Cement and Concrete Research 31 (2001) 767 ± 774

Mechanical properties of calcium-leached cement pastes

Triaxial stress states and the influence of the pore pressures
F.H. Heukamp*, F.-J. Ulm, J.T. Germaine
Department of Civil and Environmental Engineering, Room I-215, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge,
MA 02139, USA
Received 21 April 2000; accepted 1 February 2001

Abstract

Application of concrete in nuclear waste containments requires knowledge of its mechanical behavior when subjected to calcium leaching.
In order to address real-life situations, multiaxial stress states of leached material must be considered. This paper reports results from a series
of triaxial tests of calcium-leached cement paste obtained from accelerated leaching tests that operate on an acceleration rate of 300,
compared with natural calcium leaching. Along with the global strength loss due to chemical decohesion, an important loss of frictional
performance is reported. Environmental scanning electron microscope (ESEM) pictures of both leached and unleached material are
presented, and they indicate that this loss of frictional performance can be associated with a highly eroded microstructure perforated by the
leaching process. In addition, the frictional behavior of leached cement pastes is found to be strongly dependent on the drainage conditions of
the material and thus, on the interstitial pore pressure. Through a poromechanical analysis, it is shown that this high pore pressure sensitivity
of leached cement paste can be attributed to the low skeleton-to-fluid bulk modulus ratio, Ks/Kf, of the degraded material, which, together
with the increase in porosity, leads to the high compressibility of calcium-leached materials. This low Ks/Kf ratio is the consequence of an
intrinsic chemical damage of the solid skeleton, which occurs during calcium leaching. D 2001 Elsevier Science Ltd. All rights reserved.

Keywords: Nuclear waste storage; Accelerated calcium leaching; Triaxial testing; Frictional behavior; Chemical damage; Pore pressure; ESEM

1. Introduction ized by sharp dissolution fronts [1,8,15]. This calcium

leaching leads to a degradation of the mechanical properties
Concrete is commonly employed in radioactive waste of concrete. Moreover, as leaching by deionized water is a
disposal as an effective and economical construction mate- very slow process, monitoring the durability of nuclear
rial for containment barriers, liners, and encasement of waste storage structures involves large time scales that
containers. Because of the critical nature of nuclear waste, complicate experimental assessment.
the load-bearing capacity of concrete containment structures In terms of chemo-mechanical effects, the loss of elastic
must be ensured over several hundred years. A widely stiffness (chemical damage) and the strength loss in uniaxial
accepted reference scenario for the durability design of compression due to calcium leaching have been subject to
waste containers is calcium leaching by pure water [6]. This first studies [7,11,18]. In contrast, little is known about the
design scenario refers to the risk of water intrusion in the behavior of leached cementitious materials under bi- or
storage system. It is assumed that the concrete is subjected triaxial stress states.
to leaching by permanently renewed deionized water acting
as a solvent. The lower calcium ion concentration in the
interstitial pore solution leads to the dissolution of the 2. Experimental program
calcium bound in the skeleton as Portlandite crystals,
Ca(OH)2, and calcium-silicate-hydrates (C-S-H) character- The objective of the experiments is to determine the
strength domain of leached cementitious materials under
* Corresponding author. Tel.: +1-617-253-0251; fax: +1-617-253-
triaxial stress states. This requires (1) an accelerated test
6044. method able to reproduce in vitro the intrinsic material
E-mail address: heukamp@mit.edu (F.H. Heukamp). response that characterizes the long-term behavior of cemen-

0008-8846/01/$ ± see front matter D 2001 Elsevier Science Ltd. All rights reserved.
PII: S 0 0 0 8 - 8 8 4 6 ( 0 1 ) 0 0 4 7 2 - 0
768 F.H. Heukamp et al. / Cement and Concrete Research 31 (2001) 767±774

titious materials, and (2) a homogeneous decalcification state high calcium equilibrium concentration of 92.9 mol/l (6 M
to assess the ``real'' material response in the mechanical tests. NH4NO3), which in fine is the key to an accelerated leaching
process. In contrast, for pH > 9.25, the lack of ammonium
2.1. Design reduces the calcium equilibrium concentration and thus, the
overall leaching process. This is why it is important to
Calcium leaching is a coupled diffusion ± dissolution monitor the pH of the aggressive solution during the
process involving sharp dissolution fronts that propagate accelerated leaching experiment. This accelerated leaching
through the structure. The sharp dissolution fronts are due to has the same characteristics as `natural' leaching:
the locally quasi-instantaneous dissolution. The time scale
The dissolution that occurs at a higher equilibrium
of the leaching process is governed by the diffusion proper- concentration is still quasi-instantaneous in comparison with
ties of the material. Small sample sizes and high calcium the diffusion of calcium ions from the dissolution front to
efflux favor a rapid leaching process. In the present study, the outside, and thus, the leaching process is still governed
the test program was carried out on pure cement pastes, the by the diffusion properties of the material.
basic constituent of concrete, allowing for small and rela-
Albeit different in kinetics, the ammonium nitrate-
tively homogeneous material samples. Moreover, the cal- based calcium leaching leads to the same mineral end
cium efflux can be artificially accelerated by increasing the products in the cementitious material [7].
chemical equilibrium concentration, i.e. the calcium solubi- As a quasi-self-similar diffusion ± dissolution problem,
lity, at the dissolution front. Leading to a higher calcium the overall acceleration rate can be assessed from the ratio
concentration gradient in the pore solution, and thus to of the similarity parameter of front propagation:
higher efflux, this can be achieved by replacing the deio-  2
nized water that is part of the reference scenario by an d1
aˆ 3†
ammonium nitrate solution (NH4NO3). Calculations based d0
on Ref. [3] show that the equilibrium calcium concentration
is shifted from 922 mmol/l (using deionized water) to 92.9 p p
where d0 ˆ xd = t0 and d1 ˆ xd = t1 are close to a multi-
mol/l in a 6-M NH4NO3 (480 g/kg) solution. plied constant the self-similar parameters that define the
The dissolution of, e.g. Portlandite [Ca(OH)2] in the position xd of the dissolution front in the normal and
ammonium nitrate solution can be written according to the accelerated leaching setting, respectively (see, e.g. Ref. [15]).
following reaction equation: Finally, it has to be mentioned that the maximum
Ca OH†2 ‡ 2NH4 NO3 ]Ca2‡ ‡ 2OH ‡ 2H‡ solubility of ammonium nitrate in water mentioned above
(680 g/kg) shows that more than 6 M ammonium nitrate can
‡ 2NH3 ‡ 2NO3 ]Ca NO3 †2 ‡ 2NH3g ‡ 2H2 O 1† be dissolved in water. The calcium equilibrium concentra-
tion, in principle, increases with the ammonium nitrate
The dissolution front corresponds to the place where concentration. However, adding ammonium nitrate beyond
Portlandite is leached. It is the first mineral to be leached; 6 M does not accelerate the leaching process, due to the
the C-S-H decalcify thereafter [1]. The high solubility of limited amount of Portlandite in the cement paste.
ammonium nitrate (680 g/kg of solution at 20°C), the
change to the gaseous phase of NH3, and the high solubility 2.2. Calcium leaching
of calcium nitrate favor the dissolution process. An
equilibrium exists between the ammonium (NH4) and the The chosen Type I Portland cement (composition, see
aqueous ammonia (NH3aq): Table 1) paste samples with a water ± cement ratio of w/
c = 0.5 are cylinders of diameter 11.5 mm and length 60
‰NH3aq Š‰H‡ Š
NH‡ ‡
4 ]NH3aq ‡ H ; ˆ 10 pKa
2† mm. After 24 h, the specimens were demoulded and cured
NH‡ 4 in a saturated lime solution for 27 days at 20°C. One-half of
the samples was immersed in the 6-M ammonium nitrate
where [NH3aq], [H + ], and [NH4 + ] are the activities of the solution for accelerated leaching; the other half was stored
different species. The pH at which the equilibrium for control purposes in limewater. To obtain good mixing of
concentration of ammonium equals that of ammonia (NH3) the ammonium nitrate solution and most homogeneous
is equal to the pKa of NH4 + , 9.25. Hence, for a pH smaller leaching conditions possible, the tanks containing the
than 9.25, there is always more ammonium than ammonia in
solution. During the leaching process, the ammonium in the Table 1
pore solution of the cementitious material dissociates to Type I Portland cement constituents, in mass percent
ammonia and H + [Eq. (2)] due to the basic environment. H + OPC Type I CaO SiO2 Al2O3 MgO SO3 Na2O
reacts with OH , reducing the OH activity [2], and 62.3 20.8 4.4 3.8 2.9 0.39
favoring Portlandite [Ca(OH)2] dissolution. Together with Fe2O3 K2 O C3Al C3S C2S Ignition loss
the change to the gaseous phase of NH3 and the high 2.4 1.28 8 53 20 0.66
solubility of calcium nitrate, this qualitatively explains the Data from producer.
 F.H. Heukamp et al. / Cement and Concrete Research 31 (2001) 767±774 769

Ref. [1]), we obtain with the 6-M ammonium nitrate

solution (480 g/kg of solution) an overall acceleration of
a = 300 according to Eq. (3). For the 11.5 mm in diameter
cylinder samples, the dissolution front reached the center of
the specimens in less than 9 days. However, XRF analysis
showed that the average bulk calcium content was still
decreasing significantly thereafter (Fig. 2). The quasi-steady
state, i.e. homogeneous calcium content, required 45 days of
accelerated leaching.

2.3. Triaxial test setup

The triaxial cell used in the experiments has a high-

pressure steel chamber. The confinement pressure is applied
on the sample that is in a latex membrane by oil. The
hydrostatic confinement pressure is applied first before the
Fig. 1. Oscillating table for controlled calcium leaching.
deviator is added until failure. The displacement rate is
constant, and the vertical stress is measured by an internal
ammonium nitrate bath were mounted on a slowly oscillat- load cell. Fig. 3 gives a schematic view of the triaxial cell.
ing table (Fig. 1). In that way, the bath was constantly The test focuses on strength; strains are not of concern.
agitated, and the sample surfaces were in free contact with
the aggressive solution. Each tank of 15  15 cm quadratic
shape was filled with 2.2 kg of aggressive solution and
contained 26 specimens. In addition, carbonation by CO2
was prevented through replacing the air in the tanks by pure
nitrogen gas (Fig. 1). In parallel, the pH of the solution was
monitored. A necessary renewal of the solution due to a lack
of ammonium (NH4 + ) would have been indicated by a pH
greater than 9.25 [see Eqs. (1) and (2)]. At the chosen
combination of ammonium nitrate concentration, bath
volume and number of specimens, it turned out that the
aggressive solution had not to be renewed during the
leaching experiment; the leaching process continuously took
place at the highest possible rate. During the leaching
process, samples were taken out of the aggressive bath to
determine visually the progress of the dissolution front. A
square root of time function for the dissolution frontppro-
gress was obtained being in the order of d1 ˆ 2 mm= day
If we take for reference the ``natural'' pprogress
 of the
propagation front of d0 ˆ 0:115 mm= day (see, e.g.

Fig. 2. Bulk calcium content evolution during leaching. Fig. 3. Schematic view of the triaxial cell.
770 F.H. Heukamp et al. / Cement and Concrete Research 31 (2001) 767±774

To avoid the development of expansive calcium nitro-

aluminate products [7,14], the degraded samples were kept
in the bath until being tested. To have smooth surfaces and
parallel ends, the specimens were cut with a diamond saw to
a length of 23.5 mm. Confinement pressures up to 10 MPa
were applied.
An important issue arises when it comes to triaxial
testing of calcium-depleted cement pastes, related to chemi-
cal damage, i.e. the irreversible loss of elastic stiffness, due
to calcium dissolution. This chemical damage leads to a
bulk modulus of the degraded material sample, say 1 GPa
[7], which is of the same order as the compressibility
modulus of the interstitial solution, i.e. Kf = 2.3 GPa. Hence,
in contrast to undegraded cementitious materials, the pore
pressure in the interstitial space may affect both deform-
ability and strength of the material. To account for this
effect, the triaxial tests were carried out under drained and
undrained conditions. The drained conditions were assured
by a dry filter stone at the base of the specimen. Fig. 5. Degraded paste in normalized deviator mean stress plane (drained
and undrained).

p
3. Results setup. The slope dc ˆ d J2 =dm corresponds to the friction
coefficient on the compressive stress meridian (see, e.g. Ref.
Figs. [9]). In Fig. 5, the invariants defined by Eq. (4) are
p 4 and 5 show the results of this test campaign in the normalized by the uniaxial compression strength obtained
3J2 m stress invariant half-plane.
p In the triaxial test, the
second deviator invariant J2 and the mean stress m are on control specimens kept under same chemo-hygral
defined by: conditions as the tested one. The uniaxial compression
r strengths for the leached and the unleached paste are
p 1 1 1 summarized in Table 2.
J2 ˆ sij sij ˆ p jz r j; m ˆ ii
2 3 3
3.1. Unleached cement paste
1
ˆ z ‡ 2r † 4†
3 Fig. 4 shows the well-known frictional behavior of
cementitious materials under triaxial stress states (cf. Ref.
where sij = ij mdij is the stress deviator of stress tensor [20]): Increasing the confinement pressure leads to a sig-
ij, and dij denotes the Kronecker Delta; z is the vertical nificant higher second deviator invariant that the material
stress, and r the radial stress, controlled in the triaxial test can support. The friction coefficient that characterizes this
property is on the order of dc = 0.82.

3.2. Leached cement paste

The leached samples show a more diverse behavior. We

note the important overall strength loss related to a
chemical decohesion. Both the drained and undrained
experiments show this significant strength loss: In uniaxial
compression, the chemical decohesion leads to a strength
loss of about 90% (see Table 2). In addition, a significant
difference in the frictional behavior of drained and
undrained samples is observed.

Table 2
Compressive strength fc, coefficient of variation var, friction coefficient dc,
and number of tests N
fc[MPa] var[%] dc N
Unleached 54.1 5.6 0.82 5
Fig. 4. Unleached paste deviator mean stress plane. Leached 5.1 8.3 0.23 8
 F.H. Heukamp et al. / Cement and Concrete Research 31 (2001) 767±774 771

between 300 and 10,000. In contrast to other microscope

techniques, e.g. SEM, an ESEM can be operated with wet
samples, avoiding cracking due to drying. The material
surfaces shown in Figs. 8 and 9 are freshly fractured and
nitrogen-cleaned.
The pictures indicate that the significant chemically
induced loss of frictional performance can be attributed to a
highly eroded microstructure perforated by calcium leaching.

While the microstructure of the unleached cement
paste remains compact throughout the different scales, the
leached material appears discontinuous already at a mag-
nification of 300 (Fig. 8A1). In comparison to the
undegraded paste (Fig. 8A2), the microstructure of the
leached material is perforated in a regular manner by
Fig. 6. Leached paste under drained conditions in normalized Mohr large pores. In the 1000 magnification, this porotic
stress plane. microstructure shows a cauliflower-like structure (Fig.
8B1), perforated by pores of the same size as the solid
3.2.1. Drained experiments skeleton walls.
In the drained case (Fig. 5), the linearized friction
coefficient is considerably smaller than the one of the

Details of the cauliflower structure, in a 5000
magnification (Fig. 9A1) and a 10,000 magnification
undegraded material, in average dc = 0.23. Fig. 6 illustrates (Fig. 9B1) show the remnants of an initially highly
the drained residual frictional behavior in the Mohr stress heterogeneous and disordered microstructure (Figs. 9A2
plane; the stresses being normalized by the uniaxial com- and B2). Smooth plated solid remnants are separated by
pression strength, fc. The Mohr circles for drained experi- large pores, which become the main part of the pictures.
ments have an increasing diameter with increasing The pore size at this scale may be attributed to the
confinement pressure. The envelope with an initially con- dissolution of Portlandite crystals, which are the first to
stant slope is flattening at confinement pressures greater dissolve during calcium leaching, followed by the C-S-H
than the uniaxial compression strength. The applied con- decalcification. The remaining C-S-H sheets have a char-
finement pressures go up to twice the uniaxial strength of acteristic size 10 ± 100 times smaller than the one of
the degraded material. Similar confinement levels could not Portlandite [12,17]. The friction that is mobilized during
be obtained on undegraded samples. deviator loading in sane materials at the Portlandite crystal
interfaces and in the highly heterogeneous compact micro-
3.2.2. Undrained experiments structure is strongly reduced. Under triaxial loading of
Under undrained conditions, no apparent frictional degraded materials, the remaining solid walls break and
strength enhancement is observed (Fig. 5). This can be collapse without much frictional interaction with neighbor-
ascribed to pore pressure effects. The volumetric strain ing solid walls. Consequently, the stabilizing effect of a
induced by the externally applied confinement pressure hydrostatic confinement on the overall shear resistance is
results in pore pressures, which cannot escape in the reduced, leading to the observed chemical-induced loss of
undrained case. This internal pressure buildup reduces the frictional performance.
`effective' confinement stress of the skeleton, which there-
fore, cannot mobilize friction in the material during deviator
loading. In the Mohr stress plane, this behavior translates
into circles of nearly constant diameter (Fig. 7). The constant
second deviator invariant for different confinement levels
suggests that the pore pressures are of the order of magnitude
of the applied confinement pressure, that is up to 10 MPa.

4. Discussion and analysis

4.1. Environmental scanning electron microscope

(ESEM) analysis

Figs. 8 and 9 show the microstructure of degraded and

the undegraded microstructure of the cement paste Fig. 7. Leached paste under undrained conditions in normalized Mohr
obtained with an ESEM at a magnification varying stress plane.
772 F.H. Heukamp et al. / Cement and Concrete Research 31 (2001) 767±774

Fig. 8. ESEM picture of leached and unleached paste. Magnification 300 and 1000. A1: Leached paste, Mag.: 300. A2: Unleached paste, Mag.: 300. B1:
Leached paste, Mag.: 1000. B2: Unleached paste, Mag.: 1000.

4.2. Pore pressure sensitivity can be assessed from:

K0 1 b f0 f0
The reported difference in the drained and undrained bˆ1 ; ˆ ‡ 6†
Ks M Ks Kf
behavior of leached cement paste indicates a nonnegligible
role of the pore pressure on both deformability and strength with f0 the initial porosity of the material, Kf the bulk
domain of the material, but they were not measured in the modulus of the fluid, and Ks the bulk modulus of the
experiments. This pressure sensitivity may be attributed to the skeleton. The skeleton bulk modulus Ks can be estimated
increased porosity of the degraded material, but also to the from the homogenization theory that Kendall et al. [13]
chemical damage of the solid skeleton, as shown in the ESEM applied to the Young's modulus of porous materials:
pictures. Our argument is a poromechanical one. During
elastic loading, cement paste can be considered as an isotropic K0 ˆ Ks 1 f0 †3 7†
saturated porous material. Within the framework of the Biot ±
Coussy theory of poromechanics, the mean stress m = 1/3ii, In the case of an undrained experiment, for which the fluid
and the pore pressure p are related to volume strain  = "ii mass variation is zero, vf = 0, Eqs. (5) ±(7) give [Eq. (8)]:
and the relative change in fluid mass vf by (e.g. Ref. [10]):    
1 m K0 Ks f0 1 f0 †3
 ˆ ˆ ‡bˆ1 1 8†
m ˆ K0  bp B p Mb K f 1 1 f 0 †3
5†
p ˆ M b ‡ vf †
B = p/m is often referred to as compressibility coefficient
K0, b, and M denote the drained bulk modulus, the Biot or Skempton factor [4,5,16]: It quantifies the amount of the
coefficient, and the Biot modulus. The coefficients b and M macroscopically applied stress m, which is carried in an
 F.H. Heukamp et al. / Cement and Concrete Research 31 (2001) 767±774 773

Fig. 9. ESEM picture of leached and unleached paste. Magnification 5000 and 1000. A1: Leached paste, Mag.: 5000. A2: Unleached paste, Mag.:5000. B1:
Leached paste, Mag.: 10500. B2: Leached paste, Mag.: 10000.

undrained test by the saturating fluid pressure. Fig. 10

shows this function for two different Ks/Kf ratios. The lower
value, Ks/Kf = 3, corresponds roughly to the value of a
degraded sample; the upper value Ks/Kf = 20, to the one of
the sane cement paste. The figure shows the two parameters,
which govern the pressure sensitivity of cementitious
materials: the porosity f0 and the skeleton-to-fluid bulk
modulus ratio Ks/Kf. The lower the Ks/Kf ratio, the less
sensitive is the function p/m to a change in initial
porosity. This shows that the dominating parameter
governing the pressure sensitivity of degraded cementitious
materials is the Ks/Kf ratio, associated with an intrinsic
chemical damage of the solid part of the eroded porotic
microstructure (Fig. 9A1 and B1). For a typical value of the
initial porosity of the degraded material of f0 = 0.5, the part
of the total applied stress taken over by the pore pressure is
on the order of p/m = 90%. This means that the
``effective'' confinement stress m0 = m + bp = 0.1m in an Fig. 10. Skempton coefficient B as a function of the initial porosity f0 and
undrained test, to which the chemically eroded skeleton is the Ks/Kf ratio.
774 F.H. Heukamp et al. / Cement and Concrete Research 31 (2001) 767±774

subjected, is small. In the Mohr stress plane (Fig. 7), all the collaboration with the C.E.A., Saclay, France, through Dr.
tests at different confinement pressure levels are, in terms of J. Sercombe. We thank Professor B. VoÈlker of the MIT Civil
effective stress, only one triaxial test equivalent to the and Environmental Engineering Department for fruitful
uniaxial compression test. In return, p/m  f090.2 for discussions and helpful comments about the chemical
the undegraded material, and shows that the pore pressures aspects of this paper.
that develop in the highly disordered compact cementitious
microstructure (Fig. 9A2 and B2) are small in comparison to
the hydrostatic stress mobilizing the frictional behavior. References

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FG03-99SF21891/A000 of the US Department of Energy
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