980
Proceedings of the 18
th
International Conference on Soil Mechanics and Geotechnical Engineering, Paris 2013
As expected during the first phase of permeation with pure
water, a gradual decrease of k with increasing time was
observed. This feature of cement-bentonite samples has been
reported before in the literature (e.g. Fratalocchi et al. 1998, ICE
1999) and an equation describing such trend has also been
proposed:
k(t) = k
28d
(t / 28)
-n
(3)
where k
28d
is the hydraulic conductivity at an age of 28 days, t is
the age of the sample in days and n is a constant. Based on
existing data and Eq. 3, it was possible to predict the
permeability to water of the CB sample vs. time (Fig. 4a).
In the second phase of the test, the CB sample was
permeated with a Na
2
SO
4
solution. As a result of deterioration
of the sample an immediate increase of
k
was expected.
However, in the early phase of sulphate attack (age between 30
to 70 days) the permeability to sulphates seems to be lower than
the expected permeability to water. This feature may have been
caused by formation of gypsum in the pores within the sample
(Santhanam et al. 2003) which may result in a gradual clogging
of the pores. Gypsum primarily deposits in the fissures and in
voids, because these provide the best sites for nucleation. After
a while, when the formation of ettringite becomes significant,
the affected areas tend to expand, then, fissures start to appear
which will gradually lead to an increased hydraulic
conductivity. In fact, figure 4 shows that expansion of the
sample and increase of hydraulic conductivity start at
approximately the same time.
The hydraulic conductivity of a cement-clay mix is not a
simple function of the porosity, but depends also on the size,
distribution, shape, tortuosity and continuity of the pores that
change during the cement hydration and sulphate attack.
5 CONCLUSIONS
Traditionally, the mechanical and hydraulic behaviour of
cement bentonite samples are studied separately on different
specimens. In this research, a flexible-wall hydraulic
conductivity cell was provided with bender elements to measure
the hydraulic conductivity as well as the small-strain shear
modulus.
Monitoring of G
0
was shown to provide valuable
quantitative information to study the deterioration effects of
sulphate attack on a cement-clay mix. As expected, during
permeation with deionized water an increase of G
0
and a
decrease of permeability were observed due to normal cement
hydration. On the other hand, after sustained contact with
sulphates a significant decrease of G
0
and increase of k were
measured. These observations suggest that the sulphate attack
produces severe degradation of interparticle cementation and
perhaps also severe fissuring affecting the macrostructure of the
CB sample which in turn may lead to a loss of strength.
6 ACKNOWLEDGEMENTS
The authors would like acknowledge Jonathan Mawet for his
assistance during this research.
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