2998
Proceedings of the 18
th
International Conference on Soil Mechanics and Geotechnical Engineering, Paris 2013
cations replace exchangeable monovalent cations, thereby
reducing or eliminating osmotic swelling of the bentonite and
the ability of bentonite to function effectively (e.g., Gates et al.
2009).
In response to the susceptibility of natural Na-bentonites to
chemical attack and poor hydraulic performance, recent
research has been undertaken to investigate novel bentonites
that have been chemically modified to achieve greater
compatibility with the surrounding environment, such that the
desirable engineering properties of the bentonites are not
compromised. For example, anionic polymers have been added
to Na-bentonite to attain rheological properties needed for
drilling fluids, and organobentonites (i.e., bentonites with
organic cations on the exchange sites) have been created for
applications where an organophilic material is needed for
containment of organic compounds (e.g., Lee et al. 2012). Also,
Na-bentonites complexed with various organic chemicals (e.g.,
polymers) have attracted considerable interest as potential
substitutes for natural Na-bentonites in containment barriers,
such as SB cutoff walls and geosynthetic clay liners (GCLs)
(e.g., Katsumi et al. 2008, Mazzieri et al. 2010). The objective
of this paper is to illustrate the potential improvement in
engineering properties that can be attained with some of these
novel bentonites when used in containment barriers. This
objective is achieved by comparing engineering properties of
the novel bentonites, including hydraulic conductivity to water
and chemical solutions, with those of natural Na-bentonites.
2
NOVEL BENTONITES
Three novel bentonites are considered herein. These bentonites
include (1) a natural Na-bentonite polymerized with acrylic acid
to form polyacrylate polymerized bentonite, referred to as a
bentonite polymer nanocomposite or BPN, (2) a propylene
carbonate (PC) modified Na-bentonite, referred to as
"multiswellable bentonite" or MSB, and (3) Na-bentonite
amended with sodium carboxymethyl cellulose (Na-CMC),
referred to as "HYPER clay" or HC.
The BPN was supplied by Colloid Environmental
Technologies Co. (CETCO, USA), and was produced with
polyacrylic acid (PAA) using methods similar to those used for
the production of polymer nanocomposites (Bohnhoff 2012,
Scalia 2012). A monomer solution was prepared by dissolving
acrylic acid in water, and then the solution was neutralized with
sodium hydroxide. Next, a natural (unmodified) Na-bentonite
was added to the monomer solution in concentrations ranging
from 30 to 50 % by weight to form bentonite-monomer slurry,
followed by the addition of sodium persulfate, which served as
an initiator. Polymerization then was induced by raising the
temperature of the slurry above the decomposition temperature
of the initiator molecule. Following polymerization, the PAA
polymerized bentonite was oven dried, milled, and screened.
The MSB, supplied by Hojun Corp. (Japan), was created
by compounding Na-bentonite with PC that expands the clay
lattice and forms a hydration shell around the interlayer cations.
The resulting Na-bentonite-PC complex can undergo osmotic
swell in both fresh water and electrolyte solutions (hence the
term "multiswellable"), including sea water (~500 mM NaCl)
and solutions containing multivalent cations (e.g., Onikata et al.
1999). The MSB contained 25 % PC by dry weight.
The HC was created by combining Na-bentonite with Na-
CMC, the sodium salt form of the anionic polymer CMC that
has been used in industrial applications as a thickener (Di
Emidio 2010). The base Na-bentonite was combined with a Na-
CMC solution to form slurry. The slurry was dried at 105 °C,
and the resulting solids were ground with a mortar grinder. The
Na-CMC penetrates the interlayer regions between the clay
platelets, resulting in expansion of the clay lattice and enhanced
osmotic swell. Test results for HYPER clay containing either 2
% Na-CMC or 8 % Na-CMC are presented herein, where the
former is designated as HC2 and the latter is designated as HC8.
In both cases, the base Na-bentonite was the same base Na-
bentonite used to create the MSB (designated herein as NB3).
The index properties of the novel bentonites (BPN, HC2,
HC8, and MSB) are summarized in Table 1 and compared with
those of two natural Na-bentonites designated as NB1
(NaturalGel
®
, Wyo-Ben, Inc., USA) and NB2 (Volclay
®
,
American Colloid Company, USA) and the base Na-bentonite,
NB3, used to create HC and MSB. Both NB1 and NB2 are
commonly used for SB cutoff walls. The unusual behavior that
can be exhibited by polymer modified bentonites is illustrated in
the case of the BPN, wherein the BPN exhibits the highest free
swell and cation exchange capacity (
CEC
), despite the lowest
liquid limit (
LL
) and an overall nonplastic behavior. This
unusually high swell and
CEC
have been attributed to the high
swelling potential of the hydrophilic polymer used in the BPN
(also used in baby diapers) and the tendency of the polymer to
bind cations (Bohnhoff 2012, Scalia 2012). In contrast, the
CECs
for HC2, HC8, and MSB are only marginally higher than
that of NB3. However, HC2, HC8, and MSB exhibited
markedly higher
LLs
relative to NB3, presumably due to the
addition of Na-CMC and PC, respectively.
Table 1. Index properties of bentonites examined herein (NB1 = NaturalGel
®
; NB2 = Volclay
®
; NB3 = Hojun Na-Bentonite; BPN = Bentonite
Polymer Nanocomposite; HC2 and HC8 = HYPER Clay with 2 % and 8 % Na-CMC, respectively; MSB = Multiswellable Bentonite).
Value
Property
Standard
NB1
NB2
NB3
BPN
HC2
HC8
MSB
Soil classification
ASTM D 2487
CH
CH
CH
CH
CH
CH
CH
Liquid limit (%)
583
420
466
255
650
742
547
Plasticity index (%)
ASTM D 4318
530
381
421
NP
594
681
502
Distilled water swell index (mL/2 g)
ASTM D 5890
35
32
26
73
37
c
48
c
29
Montmorillonite content (%)
a
69
91
77
76
78
78
74
Cation exchange capacity,
CEC
(cmol
c
/kg)
b
83.4
78.0
44.5
143
47.3
46.7
49.8
Exchangeable metals (cmol
c
/kg):
b
Ca
2+
4.9
28.1
5.6
9
11.4
12.7
7.7
Mg
2+
8.8
13.3
7.9
3
6.2
5.5
6.1
Na
+
73.4
34.3
26.3
128
34.2
44.5
33.3
K
+
1.1
1.6
0.2
3
0.3
0.2
0.5
Sum
88.2
77.3
40.0
143
52.1
62.9
47.6
a
Based on energy dispersive X-ray diffraction analysis;
b
Procedures for NB1, NB2, and MSB given by Shackelford and Redmond (1995); procedures for BPN given by Scalia (2012); procedures for HC2
and HC8 given by Di Emidio (2010);
c
Values likely underestimated (see Di Emidio 2010).
