3001
Long term performance of cement-bentonite cut-offs in saline and acidic solutions
Perméabilité à long terme des parois ciment-bentonite en solutions acides et salines
Brianzoni V., Fratalocchi E., Pasqualini E.
Università Politecnica delle Marche, Ancona, Italy
ABSTRACT: The paper presents the results of a research aimed at evaluating and predicting the long term performance of CB
mixtures in terms of hydraulic conductivity, k. A low k value is the necessary (even if not sufficient) requirement for a cut-off wall to
isolate polluted sites. Aqueous sulphate solutions of different nature (acidic, saline) and concentrations were used as permeants in
column tests performed for long time (one year or more) in order that full effects of physical and chemical reactions may occur.
Results of these tests are here analysed mainly with the aim to find a criterion to predict how long a cement-bentonite cut-off wall can
keep a proper hydraulic performance.
RÉSUMÉ: L’article présente les résultats d’une recherche ayant pour objectifs l’évaluation et la prévision de la perméabilité (k) à
long terme des mélanges ciment-bentonite (CB). Une perméabilité faible est une condition nécessaire (bien que non suffisante) pour
jouer le rôle d’écran de confinement sur les sites pollués. On a utilisé des solutions aqueuses de sulfates de différents natures (acide,
saline) et concentrations dans des essais d’infiltration de longue durée (une année voire plus), pour s’assurer du développement
complet des interactions chimiques and physiques. Les résultats de ces essais sont analysés avec l’objectif principal de trouver un
critère pour prévoir les performances hydrauliques à long terme des parois en ciment-bentonite.
KEYWORDS: cut-off, acid, sulphate, cement-bentonite
1 INTRODUCTION
Few data are available in literature on the long term
performance and durability of cement-bentonite (CB) cut-off
walls (e.g. Jefferis 1992, 1993; Jefferis and Fernandez 2000;
Fratalocchi et al. 2006, 2010) although this kind of vertical
barriers (single or composite with geomembrane sheets) is
frequently used to isolate polluted soils and groundwater (e.g.
Manassero et al. 1995).
The authors are being performing a research aimed at evaluating
and predicting the long term performance of CB mixtures in
terms of hydraulic conductivity, k. A low k value is indeed the
necessary (even if not sufficient) requirement for a cut-off wall
to isolate polluted sites. Aqueous sulphate solutions of different
nature (acidic, saline) and concentrations were used as
permeants in column tests performed for long time (one year or
more) in order that full effects of physical and chemical
reactions may occur. Results of these tests are here analysed
mainly with the aim to find a criterion to predict how long a
cement-bentonite cut-off wall can keep a proper hydraulic
conductivity.
2 MATERIALS AND TESTING METHODS
The CB mixture was selected in order to get a good hydraulic
performance when permeated with water. To this purpose blast
furnace slag cement was chosen (70% slag) as well as Na-
bentonite (activated) and a special additive. The mix design is:
cement/water = 0.22, bentonite/water = 0.0456, additive = 2
l/m
3
of mixture. Details on the mixing and sampling procedures
are given in Fratalocchi et al. (2010). Typical values of density
(11.4 kN/m
3
), Marsh viscosity (50 s) and bleeding (1.0%) were
obtained on the mixture at the fluid state, according to the
practice requirements.
After a curing time of 10-14 days, each sample was permeated
with a different saline or acidic solution of K
2
SO
4
or H
2
SO
4
.
Details of the permeant solutions are given in Table 1. The
thickness of all the samples was in the range of 3.0-5.7 cm
except two samples permeated with the same less concentrated
acidic solution (sample A was 9.0 thick, sample B was 2.2
thick).
Flexible wall permeameters with bladder accumulators were
used in order to control the effective confining pressure (40
kPa), the sample volumetric strains and to collect the effluent
liquid. Constant head test were performed at hydraulic gradient,
i, ranging from 25 to 100. Samples of the effluent liquid were
periodically taken for concentration measurements to get the
breakthrough curve (Fratalocchi et al. 2010; Brianzoni 2012)
and to know the starting time at which the pore liquid fully
consists of the inlet liquid. Permeation was kept for long time
(months up to almost 2 years) in order to verify the long term
performance.
Table 1. Main data of the permeant solutions
Solution
SO
4
2-
(mg/l)
pH
K
2
SO
4
(5.0)
2756
6.8
K
2
SO
4
(27.5)
15159
7.0
K
2
SO
4
(50)
27562
7.3
K
2
SO
4
(95)
52368
8.0
H
2
SO
4
(1.0)
9789
1.0
H
2
SO
4
(1.5)
1176
1.5
H
2
SO
4
(2.0)
310
2.0
