
fresh state (plastic shrinkage), drying of concrete elements in the hardened
state under restraint (drying shrinkage), freeze-thaw attack, alkali-silica
reaction, sulphate attack, and the corrosion of steel reinforcement [82].
Detailing suitable approaches to designing concrete mixes to prevent these
forms of deterioration is beyond the scope of this book, but clearly adequate
design of concrete for durability is a desirable prerequisite for structures
which are to resist plant colonisation.
Where cracks are already present, they will need to be repaired,
not only to prevent plant establishment, but to prevent other forms of
deterioration, and to satisfy serviceability requirements of the structure.
The repair of cracks will normally involve either the use of structural
repair materials, which are applied by hand, or the use of concrete injection
methods. Structural repair materials are normally either inorganic cements
or polymer modified cements. Injection formulations are typically based
on polyurethane or epoxy resins.
Joints are a necessary feature in many parts of concrete structures,
since they prevent cracking resulting from volume change. Moreover,
discontinuities between different structural elements are often an inevitable
aspect of the construction process. However, they clearly offer an opening
in the concrete surface in which plants can be established (Figure 5.16).
Therefore, sealing of joints is a prudent strategy. Indeed, there are more
general reasons for sealing joints, since they also present a means for
 Plants and Biodeterioration 255

substances which are likely to compromise the durability of concrete to make

their way beneath the surface. Sealants used for joints are normally highly
elastic materials which are impermeable to water. Normally it is anticipated
that sealants will begin to deteriorate within the intended service life of a
concrete structure, and so replacement at some point is usually inevitable.
Thus, seals should be inspected as part of the maintenance programme of
a structure.
Where precast elements are placed alongside each other, a joint is
created which can also accomodate plants. An interesting example of this
is reported in the form of the penetration of Japanese knotweed (Fallopia
japonica) through precast concrete river revetment panels [18]. This problem
was ultimately solved by fabricating the panels with interlocking edges
which resisted penetration.
The presence of moss on concrete surfaces is, like lichen, quite often
aesthetically appealing. However, its potential role as a mechanism by
which a growth medium for vascular plants can be established, means that
it may not always be desirable. It should be noted that for vascular plants

Figure 5.16 Plant growing in a joint in a concrete structure.

256 Biodeterioration of Concrete

to damage the underlying concrete, cracks or joints will still probably need
to be present.
Removal of moss is most effectively achieved through physical methods
such as scraping or water jetting—removal is the important part, and
application of herbicide will not make this process any easier.
The design of concrete mixes to resist damage from plant damage
can be done with less certainty compared to the organisms examined in
previous chapters. A reduced water/cement ratio is certainly unlikely to
do any harm, could potential reduce the availability of water, and would
have the side-effect of improving concrete durability in more general terms.
Furthermore, assuming that the worst-case scenario for acid exudation from
roots is citric acid, the results reported in Chapter 4 would point to either
Portland or calcium aluminate cement.
Where vascular plants have already become established on a structure,
their removal is necessary, followed by filling of cracks with appropriate
repair materials, and joints with sealants, to prevent re-establishment. In
some cases the application of herbicides may be beneficial.
Herbicides can be categorised in terms of the manner in which they
work and can be selected accordingly. Hormone type herbicides mimic the
growth hormones of plants. They selectively target broad-leafed plants, but
do not harm narrow-leafed plants, such as grasses.
Moss and algae herbicides are particularly suited for acting on these plants.
They include acetic acid and ferrous sulphate, both of which are not
suited for use on concrete. Ferrous sulphate will stain concrete, whilst the
potentially damaging effect of acetic acid on concrete is covered in Chapter 3.
However, acetic acid products marketed as both algae/moss herbicides and
cleaning agents are sometimes marketed specifically for cleaning concrete,
on the grounds that the acid will remove the outer surface of the paving as
part of the cleaning process.
Contact herbicides kill only the part of the plant that they come into contact
with, whilst systemic herbicides will move through the plant to affect
all parts. Residual herbicides take some time either to be dispersed or
break down, thus leaving soil uninhabitable by plants for some time after
application.
Table 5.5 lists herbicides which are likely to potentially be usable on
concrete. It should be noted that most commercial products are combinations
of more than one herbicide, to tailor the product to specific applications.

Climbing plants

The establishment of climbing plants on concrete walls is, again, possibly an

agreeable development, and the potential for damage should be carefully
 Plants and Biodeterioration 257

Table 5.5 Herbicides likely to be suitable for use on concrete.

Herbicide Residual? Type Suitable For

Triclopyr Slightly Hormone Broad-leafed plants
2,4-D Slightly Hormone Broad-leafed plants
Dicamba Slightly Hormone Broad-leafed plants
Mecoprop-P Slightly Hormone Broad-leafed plants
MCPA Slightly Hormone Broad-leafed plants
Clopyralid Slightly Hormone Broad-leafed plants
Fluroxypyr Slightly Hormone Broad-leafed plants
Benzalconium chloride No Moss and algae Moss, algae
herbicides
Fatty acids (including No Moss and algae Moss, algae, vascular
pelargonic acid) herbicides + Contact plants
Diquat No Contact Vascular plants
Glyphosate No Systemic Vascular plants
Flufenacet Yes Residual Vascular plants
Metosulam Yes Residual Vascular plants
Diflufenican Yes Residual Vascular plants

considered prior to deciding to remove such plants. This is because concrete

walls are likely to possess fewer features which would allow roots or tendrils
to intrude beneath the surface, and where these are absent, there is little
argument for removal.
Where the plant is ivy, and damage is a real possibility, guidance has
been developed for removal [83]. The approach recommended takes into
account the fact that the adventitious roots of ivy may continue to grow–
possibly more aggressively–after the main stem has been cut. The advised
sequence of events is as follows:
1. The main stem of the plant is cut.
2. Where the plant is well established, it should be sprayed with a
systemic herbicide.
3. The plant should be left for a period of time until the stems have died
and show evidence of shrinkage.
4. The roots should either be dug up, or an herbicidal gel applied to the
stump.
5. The plant should be removed from every point of entry into the wall,
and the wall repaired if necessary.
258 Biodeterioration of Concrete

Tree roots and pavements

The extent to which roots are able to grow under pavements can be limited
by the use of root barriers. Root barriers are layers of material—running
vertically between the tree and the pavement to be protected—whose
purpose is to deflect or constrict the growth of roots [58]. Materials able to
deflect growth include plastic and glass fibre composite panels, preserved
plywood and tar paper or asphalt impregnated felt. Constricting barriers
are normally geotextiles, but can also include copper mesh. This material
possibly has an enhanced control over roots, in that copper released into
the soil is likely to have an inhibitory effect on growth. Indeed, geotextiles
impregnated with copper sulphate have also been successfully used as a
root barrier.
It has been argued that the use of root barriers compromises the stability
of trees, but a study simulating the effect of wind on trees grown with and
without barriers found that the presence of barriers actually appeared to
increase resistance to being uprooted [84]. This was attributed to the deeper
roots of these trees.
Another possible approach to limiting upward growth of roots beneath
pavements is to allow moisture in soil to evaporate, leading to lower
moisture levels at the pavement–soil interface. This can be achieved through
permeable paving. Where the paving is concrete, this can take the form
either of paviers with gaps which allow the movement of water vapour—
but still support foot traffic—or pervious concrete. Pervious concrete is
concrete containing coarse aggregate, but little or no fine aggregate [85].
This produces a material with a volume of large pores, which allow the
movement of liquid water and oxygen downwards, and water vapour
upwards. For permeable pavements to be successful, a soil which permits
percolation of water is essential, since an impermeable soil will still lead
to high moisture levels close to the surface [86].
The success of permeable pavements, however, is reported as being
somewhat inconsistent, with levels of moisture and oxygen beneath test
pavements showing little correlation between permeable and impermeable
surfaces [87].
When compacted subgrade is located below pavements, there exists
the possibility of including ‘root paths’ [86]. These are trenches in the sub-
grade lined with strip drain material leading to the other side of the paved
area. The trenches are backfilled with good quality soil before the rest of the
pavement construction is completed. The theory behind this approach is that
it offers a low-resistance path for the root to grow beneath the pavement into
an area where growth can continue without disruption of infrastructure.
Such systems are also sometimes referred to as ‘root breakout zones’ [58].
There is currently an absence of data with regards to the effectiveness of
this approach.
 Plants and Biodeterioration 259

The compaction of soils beneath pavements can be avoided through

the use of concrete pavement slabs suspended over the soil, and thus not
requiring any support from the underlying ground. The approach has
been demonstrated as promoting growth of trees, but its effectiveness at
preventing pavement damage is currently unclear [87].
The growth of roots away from trees is stimulated by the absence
of either water or nutrients. If adequate quantities of both are provided,
growth will be suppressed. This approach clearly needs a degree of planning
and resources, and is probably best suited to urban areas. However, the
introduction of tree pits has made this a more realistic option. Tree pits
are concrete lined chambers located beneath the ground and covered by
a suspended concrete slab [58]. The bottom of the pit consists of a bed of
gravel directing water into a drain. The chamber holds the root ball of the
tree, plus a volume of soil. The tree trunk projects through an opening in
the concrete cover, and is often encircled by a cast iron grille. A pipe also
runs through the interior of the chamber to deliver water and, possibly,
nutrients. This pipe will deliver water to a series of tree pits, thus making
irrigation relatively easy and automatable. The tree pit has the advantage
not only of allowing irrigation and fertilization, but also allows for the
introduction of root paths, avoids compaction through the suspended slab,
and also permits control over the size of the chamber, which—as will be
discussed later in this chapter—can be useful in limiting damage to pipes.

Tree roots and foundations

There are two effective approaches to limiting damage to foundations as

a result of desiccation of soil by tree roots. Firstly, statistical analysis of
instances of damage allows likely safe distances that buildings can be located
away from specific tree species. For instance, the maximum distance from
oak trees where 90% of the instances of damage were reported in the UK
is 18 m [59]. This approach can also be applied when planting new trees.
Secondly, where relocation of a planned structure is not possible, deeper
foundations can be used to take the foundation below the zone of soil
affected by the drying effects of the roots [60].

Pipes

The type of seal used at pipe joints will define the ease with which root
intrusion can occur. The previous surveys of root intrusion into pipes have,
in part, examined pipes with more traditional seals, such as textile strips
impregnated with bitumen and then sealed in place with cement. This
sealing technique has been superseded, firstly with natural rubber rings and
secondly with synthetic rubber gaskets designed to making fitting easier. A
study examining the more modern seal found that despite its composition
260 Biodeterioration of Concrete

and design, roots had little problem in breaching it [63]. Improvement was
observed, however, when the outside of the joints was further sealed with
self-vulcanising tape.
The recommendations which have arisen from this study are that care
should be taken in planting trees near pipelines, with consideration of
distances and tree species. Tree species should be those having a low ‘root
energy’—i.e., being less intrusive in nature. Common lime (Tilia europaea)
is suggested as an example by the researchers. It was also proposed that
the plant bed (the volume of soil which is excavated and replaced when a
tree is planted) should be made larger than normal. The reason why such
an approach is likely to be effective is that this volume of soil tends to be
more porous. Root growth will typically initially be limited to this zone
prior to infiltrating the denser surrounding soil, thus extending the period of
time before roots reach the pipe. Finally, the use of root barriers geotextiles
was proposed as a means of slowing the progress of roots, although it was
stressed that such barriers will typically hinder root growth rather than
stop it.
Where root intrusion has occurred, specialised cutting equipment can be
deployed in pipes to remove blockages. Root intrusions can also be removed
through the application of formulations containing the contact herbicide
metam sodium and a growth inhibitor [86]. By using a contact herbicide, the
intruding roots can be destroyed without killing the entire plant. However,
since it requires releasing quantities of herbicide into wastewater, in many
countries this approach is not permitted.

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[66] Javaherdashti R, Nikraz H, Borowitzka M, Moheimani N and Olivia M (2009) On the
impact of algae on accelerating the biodeterioration/biocorrosion of reinforced concrete:
a mechanistic review. European Journal of Scientific Research 36(3): 394–406.
[67] Jayakumar S and Saravanane R (2010) Detrimental effects on coastal concrete by Ulva
fasciata. Construction Materials 163(4): 239–246.
[68] Coombes MA, Naylor LA, Viles HA and Thompson RC (2013) Bioprotection and
Disturbance: seaweed, microclimatic stability and conditions for mechanical weathering
in the intertidal zone. Geomorphology 202: 4–14.
[69] Hughes P, Fairhurst D, Sherrington I, Renevier N, Morton LHG, Robery PC and
Cunningham L (2013) Microscopic examination of a new mechanism for accelerated
degradation of synthetic fibre reinforced marine concrete. Construction and Building
Materials 41: 498–504.
[70] Hughes P, Fairhurst D, Sherrington I, Renevier N, Morton LHG, Robery PC and
Cunningham L (2014) Microbial degradation of synthetic fibre-reinforced marine
concrete. 86(A): 2–5.
[71] Suseela M and Toppo K (2007) Algal biofilms on polythene and its possible degradation.
Current Science 92(3): 285–287.
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[72] Lane RA (2005) Under the microscope: understanding, detecting, and preventing
microbiologically influenced corrosion. Journal of Failure Analysis and Prevention
5(10-12): 33–38.
[73] Qin J (2005) Bio-Hydrocarbons from Algae: Impacts of temperature, light and salinity
on algae growth. Australian Government Rural Industries Research and Development
Corporation, Barton, Australia.
[74] Barberousse H, Lombardo RJ, Tell G and Couté A (2006) Factors involved in the
colonisation of building façades by algae and cyanobacteria in France. Biofouling 22(2):
69–77.
[75] Singh SP and Singh P (2015) Effect of temperature and light on the growth of algae
species: A review. Renewable and Sustainable Energy Reviews 50: 431–444.
[76] Navío JA, Macias M, Garcia-Gómez M and Pradera MA (2008) Functionalisation versus
mineralisation of some N-heterocyclic compounds upon UV-illumination in the presence
of un-doped and iron-doped TiO2 photocatalysts. Applied Catalysis B: Environmental
82(3-4): 225–232.
[77] Maury-Ramirez A, De Muyncka W, Stevens R, Demeestere K and De Belie N (2013)
Titanium dioxide based strategies to prevent algal fouling on cementitious materials.
Cement and Concrete Composites 36: 93–100.
[78] Linkous CA, Carter GJ, Locuson DB, Ouellette AJ, Slattery DK and Smith LA (2000)
Photocatalytic inhibition of algae growth using TiO2, WO3, and co-catalyst modifications.
Environmental Science and Technology 34(22): 4754–4758.
[79] Building Research Establishment (1992) BRE Digest 370: Control of Lichens, Moulds
and Similar Growths. Building Research Establishment, Watford, UK.
[80] The Concrete Society (2013) Visual Concrete—Weathering, Stains and Efflorescence.
The Concrete Society, Camberley, UK.
[81] De Graef B, Dick J, De Windt W, De Belie N and Verstraete W (2004) Cleaning of
concrete fouled by algae with the aid of thiobacilli. pp. 55–64. In: Silva MR (ed.). Second
International RILEM Workshop on Microbial Impact on Building Materials. RILEM,
Paris, France.
[82] Dyer TD (2014) Concrete Durability. CRC Press, Boca Raton, FL, USA.
[83] Building Research Establishment (1996) BRE Digest 418: Bird, Bee and Plant Damage
to Buildings. Building Research Establishment, Watford, UK.
[84] Smiley ET, Key A and Greco C (2000) Root barriers and windthrow potential. Journal
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[85] Tennis PD, Leming ML and Akers DJ (2004) Pervious Concrete Pavements. Portland
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Spring, MD, USA.
[86] Watson GW, Hewitt AM, Custic M and Lo M (2014) The management of tree root
systems in urban and suburban settings II: a review of strategies to mitigate human
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[87] Smiley ET, Calfee L, Fraedrich BR and Smiley EJ (2006) Comparison of structural and
noncompacted soils for trees surrounded by pavement. Arboriculture and Urban
Forestry 32(4): 164–169.
Chapter 6

Damage to Concrete from

Animal Activity

6.1 Introduction
This chapter examines the impact animal activity has on the durability
of concrete. Animals are members of the kingdom Animalia. They are
eukaryotes, like fungi, but differ significantly in that they are exclusively
multicellular and are motile. This chapter differs from previous chapters in
that, given the variety of organisms within this kingdom, the complexity
of their life-cycles, plus the relatively small size of the sub-set which
cause deterioration of concrete, there is little benefit in understanding
the organisms themselves in detail. Thus, the approach taken will be to
identify types of animal documented as having caused damage to concrete
structures, with a discussion of approaches which may protect structures
vulnerable to such attack.
Additionally, the chapter focuses largely on biodeterioration resulting
from concrete being specifically targeted by an animal to satisfy some
requirement for survival. For instance, whilst it is likely that abrasion of
concrete surfaces by the hooves of cattle compromises the durability of
concrete worldwide, the cattle do not benefit from this abrasion and the
concrete is not specifically selected by the animals because it can be abraded,
or contains something that they need to live.
The majority of this chapter involves the marine animals, since these
are the most damaging to concrete, albeit under specific conditions. In most
cases the manner in which these organisms attack concrete is comparable,
and common solutions exist.
The chapter ends with a discussion of the potential for damage to
concrete from bird droppings. Whilst this falls outside of the chapter remit
defined above, this topic has been the subject of some speculation in recent
years, and this discussion aims to evaluate the facts and draw balanced
conclusions from them.
266 Biodeterioration of Concrete

6.2 Marine Animals

Submerged concrete surfaces in a marine environment will typically
rapidly acquire an attachment of marine life. An example of how mixed
communities of marine organisms develop on a concrete pile located off the
West coast of Sweden is shown in Figure 6.1. Whilst the manner in which
colonization occurs is dependent on geography, the sequence is reasonably
typical. Initially the surface is colonized by algae and the genus Laomedea
—small plant-like animals which are members of the Hydrozoa class of
animals. The surface gradually becomes more diverse with the appearance
of Ciona intestinalis (a sponge) and at least two species of ascidians, or sea
squirts: a species of the genus Ascidiella and Corella parallelogramma. Lesser
coverage by the calcareous tube worm Pomatoceros triqueter was briefly
observed, but this species is seemingly gone after 12 months. These reside
against the surface within a calcium carbonate tube which the worm forms
around itself. Finally, the appearance of barnacles in the form of Balanus
improvisus was observed.
This assemblage of marine species is far from comprehensive. Other
organisms which may be found growing on submerged marine concrete
can include the Bryozoa (‘moss animals’), hard and soft corals, anemones,
sea urchins and various molluscs [2, 3]. Molluscs will typically appear more
than 12 months after placing.
From this range of organisms, only a few are known to cause
deterioration of concrete. Specifically, various molluscs and sponges have

100

80

;12.
u.i 60
~
<(
0::
w
>
0 40
u

20

0
0 2 4 6 10 12

TIME, months

Figure 6.1 Colonisation of a concrete pile off the West coast of Sweden [1].
 Damage to Concrete from Animal Activity 267

been documented as causing damage. It has also been found that some
marine worms and sea urchins may also be capable of causing damage.
It has been observed previously that, in some cases, the colonization
of a concrete surface may at least partly be determined by the presence of
substances in the material which can be used as nutrients. In the case of
marine organisms, this is not the reason for causing damage—all of the
nutrients required by such marine animals are likely to be present in the
seawater itself either as dissolved chemical species or as other organisms or
particles of debris. Instead, in most cases, these animals are boring beneath
the surface for shelter, or the damage is a side-effect of feeding off other
organisms on the concrete surface.
Before examining each of these types of marine animal in more detail, it
is worth briefly mentioning the barnacle, whose presence on concrete would
appear to have potentially beneficial effects with regards to durability.
Barnacles are arthropods which fix themselves permanently to a hard
substrate, feeding on particles suspended in seawater. They are encased in a
carapace which is composed of calcium carbonate. Most barnacle species are
attached to the substrate by a calcium carbonate basal plate or membrane,
which effectively isolates the animal from the substrate.
The basal plate is typically a relatively dense formation, and where
barnacles are present on concrete, it has the tendency to make the surface
less permeable to ingressing chemical species. This has particular benefits
from a concrete durability perspective in limiting the movement of chloride
ions from seawater through the concrete cover towards steel reinforcement,
where their presence will eventually initiate corrosion. The effect of barnacle
coverage on the chloride diffusion coefficient of concrete is shown in
Figure 6.2.
Barnacle colonization of concrete tends to be slower than for other
surfaces, such as steel [1]. This is largely the result of the high initial pH [5].

6.2.1 Molluscs

A number of members of the phylum Mollusca are capable of either boring

into or abrading the surface of concrete. The most frequently documented
instances of mollusc damage to concrete involve the bivalves.

Bivalves

Certain bivalves bore a cylindrical borehole into the substrate (usually rock)
and live in the resulting hole, enlarging it as the organism grows.
Table 6.1 lists bivalve species which have been observed to bore into
concrete. One location in this table is the Panama Canal, where Lithophaga
aristata was established as causing damage to concrete piers. However, the
same report also suggests that other species in this location may also be
268 Biodeterioration of Concrete

ro Q)
2.0
N~ 1.8
E
(.)

r-- 1.6
z
L1J
Q
LL
LL
1.4

1.2 I',
•
L1J
''
0
(.)
z
1.0
' ' •
' .....................
0 ',
0.8 ' '
(i5 ' '-,
::::> '-,
LL
LL 0.6 • ' ,,
0 ••
• ••
L1J 0.4
0
(i:
0.2
0
--'
I
(.) 00
0 20 40 60 80 100

COVERAGE,%

Figure 6.2 Chloride diffusion coefficients of concrete covered to different

extents by barnacle communities [4].

Table 6.1 Bivalve species confirmed as boring into concrete.

Genus Species Order/Family Boring Location References

Mechanism
Lithophaga – Mytiloida/ Chemical Saudi Arabia/ [6, 7]
Mytilidae Jamaica
–
aristata Panama Canal [8]
Pholadidea penita Myoida/ Mechanical Los Angeles [8]
Pholadidae Harbour
Platyodon cancellatus Myoida/Myidae Mechanical Los Angeles [8]
Harbour

responsible, namely Carditamera affinis and Hiatella solida [8]. Additionally,

a review paper on marine borers also identifies the Lithophaga, Pholadidea,
Platyodon and Hiatella genera as being known to bore into concrete, and
adds Zirfaea, Petricola, and Carditamera to this [9].
Most bivalves use mechanical action to bore into rock or concrete. This
is achieved using a filing/rasping action of the shell to remove material.
Lithophaga, however, does not have a shell suitable for mechanical boring
and secretes a substance from a gland known as the pallial gland which
contains a mucoprotein capable of chelating calcium (see Chapter 2) [10].
As a result, Lithophaga are only capable of boring into calcareous rock such
 Damage to Concrete from Animal Activity 269

as limestone. The high calcium content of the cement matrix of concrete

makes it a suitable substrate for Lithophaga boring.
In many instances, observation of mechanical borers in concrete have
been associated with poor mechanical properties in the material. For
instance, in the case of Pholadidea and Platyodon (Figure 6.3), the concrete
was found to be extremely weak, which was blamed on the manner in which
the concrete had been placed underwater leading to a high water/cement
ratio [8]. However, it would appear that the strength of concrete is not a
barrier when the boring mechanism is chemical: in a study of concrete piles
off the coast of Saudi Arabia Lithophaga were found to be able to penetrate
concrete piles with a strength of 50.7 MPa [6].
In this case, the diameter of the boreholes was found to be around 5
mm and the holes extended up to 15 mm into the concrete. The species can
bore at a rate of up to 10 mm per year and the diameter of boreholes can
be as large as 15 mm for a mature animal [11].
It is notable that, where stated, concrete attacked by Lithophaga
contained calcareous aggregate [6, 7]. Thus, the entire mass of concrete was
presumably wholly vulnerable to the type of chemical boring employed by
Lithophaga. Indeed Lithophaga has been observed to bore through cement
and calcareous aggregate with equal ease [7]. The same study, involving a
concrete block ‘reef’ colonised by Lithophaga (albeit alongside other boring
organisms) had encountered annual rates of bioerosion of around 4.5% by

Figure 6.3 Bivalves of the genus Pholadidea in concrete taken from

concrete pile jackets off the Californian coast [8].
270 Biodeterioration of Concrete

volume. Based on the descriptions of damage made by the researchers, the

majority was from the bivalve.
The number of reported cases of this type of biodeterioration is very
small, and it is likely that concrete containing siliceous aggregate—which
cannot be removed by the chemical boring mechanism—is more resistant
to Lithophaga.
Despite their name, shipworms are bivalve molluscs. They belong to
the genera Tenera and Banksia which both belong to the family Teredinidae
and the order Teleodesmacea. Their name refers to their ability to bore
into timber (historically, that of ships’ hulls) to make their home, and
their elongated body with two small shells located at their head. These
shells are used as rasping tools for boring. One literature source states that
shipworms have been known to bore into concrete, although no specific
evidence is provided [9]. Given that these animals utilize a mechanical
boring mechanism, it is reasonable to suppose that —as for the other
bivalves that bore in this way—only weaker and/or deteriorated concrete
is likely to be vulnerable.

Chitons and other grazing molluscs

Chitons are molluscs which live on rock surfaces in marine environments,

often in intertidal or subtidal zones. They are covered in eight articulated
shell plates which protect them, whilst giving them considerable freedom
of movement. Chitons are able to move over rock surfaces, feeding on
algae and other organisms, such as bryozoans. They are equipped with a
rasp known as a radula which is used to scrape these organisms from the
surface. The radula is coated in a layer of extremely hard magnetite (Fe3O4)
and the strategy the chiton employs to maximize the efficiency of grazing is
to abrade not only the food source, but also a thin layer of the underlying
rock. The particles of rock pass through the animal’s digestive system.
Chitons are commonly found on concrete surfaces in coastal locations.
Examples include the species Sypharochiton pelliserpentis on seawalls in
Sydney Harbour [12] and Mopalia hindsii on concrete piles in Monterey
Harbor in the United States [13]. Whilst there appear to be no estimates of
abrasion rates for chitons feeding from concrete surfaces, abrasion rates
for limestone have been estimated. Two studies of chitons (Acanthopleura
gemmata) living on limestone around the coast of an island in the Great
Barrier Reef estimated erosion rates of 0.2–0.7 mm/year and 0.16 mm/
year [14, 15].
These rates were considered significant by the researchers whose
primary interest was the effect of bioerosion processes on coral reefs.
However, assuming that concrete would typically display erosion rates at
the lower ends of this scale, due to the presence of more resistant aggregate
particles, rates are likely to be considerably lower than those observed
 Damage to Concrete from Animal Activity 271

for bivalves such as Lithophaga. Thus, it is unlikely that chitons present a

significant threat to concrete durability.
Other grazing molluscs include the gastropods such as limpets
(e.g., members of the families Nacellidae, Lottiidae, Siphonariidae and
Fissurellidae), sea snails (e.g., Trochidae, Littorinidae and Neritidae).
Limpets possess radula with silica-rich teeth, which are also implicated in
bioerosion processes on rocks [16]. However, rates of erosion would not
appear to exceed those of chitons. Rates from the grazing of sea snails are
thought to be even lower.

6.2.2 Sponges

Like the mollusc Lithophaga, boring sponges (phylum Porifera, class

Demospongiae) utilise chemical secretions to dissolve calcium in rock
substrates. Where this has occurred in concrete the effect has been the
formation of networks of holes around 1 mm in diameter (Figure 6.4). These
honeycomb-like structures are used by the sponge to anchor itself to rock
surfaces, and can increase in depth at a rate of around 1 mm per year [17].
Table 6.2 lists reported instances of concrete attack by boring sponges.
In all cases, the coarse aggregate used was calcareous, and in most instances
it is noted that boring was entirely limited to the aggregate [6, 7, 17].
Whilst the borehole diameter and rate of increase in depth resulting
from boring sponges is notably less than the values for Lithophaga, rates of

Figure 6.4 Holes formed in calcareous aggregate in concrete by a

boring sponge [6].
272 Biodeterioration of Concrete

Table 6.2 Boring sponges found in concrete structures.

Genus Species Order/Family Location Reference

Cliona – Hadromerida/Clionaidae Saudi Arabia [6]
–
Jamaica [7]
–
Off coast of Georgia, USA [17]
Damiria – Poecilosclerida/Acarnidae Jamaica [7]

deterioration have the potential to be significant, if coverage of a surface is

complete. One study examining the erosion rate of coral reefs which had
been colonised by boring sponges estimated rates between 0.84 and 23.0 kg
CaCO3/m2 year [18] (where the area term in the units is the surface area of the
coral) with the highest rate being observed for colonies of the species Cliona
lampa. However, it is unlikely that coral is wholly comparable to calcareous
aggregate in concrete, since it has a very different microstructure, and the
presence of cement around the aggregate may hamper boring rates. Thus,
the lower end of this range is likely to be more representative of concrete.
Boring sponges tend to be at their most destructive in regions close
to the equator. It has been found that whilst temperature does not have
a direct influence on the rate of boring [19], the productivity of the water
in which the sponge lives does [7], presumably because there is a readily
available supply of nutrients. Interestingly, a study looking at the boring of
sponges into the shells of molluscs has found that, as the pH declines, the
rate of boring increases [19]. This has potentially significant implications,
since reduced pH of the oceans is predicted as a result of climate change.

6.2.3 Sea urchins

Some sea urchins are capable of making quite substantial burrows in rock.
The burrow serves to provide shelter, and is formed by the animal securing
itself to the rock with its feet and abrading the surface with its five jaws
and rubbing with its test (shell) and spines. The motion is usually back and
forth in only one direction, with the ultimate effect of producing a burrow
which consists of a trench which is triangular in profile, with the urchin
normally located at the lowest point [16].
The purple sea urchin (Strongylocentrotus purpuratus)—which can have
a diameter of around 65 mm—has been reported to have burrowed to
significant depths into concrete breakwaters on the Californian coast [20].

6.2.4 Marine worms

Another marine borer identified as being capable of boring into concrete are
the marine worms of the class Polychaeta. Many of these worms are capable
 Damage to Concrete from Animal Activity 273

of boring into calcareous rock. Initially it was believed that this was achieved
through a combination of chemical and mechanical processes achieved
through the use of calcium-chelating mucopolysaccharide secretions from
glands and the use of brush-like structures known as setae [21]. However,
experiments in which the setae were removed found that the worm could
bore without difficulty into calcareous rock, presumably through purely
chemical means [22].
A study which examined the biodeterioration of concrete blocks in
an artificial reef-type structure found that the worms appeared to favour
boring into the cement matrix rather than into the calcareous aggregate
used in the blocks [7].

6.2.5 Limiting damage from marine animals

It is firstly worth noting that the most aggressive forms of marine animal
attack are limited to warmer zones, presumably where the productivity of
the sea is sufficiently high to support the sort of energy requirements of
boring on the scales reported. Thus, damage from marine animals in many
parts of the world is likely to be on a scale which is of little concern.
The mechanisms employed by marine animals to bore or burrow into
concrete fall into two categories: mechanical and chemical, and different
options are available in limiting damage depending on what mechanism
is used.
The results discussed in this section point to the likelihood that using
non-calcareous aggregate in concrete is likely to limit the magnitude of
attack. However, different animal types showed different preferences. Thus,
boring sponges, which seem to exclusively bore into calcareous aggregate
would probably be severely limited in its ability to bore by the presence of
wholly siliceous aggregate. Lithophaga appears to be able to bore through
both cement and calcareous aggregate. Nonetheless, use of non-calcareous
aggregate might be expected to hamper or entirely prevent boring. This
appears to be supported by the apparent complete absence of reported
incidences of Lithophaga in concrete without calcareous aggregate.
The only detailed report of Polychaeta worms boring into concrete
appears to indicate less sensitivity to aggregate type and a possible
preference for the cement matrix, indicating that sourcing specific aggregate
types to control this form of attack may not be an option.
Where mechanical damage is observed, it is often associated with
concrete of a low quality, and simply achieving appropriate performance
through mix design and good practice in mixing, placing and curing is
likely to limit biodeterioration considerably.
Whether the mechanism used by marine animals is chemical or
mechanical, the use of physical barriers to prevent boring is likely to be an
274 Biodeterioration of Concrete

effective option. The report of attack by Lithophaga and Cliona off the coast
of Saudi Arabia examined possible barrier options [6]. These included
polymer surface wraps, underwater epoxy paints, polyester bags deployed
around the concrete structure surface into which cement grout is pumped,
and moulded fiberglass jackets. The solution that was eventually selected
was the jackets, which were secured using an epoxy grout introduced into
the annulus between the jacket and the concrete surface.
Barriers of these types are effective through the provision of an
additional layer of protection between the concrete surface and the seawater.
However, it should be noted that purple sea urchins have been reported to
have abraded through steel casings around concrete piles [20], indicating
that even a strong barrier may not entirely resolve the problem. How
much of this damage was carried out by the sea urchin and how much
was the result of abiotic corrosion—which would be likely to be occurring
in parallel—is not clear.

6.3 Deterioration from Exposure to Bird Droppings

Contact with bird droppings is sometimes cited as a source of damage to
concrete and other construction materials [23]. The reason usually cited is
that they contain acid. Bird droppings contain uric acid, present as crystals
dispersed in a saturated solution of the compound—its solubility is low:
around 0.068 g/l [24], although it becomes more soluble at higher pH [25]. It
can also exist in a dihydrate form [26], which is slightly less soluble at 25°C.
The compound is a relatively weak diprotic acid, having a pKa1 value of 5.4
and a pKa2 of 11.3 [27]. Uric acid forms a calcium salt—(calcium hydrogen
urate hexahydrate, Ca(C5H3N4O3)2.6H2O). This is of low solubility (log Ksp
at 25ºC = –9.81) [28], and so contact between uric acid and cement may lead
to the precipitation of the salt. This salt has a high molar volume (267 cm3/
mol) [29], indicating the possibility of damage to concrete.
There is a lack of data regarding complex formation by the urate
and hydrogen urate ions, with the exception of iron, and so prediction
of the likely outcome of contact between cement and bird droppings
must be done without this data. However, using geochemical modelling
techniques (Figure 6.5) it is evident that the calcium urate salt is in fact
likely to be precipitated at a location just outside the cement paste itself.
Thus, deterioration by the mechanism of expansive salt precipitation is
unlikely. Damage through conventional acidolysis still occurs, but is limited
significantly by the lower concentrations of acid—the pH of bird droppings
is typically only slightly below 7 [30].
A study examining the effect of solutions leached from bird droppings
on crushed concrete samples held in test tubes for a period of five weeks
found slightly higher levels of mass loss in some cases compared to a control
 Damage to Concrete from Animal Activity 275

/·-~;·-·-·-·- · -·-·-·-·-·-·-·-·- · -·-·-·- · -·- 12

10
0.03
8 I
0.

en
Q) 6
0
E Original edge of cement paste
- 0.02 4
~ 2
i=
z hydrogen urate hexahydrate
<t: Ettringite (AI)
::::l Monosulfate (Fe)
a

0 2 3 4 5 6 7 8 9 10

DEPTH FROM SURFACE, mm

Figure 6.5 Quantities of solid phases versus depth obtained from geochemical
modelling of uric acid attack of hydrated Portland cement. Model conditions: Excess
of solid uric acid located in acid solution around cement paste; volume of acid solution
= 4 l; mass of cement 80 g; diffusion coefficient = 5 × 10–13 m2/s.

exposed to distilled water [30]. However, the differences were of the order
of tenths of a percent, and the influence of such a difference on concrete
performance is likely to be small. It should also be pointed out that by using
leached solutions, the concentration of uric acid would have been low, due
to its low solubility. Thus, there exists the possibility that the damaging effect
of uric acid would have been greater if solid uric acid had been retained,
since it would continue to dissolve during the experiments to replace any
which had reacted with the concrete, as would happen in reality.
The chemical effect of concrete to bird droppings and uric acid
specifically is still an area that needs further investigation, but there is little
evidence of there being a serious problem.
However, one manner in which bird droppings may cause deterioration
is as a source of nutrients for other species—such as fungi, bacteria and
plants—which can damage concrete, in part through their production of
other organic acids [31]. When fungi metabolize uric acid, the end products
are urea and glyoxylic acid [32]. Urea does not affect the durability of
concrete. Whilst experimental data relating to the interaction of glyoxylic
acid with concrete is not available, it is likely that the compound behaves
in a similar manner to other monocarboxylic acids, such as acetic acid (see
Chapter 3).
It should be stressed that there are many other reasons for limiting the
deposition of bird droppings on the built environment, including those
276 Biodeterioration of Concrete

of health and aesthetics. These reasons should, therefore, take priority in

deciding whether measures are required. There exists much guidance on
this issue, including a document by the Building Research Establishment
in the UK [23].

6.4 References
[1] Andersson MH, Berggren M, Wilhelmsson D and Öhman MC (2009) Epibenthic
colonization of concrete and steel pilings in a cold-temperate embayment: a field
experiment. Helgoland Marine Research 63(3): 249–260.
[2] Oil and Gas UK (2013) The Management of Marine Growth During Commissioning.
Oil and Gas UK, Aberdeen UK.
[3] Agatsuma Y (2013) Chapter 15: Stock enhancement. pp. 213–224. In: Lawrence JM (ed.).
Sea Urchins: Biology and Ecology, 3rd Ed. Academic Press, London.
[4] Yokota H, Hamada H and Iwanami M (2013) Evaluation and prediction on performance
degradation of marine concrete structures. In: Third International Conference on
Sustainable Construction Materials and Technologies, Kyoto, Japan.
[5] Ido S and Shimrit P-F (2015) Blue is the new green—Ecological enhancement of concrete
based coastal and marine infrastructure. Ecological Engineering 84: 260–271.
[6] Wiltsie EA, Brown WC and Al-Shafei K (1984) Rock borer attack on Juaymah trestle
concrete piles. pp. 767–772. In: First International Conference on Case Histories in
Geotechnical Engineering, Missouri University of Science and Technology, Rolla, MO,
USA.
[7] Scott PJB, Moser KA and Risk MJ (1988) Bioerosion of concrete and limestone by marine
organisms: A 13 year experiment from Jamaica. Marine Pollution Bulletin 19(5): 219–222.
[8] Atwood WG and Johnson AA (1924) Marine Structures—their Deterioration and
Preservation. National Research Council, Washington DC, USA.
[9] Castagna M (1973) Shipworms and other marine borers. Marine Fisheries Review 35(8):
7–12.
[10] Jaccarini V, Bannister WH and Micallef H (1968) The pallial glands and rock boring in
Lithophaga lithophaga (Lamellibranchia, Mytilidae). Journal of Zoology 154(4): 397–401.
[11] Warme JE and Marshall NF (1969) Marine borers in calcareous terrigenous rocks of the
pacific coast. American Zoologist 9(3): 765–774.
[12] Moreira J (2006) Patterns of occurrence of grazing molluscs on sandstone and concrete
seawalls in Sydney Harbour (Australia). Molluscan Research 26(1): 51–60.
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Index

A aggressive CO2 33, 125

Aglaothamnion 233
Abietinella 236 Ailanthus altissima 223
abundance ratio 8 Albizia lebbeck 224
Acanthopleura 270 algae 4, 85, 155, 157, 171, 203, 212–214, 217,
Acarospora 176, 195 219, 225, 228, 230–233, 245–253, 256,
Acer pseudoplatanus 223 257, 266, 270
acetate 14, 15, 39, 41, 42, 142, 143, 160, 250 cryptoendolithic 230
acetic acid 8, 9, 14, 39, 41–44, 115–121, 127, diatoms 230, 235
142, 181, 185, 189, 256, 275 organic acid production 179
acid attack 22, 71, 94–96, 98, 102, 104, 105, alkaliphilic 83, 225, 226
107–109, 111–113, 116, 117, 119, 129, Alternaria 160, 172, 174, 194
136–139, 141, 179, 185, 187–189, 275 aluminium butyrate 63
effect of abrasion 99, 100, 108 aluminium citrate 68
acid dissociation constant 8, 13, 24, 29, 31, aluminium diacetate 42
32, 38, 41, 44, 46, 47, 49, 52, 53, 57, 60, aluminium gluconate 65
63–65, 67, 118, 119, 190, 191 aluminium glycolate 47
acidic strength 7, 190 aluminium lactate 45
acidolysis 70, 71, 93, 95, 98, 100, 107, 111, aluminium monoacetate 42
115–117, 138, 139, 169, 188, 200, 242, aluminium oxalate 50
274 aluminium triacetate 42
acidophilic 83, 96, 225 aluminium formate 34, 39
Acremonium 174 aluminium nitrate 28, 29
activity 11, 12, 91, 93, 96, 105, 106, 115, aluminium toxicity 229
125–128, 135, 138, 141, 159, 175, 191, bacteria 228
192, 203, 247, 265 plants 229
admixtures 86, 123, 137, 201, 202 aluminium tritartrate 62
biocides 135, 139, 203 Amblystegium 236, 239
plasticizers 137 ammonia 105–107, 114, 160, 161, 228
super-plasticizers 137 ammonia-oxidising bacteria 106
aerobe 78, 90, 107, 155 ammonium 28, 107, 113, 114, 135, 156, 201,
aerotolerant 78, 79 202, 228, 250, 253
AFm 17, 22, 28–31, 33, 35, 39, 70, 89 anatase 200, 249, 250, 252, 253
AFt 17, 22, 33, 35, 39, 70, 89 anaerobe 78, 90, 155
Agaricus 196 anemone 266
aggregate 70, 77, 87–90, 98, 101, 104, 105, animals 3, 5, 153, 158, 217, 218, 222, 223,
121, 138, 200, 202, 229, 230, 245, 246, 265–267, 269, 270, 272, 273
258, 269–273 Anomodon 236
dolomite 105 Apatococcus 233
limestone 90, 101, 102, 104, 121, Aphanocapsa 124
269, 270 Aphanothece 124
synthetic 105, 246, 259 apple of Sodom 224
280 Biodeterioration of Concrete

aragonite 32, 35, 80 Bryoerythrophyllum 236

Arthrobacter 204 bryophytes 215, 217–219, 232, 236, 245
asbestos cement 86, 87, 107, 114, 195, 204, bryozoa 266
205 Bryum 237
ascidians 266 buckthorn 223
Ascidiella 266 buddleia 223, 226, 227
ascorbic acid 179 Buddleja 223
Ascophylum 245 butyrate 63
ash (plant) 223 butyric acid 57, 63, 115, 119–121, 128, 179
Aspergillus 157, 158, 169, 170, 172, 174, 179,
180, 191–195, 199, 202, 205 C
Aspicilia 176
calcicole 221–223, 229
atranorin 166
calcifuge 221, 222, 229
Atrichum 236
calcite 32, 35, 80, 87, 206, 245
Aulacoseira 235
calcium butyrate 63
Aureobasidium 172, 174
calcium carbonate 32, 82, 83, 87, 101, 121,
autotrophic 78, 87, 88, 106, 213, 230
125, 205, 266, 267
Azadirachta indica 224
calcium citrate 3, 66, 68, 123, 163, 181, 185,
B 195
calcium diacetate 42
Bacillus 79, 80, 93, 135, 204 calcium formate 39
bacteria 1–3, 5, 77–93, 95, 96, 98, 100, calcium fumarate 64
106–110, 112–115, 119, 122, 123–125, calcium gluconate 65, 188, 190
127–132, 134–140, 142, 153, 157, 159, calcium glycolate 47
160, 165, 204, 205, 206, 228, 253, 275 calcium hydrogencarbonate 32, 33
antifungal 201, 202, 204 calcium hydrogen citrate 68
used to clean concrete surface 253, calcium hydrogen malate 59
254 calcium hydrogen tartrate 54, 62
Baeomyces 166 calcium hydrogen triacetate 42
baeomycic acid 166 calcium hydrogen urate 274
Banksia 270 calcium hydroxide 10, 13, 17
Barbula 236 calcium lactate 45
barnacles 266, 267 calcium malate 59, 185
basaluminite 25, 26 calcium nitrate 29
Beauveria 174 calcium nitrite 30, 31
Balanus 266 calcium oxalate 49, 50
bicarbonate 31, 32, 87, 88 calcium propionate 66
biochemical oxygen demand 130 calcium pyruvate 52, 53
biocide 4, 135, 139, 202–204, 250–253 calcium silicate hydrate gel 17
biofilm 79, 82, 85, 86, 90, 106, 107, 109, 123, calcium succinate 56, 189
124, 125, 131, 132, 134, 157, 168, 169, calcium tartrate 55, 62, 184, 190
196, 213, 214 Callicladium 237
bioprotection 210, 263 Calliergonella 237
bird 220, 265, 274, 275 Caloplaca 166, 176
droppings 265, 274, 275 Calothrix 124
black locust 222 Calotropis procera 224
Brachytheciastrum 236 Calymperes 226, 227
Brachythecium 236 Campyliadelphus 237
bramble 223 Campylidium 237
bodhi tree 224 Campylium 237
borehole 267, 269, 271 carbon dioxide 3, 30, 35, 78, 83, 156, 196,
Botyritis 174 213, 228
Bracteacoccus 233 see also aggressive carbon dioxide
brucite 84
 Index 281

carbonation 3, 83, 84, 87, 92, 93, 121, 160, citric acid 3, 8, 9, 66–69, 71, 115, 157, 163,
162, 225, 249 175, 179–182, 184, 190, 195, 229, 230,
carbonic acid 30–38, 87, 88, 122, 125, 126 235, 256
carboxylic acid 34, 39, 42, 66, 127 Cladonia 166
Candelaria 177, 178 Cladosporium 165, 172, 174, 194, 204, 205
Candelariella 177, 195 Clauzadea 177
caoxite 48 Clematis vitalba 223
caper 222, 224 cleaning 4, 134, 135, 198, 200, 201, 204, 253,
Capparis 222, 224 254, 256
Carditamera 268 Climacium 237
Catapyrenium 177 clinker 16, 17, 89, 105
Catillaria 177 coastal protection 245, 246
cellulose 127, 153, 155, 156, 175, 196, 212, coatings 137, 139–141, 203, 251–253
213, 215, 231 Coccobotrys 233
Celtis australis 223 Coccomyxa 233
cement 1, 3, 5, 7, 15–18, 20–22, 24–26, 28, colemanite 202
29, 31–33, 35, 39, 42, 45, 47, 49, 55, 58, Collema 177
62–68, 70, 71, 77, 83–90, 92–96, 98–105, Colletotrichum 170
107–121, 125–129, 136–139, 142, 143, Cololejeunea 238
160, 162, 163, 165, 168–171, 179–196, common ion effect 13
199–205, 225, 229, 230, 231, 245, 246, complex 3, 13–15, 20, 21, 24, 28, 29, 31, 32,
247, 249, 250, 252, 254, 256, 259, 269, 34, 35, 38, 39, 41, 44, 45, 47, 50, 60, 63,
272–275 64–67, 71, 86, 89, 105, 136, 154, 159, 164,
calcium aluminate 18, 20, 22, 33, 70, 165, 217, 274
93, 100, 101, 105, 111, 116, 120, 121, complexolysis 70, 71
183, 190, 200, 249, 256 Coniosporium 160, 172
calcium sulphoaluminate 101, 104, Conocephalum 238
111, 113, 181–185 copper 133, 139, 163, 201, 204, 258
cement content 88, 98–100, 109, 111, Coprinus 159
119, 121, 136–138 coral 270, 272
Portland 16–18, 20–22, 24–26, 29, Corella 266
31–33, 35, 39, 42, 45, 47, 62–66, 68, corrosion 1–3, 24, 98, 105, 109, 111, 123, 127,
70, 85, 88, 89, 93, 94, 99–101, 107–111, 128, 141, 198, 246, 247, 254, 267, 274
115, 116, 118, 125, 127, 138, 143, 160, cracks 71, 86, 96, 138, 141, 205, 219–221, 235,
162, 170, 171, 181–189, 191, 192, 201, 242, 245, 254, 256
202, 246, 247, 249, 250, 256, 275 Croton bonplandianus 224
Centranthus ruber 222 Cryptomonas 225
Ceratium 233 CSH gel – see calcium silicate hydrate gel
Ceratodon 237 Ctenidium 238
Chaetomium 174, 199 Curvularia 172
Chaetophoma 172 cyanobacteria 85, 88, 90, 124, 155, 157, 171,
chelation 15 212, 214, 228, 231, 250
chemotrophic 78 Cyanosarcina 124
Chernobyl 171, 174, 175 cypress 243
Chiloscyphus 237
chiton 270, 271 D
Chlamydomonas 230
dairy industry 115
Chlorella 230, 233, 249–252
Dalbergia sissoo 224
chlorine 132
Damiria 272
chlorophyll 197, 212–214, 250
Davies equation 11, 12
Chlorosarcinopsis 233
Debye-Hückel equation 11
Choricystis 233
decalcification 22, 26, 70, 108, 116, 138
Chroococcus 124
deprotonation 8, 24, 43, 44, 46, 50, 53, 54,
Chrysosporium 174
57, 66
Ciona 266
282 Biodeterioration of Concrete

Deschampsia flexuosia 229 fly ash 17, 88, 89, 100–103, 111, 112, 116, 117,
Desmococcus 233 119, 121, 127, 137, 138, 143, 181–184,
Desulfovibrio 90, 135, 136 199, 200, 246
Dicranella 238 Fonsecaea 173
Dicranum 238 Fontinalis 238
Didymon 238 formate 34, 35, 38, 39
Dirina 177 formic acid 34, 38, 39, 40, 230
Dirinaria 177 foundation 84, 243, 259
dissociation ratio 8 fragmentation 3, 71, 100, 158, 163, 181, 182,
Dittrichia viscosa 222 184, 185, 189, 190
Doratomyces 174 Fraxinus excelsior 223
Dreponacladus 238 Fucus 233, 245
Dryopteris filix-mas 223 Fulgensia 177
dry rot 175 fumarate 64
fumaric acid 57, 64, 229, 241
E fumarprotocetraric acid 167
Funaria 238
Eh 19, 20
fungi 3–5, 72, 153–163, 165, 169–175, 179,
elderberry 222
180, 188, 190–201, 203, 249, 265, 275
elm 223, 224, 243
calcicolous 160
Encalypta 238
silicicolous 160, 180
Endocarpon 177
corrosion of post-tensioned
Enterococcus 115
reinforcement 210
enzymes 82, 88, 127, 155, 156, 159, 196
fungicide 201, 202
Epicoccum 173
Fusarium 173, 174, 191, 193, 198
Erysimum cheiri 222
ettringite 17, 18, 24–26, 33, 93–95, 98, 100, G
122, 136, 183
Eucalyptus 245 gastropods 271
Euonymus europaeus 222 Geminella 233
Eurhynchium 238 geochemical modelling 71, 94, 95, 101, 104,
Evernia 166 107, 108, 111, 113, 183–185, 187–190,
Exophiala 160, 173 274, 275
expansive reaction products 71 geopolymer 246, 247
extracellular polymeric 82, 107, 157, 165, Geotrichum 173, 174
168, 170, 219 GGBS – see ground granulated blastfurnace
slag
F gibbsite 21, 22, 26, 39, 44, 50, 57, 66, 70, 71,
111, 183
FA – see fly ash
Gloeocapsa 124
Fallopia japonica 222, 255
Gloeocapsa-Chroococcus 124
false acacia 222
Gloeothece 124
fern 218, 223
glycolate 46, 47
male 217, 218, 223
glycolic acid 46–48
ferrihydrite 10, 21, 22, 26, 46, 50, 66, 70, 89,
glyoxylic acid 275
95, 188
Grimmia 238
fibres 82, 137, 139, 141, 195, 203, 204, 246,
ground granulated blastfurnace slag 17, 18,
258
20, 100–102, 120–122, 126, 199, 250
Ficus 223, 224
growth 3, 5, 77, 79–81, 83–85, 93, 101, 107,
fig 223
108, 113, 114, 122–125, 127, 130–132,
Fissidens 238
135, 138, 154–156, 158–161, 165, 175,
flagella 82, 238
176, 178, 192–195, 197, 199–205, 216–
Flavobacterium 123
218, 220, 221, 225, 226, 228, 231, 232,
243–245, 247–253, 255, 256, 258–260
 Index 283

fungi 3, 5, 72, 153–156, 158–161, 165, iron (II) acetate 42

175, 192–195, 197, 199–203, 249 iron (II) formate 39
plants 3, 5, 107, 114, 216–218, 220, iron (II) fumarate 64
221, 226, 228, 231, 232, 243, 245, 247, iron (II) gluconate 65
255, 256 iron (II) hydrogen malate 59
gum Arabic tree 224 iron (II) lactate 45
Gymnostomum 238, 239 iron (II) malate 59
Gypsophilia fastigata 229 iron (II) nitrate 29, 133
gypsum 16, 25, 71, 88, 93–96, 98, 100, 101, iron (II) oxalate 49, 50
105, 122, 136–138, 162, 163, 179, 180, iron (II) succinate 56
199, 254 iron (II) tartrate 55
gyrophoric acid 166 iron (III) butyrate 63
iron (III) citrate 68
H iron (III) diacetate 41, 42
iron (III) formate 34, 39
half saturation coefficient 80, 114
iron (III) glycolate 47
Halidrys 233, 245
iron (III) lactate 45
Hedera helix 222, 242
iron (III) nitrate 29
Hedwigia 238
iron (III) oxalate 50
Helminthosporium 173
iron (III) propionate 66
hematite 18, 89
iron (III) tartrate 57, 62
hemicarbonate 33, 35
isobutyric acid 115, 118, 119, 121, 122, 179
herbicide 256, 257, 260
itaconic acid 179
Heterodermia 177
ivy 222, 242, 257
Heterotrophic 78, 79, 86, 87, 114, 122, 123,
125, 127–129, 132, 157, 158, 162, 213, J
230
Hiatella 268 Japanese flowering crabapple 244
Holoptelea integrifolia 224 Japanese knotweed 222, 255
Homalia 239 joints 204, 242–244, 254–256, 259, 260
Homalothecium 239 jujube 224
Homomallium 239 Jungermannia 239
Hormoconis 198 jungle tulsi 224
Hormotila 234 jurbanite 25, 26
hydrogarnet 25
hydrogen peroxide 132, 196 K
hydrogen sulphide 1, 78, 91, 128, 132, 133,
Keratococcus 234
135, 137–139
2-ketogluconic acid 79
hydrophobic surfaces 219, 251
Klebsormidium 225, 226, 234, 251
hydrozoa 266
Ksp 7, 10–13, 15, 16, 274
Hygroamblystegium 239
Hygrohypnum 239
L
Hylocomium 239
Hyperphyscia 177 lactate 44, 45, 119
hyphae 154–158, 168–171, 175, 180, 194 lactic acid 42–46, 57, 90, 115, 117–121, 230
pressure exerted by 169, 170 Lactobacillus 115
penetration of cement paste 170, 171 Laomedea 266
Hypnum 239 leaching 3, 70, 71, 126, 127, 162, 170, 193, 225
hypochlorite 132, 201, 202 lebbeck 224
Lecania 178
I Lecanora 176
lecanoric acid 166
Indian banyan 224
Le Chatelier’s principle 12
Indian elm 224
Lecidea 176
iron (II) butyrate 63
Lepraria 176
284 Biodeterioration of Concrete

Leproplaca 176 molluscs 3, 266, 267, 270–272

Leptobryum 239 Monilinia 173
Leptolyngbya 124 monocarbonate 33–35
Leskea 239 Monod equation 80
Leucas biflora 224 monosulphate 17, 25, 26, 93, 163
Leucodon 239 Mopalia 270
Leuconostoc 115 moss 212, 216, 220, 226, 230, 231, 245,
lichen 4, 154, 155, 157–160, 163, 166, 175, 254–257, 266
176, 180, 195–198, 204, 205, 245, 255 ‘moss animals’ 266
lichenic acids 157, 163, 166, 197 motility 82, 153
lignin 215 Mucor 173, 174
lilac 223 mycelia sterilia 175
lime (tree) 260 mycelium 154, 156, 158, 175
limestone 90, 101, 102, 104, 121, 269, 270
limpet 271 N
Lindenbergia indica 224
natural pozzolana 100, 102, 103, 111, 112,
Lithophaga 267–271, 273, 274
117
lithotrophic 132
neem 224
lobaric acid 167
nettle-leaved Lindenbergia 224
M nettle tree 223
neutrophilic 83
magnetite 18, 89, 270 Nigrospora 173
maintenance 2, 3, 135, 198, 204, 255 nitrate 28–30, 78, 113, 114, 127–129, 132, 133,
malate 57–59, 185 156, 228
malic acid 54, 57, 59, 80, 156, 170, 185, 187, nitric acid 28–30, 105–116, 138
188, 190, 192 nitrifying bacteria 105, 107–110, 112–114,
Malus floribunda 244 142, 228
manure 115, 121, 135 role in nitrogen fixation for plants
Marchantia 239 228
Matthiola incana 222 nitrite 28, 30, 31, 106, 107, 114, 135, 136, 228
Melaleuca 245 nitrite-oxidising bacteria 106, 107
melanin 158, 170, 175, 196 nitrogen 19, 79, 88, 90, 127, 156, 157, 162,
metabolism 5, 77–80, 90, 155, 175, 200, 228
bacteria 5, 77–80, 90 Nitrobacter 107, 114
fungi 5, 155, 175 Nitrosomonas 106
metakaolin 100, 102, 112, 117, 199 Nitrosospira 106, 114
microaerophilic 78 Nitrospira 107
Microbacterium 93 nitrous acid 30, 31, 106
Microchaetaceae 124 North Indian rosewood 224
Microcoleus 124 Nostoc 124
Microspora 124, 234 Nostocaceae 124
micro-capsules 202, 203 nuclear waste 4, 126–128
micro-CT 116–118, 120, 181, 182, 184, 185 nutrients 79, 80–83, 86, 88, 90, 96, 115, 123,
mixotrophic 78, 93, 213, 230 127, 139, 154, 156, 157, 159, 162, 163,
Mnium 239 165, 168, 175, 200, 204, 215, 216, 218,
moisture 41, 85, 92, 114, 124, 125, 130, 141, 228–231, 242, 243, 245, 248, 259, 267,
165, 168, 203, 217, 220, 231, 248, 249, 258 272, 275
influence on algal growth 85, 231,
248, 249 O
influence on bacterial growth 85, 91,
oak 243, 259
92, 114, 124, 125, 130
Ochrobactrum 93
molar volume 21, 25, 29, 31, 35, 39, 42, 45,
old man’s beard 223
47, 50, 53, 56, 59, 62–66, 68, 71, 93,
oilwell decommissioning 4, 128
179–181, 184, 185, 190, 274
 Index 285

olivetoric acid 166 Physcia 176

Opegrapha 176 physodic acid 167
organic acid 34, 50, 79, 80, 89, 91, 114, 115, pipe 1, 86, 87, 90–93, 96, 100, 107, 123, 129,
118, 119, 123, 125, 128, 129, 138, 156, 130, 131, 134, 135, 137, 139, 140, 193,
157, 163, 170, 175, 179, 181, 191, 198, 243–245, 259, 260
229, 230, 243, 245, 275 liners 137
organotrophic 78 seals 259
Orthotrichum 239 pKa 8, 9, 91, 274
Oscillatoriales 124 Plagiochila 240
oxalate 49, 50, 156, 157, 162, 163, 165, 175, Plagiomnium 240
179, 180, 190, 192, 194–196 Plagiothecium 240
oxalic acid 46, 49–53, 79, 127, 156, 157, 162, plants (vascular) 215–224, 226, 227, 231–245,
163, 175, 179–181, 190, 192, 195, 196, 254–260
229, 243 plasticizer 137
Oxyrrhynchium 240 Platyhypnidium 240
Oxystegus 240 Platyodon 268, 269
Pleurochrysis 234, 246
P Pleurococcus 234
Pleurotus 196
Paecilomyces 173, 174, 194
Pleurozium 240
Paenibacillus 205
Pohlia 240
paint 203, 251, 253, 274
polymer-modified concrete 152
Palmellopsis 234
Polyporus 196
Parietaria Judaica 223
Polysaccharides 79, 82, 86, 213, 219
Parietin 168, 178
Polysiphonia 234
Parmelia 166
Polytrichum 240
Parthenocissus quinquefolia 243
Pomatoceros 266
passivation 123, 247
poplar 243, 244
pavements 228, 243, 258, 259
Populus candensis 244
pE 19
porosity 18, 71, 85, 86, 90, 95, 96, 98, 100,
Pediococcus 115
109, 111, 125, 170, 192–194, 202, 216,
pellitory of the wall 223
230, 245, 247, 249–251
Penicillium 158, 173, 174, 195, 198, 199, 201
Porphyra 235, 245
pervious concrete 258
portlandite – see also calcium hydroxide
Pestalopsis 173
potassium 17, 79, 89, 92, 132, 162, 204, 228
Pestalotia 173
potassium permanganate 132, 204
Petricola 268
potassium thiocyanate 92
pH 8, 9, 14–18, 20, 22, 24–26, 28, 31–34, 39,
pozzolanic reaction 18, 183
50, 53, 55, 57, 70–73, 83, 84, 87, 89,
Promicromonosporaceae 80
91–93, 95–97, 99, 101–103, 106–108, 111,
propionic acid 60, 65, 66, 118, 119, 121
112, 115, 116, 118, 119, 122, 125–127,
Protoblastenia 177
133, 134, 140, 157, 159, 160, 161, 175,
Pseudoleskeella 240
179, 183, 184, 193, 195, 199, 221, 222,
Pseudomonas 85, 93
225–227, 229, 246, 247, 249, 267, 272,
Pseudosagedia 177
274
Punctelia 177
Phaeophyscia 176
Pycnoporus 196
Phialophora 174
Pylaisia 240
Pholadidea 268, 269
pyruvate 50, 52, 53
Phoma 174
pyruvic acid 50, 52–54, 57, 230
Phormidium 124
Pyxine 177
phosphate 79, 88, 89, 162, 229, 230
photosynthesis 3, 155, 157, 158, 197, 212, R
213, 215, 225, 228, 246
phototrophic 78 Racomitrium 240
PHREEQC 71 radiation 158, 174, 175, 200,
286 Biodeterioration of Concrete

redox potential 18–20, 130 Senna occidentalis 224

reducing equivalents 77, 78 Serpoleskea 241
red valerian 222 Serpula 175
reinforcement 3, 24, 95, 123, 246, 254, 267 sewage 1, 88, 90–92, 96, 129, 134, 243
reproduction 5, 77, 81, 153, 158, 159, sewers 1, 4, 98, 101, 105, 129–131
216–218, 221 air injection 131
algae 217 cleaning 4
bacteria 5, 77, 81, 153, 159 shock dosing 133, 134
bryophtes 218 turbulence 131
fungi 5, 153, 158, 159 SF – see silica fume
vascular plants 216, 217, 221 shipworm 270
Resinicium 175 siderite 34, 35
Rhamnus alaternus 223 silage 114, 115
rhizoids 216, 217, 230, 245 silica fume 17, 18, 89, 100, 102, 111, 112,
penetration of substrate 216, 230, 116–119, 121, 122, 137, 138, 199
231, 245 silica gel 22, 26, 70, 109, 116, 184
Rhizopus 174 sodium hydroxide 13, 116, 122, 133, 134
Rhynchostegiella 240 sodium nitrate 127, 132
Rhynchostegium 241 sodium tetrathionate 92
Rhytidiadelphus 241 sodium thiosulphate 92
Rinodina 177 solubility 7, 10, 12, 13, 15–18, 20–23, 25–31,
Rivularaceae 124 33–48, 50–57, 59–66, 68–72, 91, 125, 127,
Robinia pseudoacacia 222 179, 183, 190, 230, 274, 275
root barriers 258, 260 diagram 7, 15, 16, 20, 22, 23, 25–28,
root paths 258, 259 30, 33–38, 40, 41, 43, 44, 46, 48, 51–57,
roots 3, 10, 214–217, 220–222, 226–230, 232, 59–62, 66, 68–70, 154, 179, 183, 215
235, 241–244, 245, 256–260 product 7, 10, 12, 13, 15, 17, 18, 20, 21,
damage to pavements 243, 258, 259 25, 29, 31, 35, 39, 42, 45, 47, 50, 53, 56,
exudation of organic acids 229, 230 59, 62–66, 68, 70, 71
intrusion into pipes 259 specific growth rate 80
shrinkage of soils 243, 259 Sphacelaria 234
roughness 84, 125, 165, 231 sphagnum moss 231
Rubus 223 spindletree 222
sponges 266, 271–273
S spore 81, 158, 159, 175, 217–219, 227, 246
Squamarina 177
salazinic acid 167 stability constant 13–16, 20, 21, 24, 28, 29,
salt effect 13, 15 31, 32, 38, 41, 44–47, 49, 50, 52, 55, 57,
Sambucus nigra 222 58, 60, 63–67, 71, 164, 165
sandstone 106, 107, 130 Stachybotrys 174
Sanguisorba minor 229 Staphylococcus 80
Sanionia 241 Staurothele 177
Sarcogyne 177 Steel 2, 3, 24, 95, 123, 127, 128, 141, 198, 246,
Scenedesmus 230 247, 254, 267, 274
Schistidium 241 Stereodon 241
Sciuro-hypnum 241 Stichococcus 234, 250, 251
Scytalidium 174 stock (plant) 222
Scytonema 124 Streptococcus 115
Scytonemataceae 124 Strongylocentrotus 272
sealant 255, 256 succinate 53, 55, 56, 189
sea snail 271 succinic acid 53, 56, 57, 186, 189, 230
sea squirt 266 sulphate 1, 3, 17, 24–26, 78, 86, 88–91, 93–95,
sea urchin 266, 267, 272, 274 105, 122, 125, 127, 128, 130–136, 162,
seed 217–222, 224, 254 163, 200, 253, 254, 256, 258
senna coffee 224
 Index 287

sulphate reducing bacteria 1, 88, 90, 91, 122, Ulocladium 174

128, 130–132, 134–136 ultraviolet radiation 107, 141, 158, 196, 200
sulphur 3, 19, 88, 92, 95, 96, 98, 100, 105, 122, decomposition of oxalic acid 196
123, 128–130, 132, 134, 136–140, 142, Ulva 235, 245, 246
162, 199, 200, 228, 253 Umbillicaria 166
sulphuric acid 1, 2, 22, 24–28, 35, 90–102, urea 105, 275
104, 105, 107–109, 111, 128, 129, 136, uric acid 274, 275
137, 139–141, 143, 179, 253, 254 ursolic acid 167
sulphur oxidising bacteria 92, 96, 105, 122, usnic acid 168
128, 130, 139
sunlight 78, 82, 125, 158, 196, 200, 213, 215, V
230, 243, 248
Vachellia nilotica 224
influence on algal growth 248
van’t Hoff equation 12, 15
super-plasticizer 137
vaterite 32, 35
sycamore 223
Verrucaria 177, 233
Sydowia 174
Virginia creeper 243
Syntrichia 241
Viscaria vulgaris 229
Sypharochiton 270
vulpinic acid 167
Syringa vulgaris 223
W
T
wallflower 222
tartaric acid 54, 55, 57, 60–62, 184, 185, 189,
water/cement ratio 85, 98, 100, 109–111,
190, 200, 229
120, 121, 125, 136–138, 165, 168, 182,
tartrate 54, 55, 57, 60, 62, 184, 185, 190
231, 249 256, 269
Taxus baccata 223
water distribution systems 106, 107
Teloschistes 177
water jetting 256
temperature 11, 12, 15, 16, 19, 31, 32, 81, 91,
water treatment 106
106, 114, 124, 129, 130, 198, 217, 230,
weddellite 48, 180, 194
249, 272
Weissia 241
influence on algal growth 249
whewellite 48, 179, 180, 194, 195
Tenera 270
willow 243
thathirippoovu 224
winemaking 185
thaumasite 35
Woodfordia fruticosa 224
Thiobacillus 2, 92, 93, 96, 101, 123, 253
woody fleabane 222
Thuidium 241
worm 3, 266, 267, 272, 273
Tilia europaea 260
calcareous tube 266
titanium dioxide 200
marine 3, 266, 267, 272, 273
Toninia 177
Tortella 241
X
Tortula 241
Trametes 196 Xanthomonas 123
Trebouxia 235 Xanthoria 178
tree of heaven 223 X-ray diffraction 22, 94, 192, 194, 195, 245
tree pit 259
Trentepohlia 124, 235 Y
Trentepohlia-Gloeocapsa 124
Trichoderma 174 yeasts 153–155
Trichothecium 174 yew 223
Truesdell-Jones equation 12
two-flowered leucas 224 Z
Zirfaea 268
U Zizyphus jujuba 224
Ulmus minor 223