977
Hydraulic conductivity and small-strain stiffness of a cement-bentonite sample
exposed to sulphates
Conductivité hydraulique et module de cisaillement initial d'un échantillon de ciment-bentonite
exposé aux sulfates
Verástegui-Flores R.D., Di Emidio G.
Laboratory of Geotechnics, Ghent University, Belgium
Bezuijen A.
Laboratory of Geotechnics, Ghent University, Belgium & Deltares, The Netherlands
ABSTRACT: Cement-bentonite slurries have often been used for geoenvironmental applications to isolate contaminated areas. A
potential issue with all cement-treated materials is their durability, especially when applied in chemically aggressive environments. In
this paper, the small-strain shear modulus (
G
0
) and the hydraulic conductivity (
k
) of a cement-bentonite sample in contact with water
and an aggressive sodium sulphate solution were investigated. Bender elements were installed in a flexible-wall hydraulic
conductivity cell, to simultaneously monitor both
G
0
and
k
. As expected, permeation with clean water had no significant effect on the
cement hydration, e.g.
G
0
continued to increase and
k
decreased gradually with time. However, after prolonged permeation with
sulphates, a decrease of
G
0
and a gradual increase of
k
were recorded. These observations suggest that contact with sulphates produces
degradation of the cemented structure that results in loss of strength and development of a network of interconnected fissures within
the sample that increases the hydraulic conductivity.
RÉSUMÉ : Des barrières de ciment-bentonite ont été souvent utilisées pour des applications géo-environnementales afin d'isoler les
zones contaminées. Un problème potentiel avec tous les matériaux cimentés est leur durabilité, en particulier lorsqu'ils sont mis en
place dans des environnements chimiquement agressifs. Dans cet article, le module de cisaillement initial (G0) et la conductivité
hydraulique (k) d'un échantillon de ciment-bentonite en contact avec de l'eau et une solution agressive de sulfate de sodium ont été
étudiés. Des bender elements ont été installés dans une cellule de conductivité hydraulique avec une paroi flexible, afin de surveiller
simultanément G0 et k. Comme prévu, l'infiltration à l'eau claire n'a eu aucun effet significatif sur l'hydratation du ciment, par
exemple, G0 a continué d'augmenter et k a diminué progressivement avec le temps. Cependant, après l'infiltration avec les sulfates,
soit une diminution de G0 et une augmentation progressive de k ont été enregistrées. Ces observations suggèrent que le contact avec
les sulfates produit la dégradation de la structure cimentée qui entraîne une perte de la résistance au cisaillement et la formation d'un
réseau de fissures interconnectées dans l'échantillon qui augmente la conductivité hydraulique.
KEYWORDS: clay, cement, sulphate attack, hydraulic conductivity, small-strain shear modulus.
1 INTRODUCTION
Low permeability vertical barriers (cut-off walls) are often used
to control groundwater flow and to isolate polluted soil. They
are constructed by excavating a vertical trench. During
excavation, the trench is filled with a slurry to prevent collapse.
When the slurry is a mix of cement, bentonite clay and water,
the barrier is denominated cement-bentonite (CB) cut-off wall
(Jefferis 1981). The design of a CB cut-off wall is based on the
characterization of the hydraulic conductivity, the strength of
the cement-clay mix and eventually chemical compatibility with
local groundwater.
Traditionally, the mechanical properties and hydraulic
properties of the cement-clay mix are studied separately on
different specimens (e.g. Opdyke and Evans 2005). Mechanical
properties are usually evaluated by unconfined compression
testing; however, the amount of data obtained is often limited to
a few curing times and is usually subjected to scatter. On the
other hand, hydraulic properties are evaluated by hydraulic
conductivity tests; however, it is difficult to relate the hydraulic
conductivity data alone to variations of strength, stiffness or
microstructure of the cement-clay mix.
In this paper, an advanced testing method was used to
simultaneously monitor both mechanical and hydraulic
properties of a single sample. To that aim, a flexible-wall
hydraulic conductivity cell was combined with a non-
destructive technique to monitor the hardening of cement-clay
samples. This technique uses bender elements (Dyvik and
Madshus 1985) to measure the small-strain shear modulus,
G
0
.
Such stiffness modulus is typically associated with small
shear-strain levels (lower than 10
−3
%). In general,
G
0
is
governed by a number of factors such as stress history, stress
level, void ratio, soil fabric, and the stiffness of the porous
medium skeleton (Santamarina et al. 2001). Then, an increase of
G
0
can be expected with increasing interparticle cementation
due to cement ageing. Conversely, a decrease of
G
0
can be
expected when interparticle cementation is disrupted due to
either mechanical or chemical degradation.
Experimental work was carried out on bentonite clay mixed
with blast furnace slag cement. Monitoring of the small-strain
shear modulus of cement-treated clay proved to provide
valuable additional information to study the degradation of
these materials.
2 MATERIALS AND SAMPLE PREPARATION
The samples studied in this research consist of a mixture of
clay, cement and water. A sodium-activated bentonite clay,
blast furnace slag cement of the type CEM III/B (ENV 197-1)
with a nominal strength of 42.5 MPa and purified water with an
electrical conductivity EC ≤ 2 μS/cm and a pH of about 7.6
were used. During hydraulic conductivity testing, the CB
sample was initially permeated with purified water for 1 month
to allow for further hydration of the cement products. After that
period, the sample was permeated with a 25 g/L solution of
Na
2
SO
4
. Such high Na
2
SO
4
concentration was chosen here to
accelerate the degradation process; however, it may be too high
to represent common sulphate exposure levels in the field.
