ADMIXTURES

ADMIXTURE
• Any chemical or mineral additive added to the
concrete mixture(other than cement, water,
aggregate) that enhances the properties of
concrete in the fresh or hardened state
• Does not typically include paints and protective
coatings (for steel or concrete)
• ACI 116R defines the term admixture as “a
material other than water, aggregates, hydraulic
cement, and fiber reinforcement, used as an
ingredient of concrete or mortar, and added to
the batch immediately before or during its
mixing”.
 Classification of Chemical
Admixture
• Water reducers
• Set-controlling chemicals
• Air entrainers
• Mineral admixtures
• Specialty admixtures
- Viscosity modifiers
- Corrosion inhibitors
- Shrinkage reducing admixtures
- Others
 Broad classification
Water Reducers

Normal Mid-range High range

5 - 8% water reduction 8 - 15% water reduction 15 - 25% water reduction
 Basic admixture chemistry
•Water reducers belong to the ‘dispersants’ family (like the
detergents and soaps used for washing)
•Dispersants are long-chain organic molecules have polar
(hydrophilic) and non-polar (hydrophobic) groups; these get
adsorbed on the cement particles
•Cement particles are dispersed by electrostatic repulsion
•Upon hydration, electrostatic charge diminishes and flocculation
occurs
ACTION OF PLASTICIZERS
 Normal water
reducers(plasticizers)
•Anionic surfactants-Lignosulphonate salts (Sodium salts of sulphonated
lignin)

•Nonionic Surfactants-Hydroxycarboxylic acids – Citric, gluconic acid

•Carbohydrates – Corn syrup, dextrin

•The dosage of normal WRs is 0.3 – 0.5 % by weight of cement. At higher

dosages, there is danger of excessive retardation and bleeding. Also, returns
diminish, and excessive air entrainment can occur.
 HIGH RANGE WATER REDUCERS (SP)
• For a given workability, the water demand is
reduced, thus resulting in higher strength and
durability.
• For a given w/c and strength, workability can
be increased.
• For a given w/c, strength and workability, the
quantity of cement can be reduced
 CLASSIFICATION OF SP
1st generation: Lignosulphonates at high dosages

2nd generation:
Polysulphonates
- Sulphonated melamine formaldehyde (SMF)
- Sulphonated naphthalene formaldehyde (SNF)

3rd generation:
- Polycarboxylates
- Polyacrylates
- Monovinyl alcohols

Typical dosage: 0.7 – 1.0% by weight of cement.

Also called ‘Superplasticisers’
 RANGE OF ACTION
The 1st generation HRWRs need a slump of around 75 mm for action (~0.45
w/c). The slump is increased up to 150 – 200 mm.

The 2nd generation admixtures can work at reasonably low slumps (25 – 50
mm, corresponding to w/c of 0.35 – 0.40) to increase the slump to ~ 250 mm.

The 3rd generation HRWRs, on the other hand, can even be used with no
slump concrete (0.29 – 0.31 w/c), and the slump is increased to more than
250 mm.

Concrete possessing slump above 225 – 250 mm is called ‘rheoplastic’.

 EFFECT OF SP ON FRESH STATE
CONCRETE
• Dramatic improvement in zero slump concrete
even at nominal dosages.
• A high dosage is required to fluidise no slump
concrete.
• An improvement upto 25 cm is seen based on
the initial slump, dosage and cement mix.
• Slump increases with dosage upto the
saturation dosage.
• Over dosage may harm the concrete.
 COMPATIBILITY
• Optimum fludizing effect at lowest dosage for
the same mixture with different admixtures is
considered economical.
• Composition of cement ,fineness of cement
etc. affect the maximum fludizing effect of an
admixture.
 Tests to Check Compatibility
• Marsh cone test
• Mini slump test
• Flow table test
Flow table test
Marsh cone test
Marsh cone
test

Flow table
test
Result for different time
 Factors affecting the Workability
• Type of SP
• Dosage
• Mix composition
• Variability in cement composition and
properties
• Mixing procedure
• Equipments
• Others
 COMMON PROBLEM WITH SPs

•Super plasticizing admixtures are prone to slump retention problems.

•The efficient dispersion of cement and reduced surface tension of water

leads to hydration of cement, which in turn causes the diminishing of the
electrostatic charge, and flocculation occurs.

•Moreover, w/c of superplasticized concrete is typically low.

•Cement-superplasticiser compatibility problems

 SITE PROBLEMS
• Slump of reference mix
• Inefficient laboratory mixer for trial.
• Sequence of addition of SP.
• Problem with crusher dust and crushed sand.
• Selection and compatibility of SP.
• Determination of optimum dosage.
• Slump loss.
• Compaction.
• Segregation and bleeding.
• Finishing
• Removal of formwork.
SLUMP LOSS:
• Often there is delay in mixing and placing of
concrete.
• The high slump achieved in mixer is lost.
• Slump loss can be managed by using retarders
 Retarding SPs
 Repetitive dosage of plasticizer
 Dosing at final point
 Maintaining low temperature
 Using compatible SP.
Effect of SP on Hardened concrete
 New generation SP
• Polymers with backbone and graft chains, such as PCEs,
acrylic esters, and cross-linked acrylic polymers, cause
dispersion of cement grains by steric hindrance.
• This phenomenon relates to the separation of the
admixture molecules from each other due to the bulky
side chains.
• Steric hindrance is a more effective mechanism than
electrostatic repulsion. The side chains, primarily of
polyethylene oxide extending on the surface of cement
particles, migrate in water and the cement particles are
dispersed by the steric hindrance of the side chains.
 Primary mechanisms of action

Lowering of Zeta Potential (leading to Steric hindrance

electrostatic repulsion)
Polymers with backbone and graft
Substances with functional groups chains
- Lignosulfonates
- Sulfonated condensate of naphthalene - Polycarboxylate ester
formaldehyde - Carboxylic acrylic acid with acrylic
- Sulfonated condensate of melamine ester
formaldehyde - Cross linked acrylic polymer
- Sugar refined lignosulfonates
Set-controlling chemicals

•Accelerators
•Retarders
 Applications
• Accelerators
- Earlier finishing of slabs
- Increase early age strength
- Early removal of forms
- Cold-weather concreting
• Retarders
- Hot-weather concreting
- ‘Long-haul’ applications
- Workable for longer time
 Basic admixture chemistry

• Set-controllers are organic or inorganic

chemicals that interfere with the hydration
process of the cement
• The rate of dissolution of cement compounds,
that is necessary for hydration to occur, is
either speeded up or slowed, depending on
the chemical
 Action of set-controllers
•Typically, set controllers affect cement hydration during the early stages,
namely, during the processes of dissolution cement compounds and
nucleation of hydration products

•According to Joisel, only the dissolution is affected by these admixtures. If

we consider the hydrating PC to be a mixture of cations (Ca) and anions
(silicate and aluminate), then the following scenarios can occur.

1. An accelerator should promote the dissolution of both cations and

anions. Since several anions are present, the accelerator should promote
the dissolution of that anion which has the lowest dissolving rate, i.e.,
silicate. A retarder impedes the dissolution of Ca ions and aluminates.
2. The presence of monovalent cations – K+ and Na+ - reduces the
solubility of Ca, but promotes the dissolution of silicates and
aluminates. At small concentrations, the former effect is
predominant, and at high concentrations, the latter effect is
predominant.
3. Monovalent anions – Cl-, NO3-, etc. – reduce the solubility of
silicates and aluminates, and promote the dissolution of Ca. At small
concentrations, the former effect is predominant, and at high
concentrations, the latter effect is predominant.
4. In the case of salts of weak bases and strong acids (e.g. CaCl2) or
strong bases and weak acids (e.g. K2CO3), at low concentration, the
dominant effect is the retardation of Ca and aluminate dissolution; at
high concentration, acceleration of the reaction occurs. Calcium
chloride (at 1 – 3% by weight of cement) is the most effective
accelerator.
 Accelerating chemicals

Chloride accelerators: CaCl2, NaCl

Non-chloride accelerators
Inorganic: Nitrates and nitrites of Ca and Na, thiocyanates, thiosulphates,
and carbonates of Ca and Na.
Organic: Amines (triethanol amine – TEA, diethanol amine – DEA),
carboxylic acids (Ca salts of formic and acetic acid), formaldehyde.
 Retarding chemicals

Organic retarders: Lignosulphonates, hydroxycarboxylic acids (citric, gluconic),

carbohydrates (corn syrup, dextrin). These are the same chemicals as normal water
reducers.

Inorganic retarders: Borates, phosphates, Zn and Cu compounds. These are not generally
used because of their high costs and low solubility.

Extended set admixtures: Phosphonates and other phosphorus containing organic acids
and salts, gluconic acid, etc. These admixtures are used for the following purposes:
- Stabilization of washwater for concrete
- Stabilization of returned plastic concrete
- Use of fresh concrete for long haul (large travel times) applications
 Common issues with set-controllers

• Essential to pay particular attention to

dosage – same chemical may behave as
accelerator or retarder depending on
concentration
• Admixtures should be added soon after
cement and water come into contact
Air-entraining agents
 Applications

• Protect against damage due to freezing and

thawing cycles
• Side effects:
1. Improve workability
2. Reduce segregation and bleeding
3. Reduce strength due to increased porosity
 Admixture chemistry
Air-entraining agents are also surface-active chemicals.

Unlike the water-reducing surfactants, the hydrocarbon chain does not

have any polar groups, and is entirely hydrophobic.

The hydrophilic polar groups are similar to water reducers.

 Mode of action
•Air bubbles are generated during the agitation and mixing of the
concrete. The air-entraining agents simply help to stabilize these bubbles
by altering the surface tension of water.

•Some common chemicals used as air entrainers are neutralized vinsol

resin, derivatized pine rosin, and fatty acids (palmitic and stearic acid),
and synthetics like dodecyl benzene sulfonate.

•Air entrainers are added to the concrete mixture either early in the
process – with the sand and coarse aggregate – or after the cement has
been added along with some of the mix water. Air entraining chemicals
should never be mixed with any other chemical additives.
Air entrainment

Small and stable air bubbles

required
Air void parameters – total
entrained air, and distance
between voids (not more than
200 micron)
 Factors affecting air entrainment
• Type and quantity of entraining agent used.
• w/c of mic.
• Type and grading of aggregate.
• Mixing time.
• Temperature.
• Type of cement.
• Influence of compaction.
• Other admixtures used.
 Effect of Air entrainment on the
properties of concrete
• Increased resistance to freezing and thawing.
• Improvement in workability.
• Reduction in strength.
• Reduces tendency to segregate , bleed.
• Decreases permeability
• Increases the resistance to chemical attack
• Permits reduction in sand content.
• Improves placeability and early finishing.
• Reduces cement content,cost.heat of hydration.
• Reduces unit weight
• Reduces water content
• Reduces alkali aggregate reaction.
• Reduces modulus of elasticity.
 Measurement of air content
• Gravimetric method.
• Volumetric method.
• Pressure method.
MINERAL ADMIXTURES
 Introduction
 Also called ‘Supplementary Cementing Materials’
 Used when special performance is needed: Increase in strength,
reduction in water demand, impermeability, low heat of
hydration, improved durability, correcting deficiencies in
aggregate gradation (as fillers), etc.
 Result in cost and energy savings: Replacement of cement leads
to cost savings; energy required to process these materials is
also much lower than cement
 Environmental damage and pollution is minimized by the use of
these by-products
 Usage depends on supply and demand forces, as well as the
market potential and attitudes
 Typical compositions
% by mass PC GGBFS F-FA C-FA SF

SiO2 21 35 50 35 90

Al2O3 5 8 25 20 2

Fe2O3 2 3 10 5 2

CaO 65 40 1 20 -

PC: Portland cement, GGBFS: Ground granulated blast furnace slag, F-FA: Type F fly
ash, C-FA: Type C fly ash, SF: Silica fume
 Classification

 Cementitious
 Highly pozzolanic: Silica fume, Rice husk ash
(controlled burning)
 Normally pozzolanic: Class F fly ash
 Cementitious and pozzolanic: GGBFS, Class C fly
ash
An additional category is also suggested by
researchers – Weak pozzolans, such as slowly
cooled and ground slag, bottom ash, and field-
burnt rice husk ash
 Pozzolans

• Pozzolans are siliceous or aluminous

materials, which possess by themselves
little or no cementitious properties, but in
finely divided form react with calcium
hydroxide in the presence of moisture at
ordinary temperatures to form compounds
possessing cementitious properties
(definition according to ASTM C595).
 Pozzolanic reaction

• CH + Reactive SiO2 (or Al2O3) + H2O 

C-S-H (or C-A-H)
• Reaction is
- Lime consuming
- Pore refining
- Interface refining (why?)
- Slow (low heat of hydration)
- Accelerated by alkalis and gypsum
Rate of pozzolanic reaction
 Pozzolanic activity
• Pozzolanic activity is evaluated using the
Pozzolanic Activity Index test, which defines the
index as:
PAI (%) = Strength (PC/pozzolan
mixture)*100 / Strength (PC mixture)
• In this test, the mix design is done using a
volumetric replacement of cement by the
pozzolan (ASTM C311) as opposed to the Slag
Activity Index test (ASTM C989) where a mass
replacement is used.
Fly Ash
 Source

• By-product obtained during combustion of

coal in thermal power plants
• The quality and composition of fly ash
depends on the type of coal being burnt
 Rank of coal

5. Lignite (brown coal)

4. Sub-bituminous coal (70 – 80% C)
3. Bituminous coal (80 – 90% C) – Soft coal, used for ordinary
purposes
2. Semi-bituminous coal – Good heating value, has a
smokeless flame
1. Anthracite (90 – 95% C) – hard coal; high temperature
needed to burn it
Low rank coals contain impurities such as clay, shale, quartz,
carbonates, and sulfides. It is these impurities which give
fly ash its composition.
 Barriers to use of fly ash

1. Difficult quality assurance

2. Poor marketing
3. Conservative attitudes
4. Storage problems
5. It is called a ‘waste’ instead of pozzolan or
cement
 Need for fly ash utilization
• Nearly 73% of India’s total power
generation is thermal (mostly using coal)
• 140 million tons of fly ash being generated
annually
• World Bank - by 2015, disposal of coal ash
would require 1000 square kilometres or
one square metre of land per person in
India
 Fly ash utilization

• Only about 30% of fly ash is being utilized

for various industries; remaining amount
gets dumped  considerable burden on
the environment
• Increased disposal implies more
environmental hazards (lead and arsenic
pollution), diseases, etc.
Steps taken to promote utilization
Ministry of Environment and Forests:
• proposal to ban excavation of top soil within a
radius of 50 kilometres from the location of
thermal power plants, making it essential for fly
ash to be used for brick manufacture
• fly ash transport costs should be borne by the
power plants
NTPC – Dry ash technology
Other agencies are also developing various uses
DST – Has set up the Fly Ash Unit
 Collection of fly ash
• During combustion of coal, 75 – 80% of the ash flies
out with the flue gas, and is thus called ‘fly ash’. The
ash that doesn’t fly out is called ‘bottom ash’. This
can be processed as aggregate, but is generally not
used in concrete.
• The collection of fly ash is done using the following
two types of precipitators:
- Bag-house precipitator
- Electrostatic precipitator
• The bag-house precipitator is found to be more
efficient, and removes out very fine material
 Precipitators

P.J.Tikalsky,"The effect of Fly ash on the Surface Resistance of Concrete”

 Uses of fly ash
· As a mineral admixture
• As a filler
· As a synthetic aggregate: Fly ash aggregate can be
produced by sintering. The resultant aggregate can be
used for lightweight concrete. However, it is very
expensive. Aggregate can also be synthesized by
agglomeration using lime or cement as binder, as in
‘cold bonding’.
• Fly ash beneficiation – Grinding of coarse fly ash to
make it suitable for use as a mineral admixtures
 ASTM Classification
 Type C: This is also called High Calcium fly ash, and
possesses both cementitious and pozzolanic properties.
10 – 15% of the material has a particle size greater than 45
μm, and the fineness (Blaine) is 300 – 400 m2/kg. The
particles are primarily solid spheres with a smooth
texture. The average particle size is less than 20 μm.

 Type F: This is also called Low Calcium fly ash, and is a

normally pozzolanic material. 15 – 20% of the material is
larger than 45 μm, and the fineness is 200 – 300 m2/kg.
Particles are solid spheres with a smooth texture, and the
average particle size is 20 μm.
 Structure of fly ash

Apart from solid spherical particles, there also

may exist hollow spheres. The small hollow
spheres with entrapped gas are called
cenospheres, while the large hollow spheres
with solid spheres inside them are called
plerospheres.

www.ctlgroup.com/group/ content.asp?
 Other issues

• The loss on ignition of fly ash can represent

the amount of unburnt carbon present.
• Too much of unburnt carbon can interfere
with the air-entraining agent, leading to
poor air void parameters.
• Restrictions are also placed on the sulfate
(SO3) content, MgO content, alkali content,
and moisture content of fly ash
 Effects on fresh concrete
 The setting time is increased when fly ash is used.
 Workability and flow of concrete are increased
due to the spherical shape of the fly ash particles,
which lends a ball-bearing type effect on the
concrete mixture.
 Bleeding and segregation are usually reduced for
well-proportioned fly ash concrete.
 The paste volume is increased when mass
replacement of cement by fly ash is done.
Effects on hardened concrete
· Strength gain of fly ash concrete is slower than normal concrete, as
stated in the previous sections. Ultimate strengths are usually
improved when fly ash is used.
· Creep and shrinkage of fly ash concrete are typically higher than
normal concrete, because of the increased amount of paste in the
concrete (when mass replacement is done).
· More air-entraining admixture is needed to entrain air in fly-ash
concrete.
· The results on the effects of fly ash on sulphate resistance are
inconclusive. (This topic will be discussed further in the chapter on
durability).
· Expansions during alkali aggregate reaction are reduced by the use of
fly ash, because of the dilution of Portland cement (implying there are
lesser alkalis available).
 Specialized applications

 In high strength concrete, as an additional

cementitious material.
 In roller-compacted concrete. Fly ash is good for
bonding in-between the layers of this concrete.
 In controlled low-strength materials (CLSM),
which are flowable mortars used as backfill
 As a synthetic aggregate
• For manufacture of bricks
 Issues with fly ash

• Due to transportation cost, the use of fly

ash beyond 40-50 km from the thermal
power plant becomes uneconomical
• Lack of appropriate technologies for
handling and transportation – no control on
the quality received from power plant
Silica fume

Or Microsilica
 Source

• By-product of ferrosilicon industry

• Purity of silica fume depends on the
ferrosilicon alloy from which Si metal is
being extracted
Ferrosilicon alloys SiO2 content

FeCrSi 18 – 48%
FeMgSi 44 – 48%
50% ferrosilicon 72 – 77%

70% ferrosilicon 84 – 88%

Silicon metal (98%) 93 – 98%

 Collection of silica fume
After being collected over the furnace, the silica
fume must be transferred, cooled, and
physically trapped.
The large pipe on the left is bringing the silica
fume from the furnaces.
The vertical elements are cyclones that are
used to remove oversize and other unwanted
materials.
The large building is the bag house where the
fume is captured.
 Silica fume available as..
 As is bulk powder: Due to the low specific gravity of silica
fume (~2.2), the bulk powder becomes very bulky and
difficult to handle and transport.
 Dry-densified silica fume: Compaction by pressure is used
to flocculate the silica fume particles. An efficient
superplasticizer is required to deflocculate and cause a
good dispersion of the silica fume in concrete.
 Slurry: 50% water + 47% silica fume + 3% chemical agent,
that keeps the particles in suspension and prevents
gelling. The slurry form is susceptible to gelling in cold
climates. However, it is a very efficient way of dispensing
silica fume. Also, storage space can also be reduced.
Forms of silica fume

D P
 Silica fume colours

Premium -- White Standard -- Grey

Whiter implies purer

 Physical properties

Particle size (typical) <1µm

Bulk density
as-produced 130 to 430 kg/m3
slurry 1320 to 1440 kg/m3
densified 480 to 720 kg/m3
Specific gravity 2.2
Surface area (BET) 13,000 to 30,000 m2/kg
 Cost & Benefits

• Cost: almost 10 times as much as PC

• Typically used at 5 – 15% replacement level
• Benefits from silica fume are due to the
pozzolanic reaction that produces
additional C-S-H, as well as due to the
particle packing (filler effect) of the fine
silica fume particles
 Effects on fresh concrete

 Because of its high fineness, the use of silica fume causes

an increase in the water demand of concrete. Typically it is
always used in conjunction with a superplasticizer.
 Silica fume causes the mix to be sticky and cohesive. Also,
concrete mixes with silica fume are prone to slump loss
problems. Because of its cohesiveness, a higher slump is
needed to place silica fume concrete.
 Bleeding is reduced drastically. In fact, most silica fume
mixes do not show any bleeding. In dry areas, if the
evaporation rate exceeds the rate at which concrete sets,
plastic shrinkage may occur.
 Fineness

100.0

Percent passing
80.0
60.0
40.0
20.0
0.0
0.1 0.3 0.5 0.7 0.9
Diameter, micrometers

cement silica fume

Plastic shrinkage problems
Pc (capillary tension) =
0.001γS/(w/c),
where γ = surface tension of
water = 0.0073 N/m, and S =
surface area of particles (20000
m2/kg for SF, 350 m2/kg for
cement).
Assuming a w/c of 0.35,
For PC concrete, Pc = 0.07 MPa
For SF concrete, Pc = 4.20 MPa
M. D. Cohen, unpublished
 Effects on hardened concrete
•Pore size refinement and reduction in
permeability occurs when silica fume is
used.
•Compressive and flexural strengths are
increased.
•Elastic modulus is increased (ESFC ~ 115%
EPCC), or, in other words, concrete becomes
stiffer with the use of silica fume

D.W.christen , E.V.Sorenson & F.F.Radjy,"

Rockbond: A New Microsilica Concrete
Bridge Deck Overlay Material"
• Creep and shrinkage are increased at high replacement levels (10
– 15%) because of an increase in the volume of the paste.
• Amount of air-entraining agent required for a particular volume
of air is increased in silica fume concrete. Freeze-thaw resistance
is reduced slightly compared to normal concrete, but damage is
usually limited owing to the extremely low permeability of SFC.
• In most cases, silica fume concrete shows better resistance to
chemical attack (exceptions being ammonium sulphate and
magnesium sulphate attack), owing to the decreased
permeability, as well as due to reduced CH in the paste.
• Expansions due to ASR are reduced in silica fume concrete.
 Corrosion rate is reduced with the use of silica
fume. This is because of two reasons: the low
permeability of SFC causes a lower availability of
moisture and oxygen at the cathodic sites, and
the high resistivity of SFC makes the flow of
electrons difficult.
 Carbonation depth is generally lowered.
 SFC has very good abrasion and erosion
resistance.
 Fire performance of SFC is not very good
Ground-granulated blast furnace
slag (GGBFS)
 Slag production
•Blast furnace slag is a by-product of the
extraction of iron from iron ore. Coke and
limestone are added as fluxes inside the blast
furnace. The impurities in iron ore combine
with the lime and rise up to the surface of the
blast furnace, while the molten iron, which is
heavier, stays at the bottom.
•1892 was the first time that Portland-blast
furnace slag cement was manufactured. In the
present day scenario, slag is used almost in
every country to varying degrees.
•The reactivity of slag depends on the rate of
cooling. In increasing order of reactivity, the
cooling processes may be ranked as: Slow
cooling (in air), Rapid cooling (by water spray),
and Quenching (dipping in water).
M.Regourd ,"Slags and Slag Cements”
 Types of slag
• Air cooled slag: Low reactivity slag that finds use as aggregate.
The strength and toughness of this aggregate makes it a very
suitable material for railroad ballast.
• Expanded or foamed slag: Low reactivity slag that is foamed
with air. Makes a very good lightweight aggregate, and is used
for thermal insulation.
• Granulated: This is a high reactivity slag, and is usually
quenched. The hardened matter is then ground to a fineness
similar to cement. Thus the name: Ground Granulated Blast
Furnace Slag (GGBFS).
• Pelletized slag: The reactivity is similar to GGBFS, but the
process of pelletization is a complex one. Typically, this type of
slag is not used as much as GGBFS.
 Pelletization

F.G.Hogan and J.W.Meusel, “Evaluation for Durability and Strength Development

of a Ground Granulated Blast Furnace Slag”
 Factors governing properties

 Chemical composition of GGBFS

 Alkali concentration of reacting system
 Glass (reactive SiO2) content of GGBFS
 Fineness of GGBFS and PC
 Temperature during early phase of
hydration
 Hydration of slag
An activator is necessary to hydrate the slag. The activation
of slag hydration can be done in the following ways:
 Alkali activation: e.g. by caustic soda (NaOH), Na2CO3,
sodium silicate, etc. The products formed are C-S-H,
C4AH13 and C2ASH8 (Gehlenite).
 Sulphate activation: e.g. by gypsum, hemihydrate,
anhydrite, phosphogypsum, etc. The products formed are
C-S-H, ettringite, and aluminium hydroxide (AH3).
 Mixed activation: When both alkali and sulphate sources
are present, such as in a cement system.
Effects on concrete properties

 Apart from delaying the initial set and

strength gain, slag does not significantly
alter the fresh concrete properties.
 The ultimate strengths with slag are
generally improved; the durability is also
improved with the replacement of cement
by slag. Especially in marine environments,
slag is the material of choice
 Rice husk ash

• This is a high reactivity pozzolan obtained by controlled

calcination of rice husk.
• Field-burnt rice husk is almost crystalline in nature, and
makes a weak pozzolan. Thus, to obtain a high degree of
pozzolanicity, a good control is needed while burning.
• RHA usually contains a large amount of unburnt carbon
which might adversely affect air entrainment.
• RHA is a fine material, with particle sizes less than 45 μm,
and a surface area of 60000 m2/kg.
• The particles are typically cellular. A high amount of
reactive silica is present in the system (> 90%).
 Metakaolin
• This is obtained from calcination of kaolinite clay in the range of
740 – 840 oC. The crystalline clay loses its structure at this
temperature by the loss of bound water. Burning should strictly
be done in this range, since beyond 1000 oC, recrystallization of
the clay occurs.
• A general formula of metakaolin can be written as AS2. This
compound reacts with CH to form additional C-S-H
• The content of C-S-H and its formation rate depends on the
mineralogical characteristics of the kaolin precursor.
• Metakaolin has a performance comparable to silica fume as a
mineral admixture in concrete.
• Since MK is not a by-product, its processing is an expensive
affair. Thus the marketability of MK is not as good as silica
fume, which is a proven by-product.
 MK reaction
The aluminosilicate compound AS2 reacts with CH
produced during cement hydration in the
following form (suggested by Murat – in Cement
and Concrete Research, Vol. 13, 1983):
AS2 + 6CH + 9H  C4AH13 + 2C-S-H
C-S-H formed in this reaction is aluminous, with a
C/S ranging from 0.83 (for crystalline forms of C-S-
H) to > 1.5 (for amorphous and semi-crystalline
forms of C-S-H).