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Self-Hardening Slurries and Stable Grouts from
Cement-Bentonite to IMPERMIX~
Gilbert Tallard, President Env~roTrench Co., Pelham N.Y.
ABSTRACT
Conventional concretes and grouts, with which most engineers are familiar, are materials
with a water/cement ratio generally below 0.5. The strength of these materials is In the tens of MPa
(thousands of psi). The structural strength of these materials is their primary quality. A less familiar
group of materials, particularly in the United States, consists of very watery slurries that set to form
a solid of relatively low strength ranging from 0.1 to 3.5 MPa (15-500 psi). The water content of
such materials may vary from 250 to over 500 percent, with a water/cement ratio as high as 10 or
more. At these water content levels, in order to have a homogenous matenal, it is necessary to
suspend the cement with a viscosifier showing thixotropic properties. The viscosifier is traditionally
some kind of clay, generally bentonite. The notion of stabilizing a cement suspension with a
tixothropic clay has made stable grouts possible. The degree of stability of such grouts is indicated
by the amount of bleed or free water at set time. The essential purpose of these materials is to
provide an economical means of controlling ground-water migration in the ground, be it soil or
rock. Whether such materials are called a slurry or a stable grout depends on the application. A
_ ~ ~
grout is used when the application is localized; a slurry is used when the application is in an open
excavation, although the term slurry grout, meaning watery stable grout, is often used. The
application field is below the ground water table and slightly above. Given the high water content,
the dehydration and destruction of the hardened slurry material would occur if exposed to the
elements, without any kind of protection. Conserved in water, these materials have an infinite life.
Originating in natural ground-water seepage control in the dam construction and repair technology,
the control of contaminated ground-water migration called for similar technology. The chemistry of
the environment and the service under which these materials are to perform has brought forth
certain shortcomings. The strongest limitation has been the lack of watertightness of mixes using
Ordinary Portland Cement with respect to the regulatory conductivity threshold of 10-7 cm/s.
Bentonite clay loses some of its properties in the presence of calcium in the cement, and as a highly
dispersive clay, it is subject to shrinkage in the presence of certain organic chemicals. in view of
these limitations, a different pair of constituents has been found and perfonns surprisingly well,
from both chemical and permeability standpoints. Named ~ERMIX~, this flexible formulation is
based on attapulgite or sepiolite clays and finely ground blast furnace slag cement. Slac cement's
long setting time is taken advantage ot In some applications like slurry trenching in difficult
ground, or shortened when strength buildup is necessary, as in sludge solidification. Given the low
viscosity and low-solids content of these suspensions, a very penetrating environmental grout for
sealing contaminated fractured rock can be found in IMPERMIX~ Whereas cement-bentonite
slurries have found limited applications in environmental remediation, due to excessive
permeability, the permeability threshold of self-hardening slurries has been lowered with
IMPERMIX'~' by one and two orders of magnitude and occasionally more. With the fact that an
I~ER~X'i' formulation can be tailored to satisfy a specific chemical condition, it is now possible
to engineer and control, with a high degree of quality assurance, the self-hardening slurry
application in site-specif~c conditions. Contaminated ground-water barriers have been created by
D-124
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APPENDIX~PAPERS PRESElITED
D-125
the one-phase slurry trenching technique using backhoe and clamshell tools, the injection vibrating
beam method, and jet grouting. Future use for ~n-situ fixation by soil mixing, pipe line
abandonment, contained sludge in-situ solidification, and horizontal jetted barriers are a few fixture
obvious applications. Solidifying low-level radioactive aqueous waste is a more engaging prospect.
Definitions
necessarily stable.
Slurp: suspension of solids in a large amount of a liquid generally water-and not
Thixotropy: a physical phenomenon occuning when a liquid builds up rigidity at rest, while
reversing to a low-viscosity fluid in a dye namic condition; such liquids are also known as shear
sensitive.
Mineral grout: suspension of mineral powders in water, not necessarily stable, and
containing at least one cementitious ingredient to create an eventual set after injection into
· .
recelvmg grounc .
Stable grout: suspension of at least one clay mineral and one cementitious powder capable
of developing sufficient viscosity to maintain all particles in suspension with a minimum of bleed
water.
Self-hardening slurry: a cliluted, stable grout used in a one-step construction process in
which the slurry in liquid phase is the slurry stabilizing the walls of a trench and in a set-~n-place
solid phase providing the desired end product.
Bentonite: a highly dispersive montmorillonite clay used extensively for viscosifying
water-based drilling or trenching fluids, and for the preparation of common stable grouts;
viscosities by adsorbing water between its structural platelets; high ion-exchange capability.
Attapu1fgite: a clay mineral of the palygorskite family with a needle-like crystalline
structure; viscosification of water occurs by mechanically stacking the needles and trapping water
between them; very little ion-exchange capability by comparison to bentonite.
Cement: common term for Ordinary Portland Cement, the most common cementitious
binder used in the world; sets by forming at least three distinct chemical compounds and some
noncomb~ned salts.
Slag cement: finely ground blast furnace slag cement is a hydraulic binder manufactured
from a residue of modern steel making; molten slag is blown into a cold-water spray, with the slag
cooling into glassy pellets; once finely ground In a cement mill, an industrial pozzolanic material is
obtained.
Cement-bentonite (CBJ: a generic mixture of Portland cement and bentonite, with a solids
content of generally less than 35 percent, mixed into a stable grout and used for alluvium and
fractured rock grouting as well as a self-hardening slurry for cutoff wall construction.
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BARRIER TECHNOLOGIES FOR ENVIRONMENTAL MANAGEMENT
IMPERMI/: a proprietary mixture of attapulgite clay and blast furnace slag cement and
other ingredients used as a self-hardening slurry, typically at a maximum solids content of 22
percent and exhibiting very low permeability, high strength, and very good stability under chemical
attack.
INTRODUCTION
Chronologically, self-hardening slurries have their origin in stable grouts. Barrier
technology, as understood in pure geotechnical construction, consists of controlling ground-water
flow under or through a man-made structure. The technology started with grout curtains and cutoff
walls having application to embankment dam construction and repair. In the waterproofing of
porous granular soils, stable grouts do a very good job in sealing these formations, by comparison
with neat cement grouts. Cement-bentonite (CB) grouts have been used extensively in alluvium
grouting around the world, except in the United States. The best permeability to be expected for the
grouted soil is in the 10-5 cm/s range. However, with grout cutoffs up to 25-feet thick, a rather low
gradient is at play, and the safety of the system can be seen as higher than in the case of the thin
cement-bentonite cutoff wall subject to a much higher gradient. This construction method was
developed in the 1970's, when tools and evolution in the rheology of the self-hardening slurries
permitted the construction of cutoff walls 2-feet thick and over 100-feet deep.
LIMITATIONS OF CEMENT-BENTONITE
For better understanding, economy, and quality assurance, design engineers generally
prefer trenched cutoff walls to grout curtains. When designing water reservoir embankments or
contemplating the repair of a leaky earth dam, a conductivity (K) in the range of 10-6 cm/s is quite
c~ti~f~t~rv in rerlllcin~ the residual seepage to an acceptable value concurrently achieving a total
~ ~ . ~ · ~ ~ 1 ~ _ _ A_ __1~ 1__ 1~ 1,.~ ~ Ill ;~11~1;- thy
cessation of Internal erosion. c;emem-oemomle selI-n=~eIluly my Am wale ~l~"ll~" ". ills
United States since the mid-1970's for the purpose of impeding the migration of a contaminated
Knifer had to get a variance from regulatory agencies with respect to the coefficient of
~ . ~ . ~ 1 _ 1 ~ 1 _ ~ ~
permeability above the norm as established by the sol1-0entomte slurry Irencn oenon mark. oo111t;
more stringent agencies, such as the California Water Board, require both compatibility and a
permeability coefficient of less than 10-7 cm/s, with no exception. In this context, 10 years ago the
Los Angeles County Sanitary District was looking to use both self-hardening slurry technology and
to meet the K < 10-/ cm/s criterion for their numerous barriers to be installed across canyons and
gullies confining its landfills. This writer was hired to formulate a cement-bentonite slurry that
could perform reliably in the field and provide a low permeability. After a few months of testing, a
cement-bentonite slurry prepared with 4.5 percent bentonite, 25 percent Portland cement, and 15
percent fly ash fluidified and retarded with an amount of lignosulfonate satisfied the criteria.
Permeabilities at 60 days in the low 10-8 cm/s range were achieved. Practically, nearly a doubling of
ingredient solids over a normal formulation were called for to reduce the permeability by one order
of magnitude. The added cost for materials and preparation on-site was quite significant. At these
solid levels, the cost for fluidifier/retarder is equal to that of the bentonite. The recourse to a high
dosage of fluidifier for viscosity control can lead (and did) to defects by excess dosage, causing
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APPENDIX~PAPERS PRESENTED
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suspended soil particles to settle in discrete bedding patterns of higher permeability. This solution
could not be generalized to the uncontrolled hazardous waste remediation industry at large.
MATERIALS
As a combination, Wyoming bentonite, which is highly dispersive and prone to ion
exchange, and Portland cement, which exhibits an early false set and some leachability, are not the
best partners for creating a Tow-permeability, high-water-content, self-hardening slurry matenal.
Hydrated bentonite will readily flocculate at the first encounter of a little amount of Portland
cement. The flocculation causes a drastic change in the filtration characteristics of bentonite slurry,
going from as low as ~ 3 cm3 on an American Petroleum Institute (APT) standard test ( 100 psi for 30
mini to over 80 cm3. Once the cement is mixed into the bentonite slurry, the filtrate, tinder API
standard, becomes total (250-300 cow. It is clear that a head differential of 210 feet of water is
unrealistic for most projects and the API standard test should be seen only as a Qualitative
procedure. Concurrently with the flocculation and filtrate loss, a s~gn~cant increase of viscosity
occurs.
The Europeans, who are not blessed with our resources in Wyoming and South Dakota,
have to permutate natural calcium bentonites into sodium bentonite. This industrial process allows
the production of all kinds of bentonites and, in particular, clays much less sensitive to calcium ion
exchange. Also, the number of cement types available in Europe is far greater than here, and certain
blended cements offer a better match to bentonitic clays, as well as intrinsically higher chemical
stability.
CHEMISTRY
When addressing hazardous waste barriers, the chemist of the contaminant can become a
major factor in the barrier's durability. Particularly with certain volatile organic compounds, such as
benzene, xylene, or methanol, the water adsorbed between the bentonite mineral platelets suffer a
change in dielectric constant (whether in a soil bentonite or cement-bentonite configuration) and
causes the bentonitic gel to contract by loss of dispersive energy. This causes an increase of
conductivity that can be over an order of magnitude. The purpose of compatibility permeability
testing under a higher than service gradient is to determine, in a few months, the degradation
potential over the life of the barrier. When permanent enclosures are involved, a 50-year service life
is far from overly ambitious.
QUALITY CONTROL/QUALITY ASSURANCE
This writer has always been adverse to the use of soil bentonite barriers for permanent
enclosures. A coarse technology and a crude construction method cannot provide the level of
quality control (QC) and quality assurance (QA) necessary to satisfy all parties having a vested
interest in the enclosure. On the other hand, the self-harden~ng slurry approach to the construction
of a barrier allows the sophistication of an engineered project: at the design stage, during
construction QC stage, and at the post-installation QA stage. With cement-bentonite self-hardening
slurries, only a number of sanitary landfill barriers could be satisfied with a K above ~ 0-7 cm/s with
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BARRIER TECHNOLOGIES FOR ENVIRONMENTALMANAGEMENT
no adverse consequences, which meant practically the end of the road for cement-bentonite In
hazardous waste remediation in the 1980's, a profound disappointment to this wnter.
~ERMIX~
The history of IMPERMIX Starts with this frustration and builds on a multiplicity of past
experiences and a taste for pluridiscipl~narity. As the steed mills of this country were closing In the
1970's, so were the sources of blended cement such as Portiand/ground blast-fumace slag-cement
blends. Today, very few sources are producing a pure blast-furnace stag-cement product that is sold
to the ready-mix concrete industry. A remarkable characteristic of hardened blast-furnace sTag-
cement In very high water/cement ratio slurry is the strength, which is many fumes that of Portland
cement mixed at the same ratio. In conventional concretes, the gain in strength is only 5-1S percent,
with the permeability and resistance to salts improved noticeably.
When, 10 years ago, this writer constructed his first soil attapulg~te slurry trench at a
chemical facility on the Gulf of Mexico, actually using seawater to prepare the slurry, a good clay
mate for the slag cement was recognized. A clay that viscosities mechanically instead of swelling
and doesn't blink at the sight of electrolytes appeared to be a good candidate indeed. The concept of
a controlled initial viscosity based on mixing energy was extremely appearing.
Further research determined the range of interesting proportions. Surprisingly, a total of 15
percent solids by weight of water could produce a hardened slurry with a coefficient of
Permeability less than ~o-7 COWS and a strength substantially above 100 psi. From this research,
it- ~
~ tin) . ~ . ~ ~ ~ 1 , · , ~ 1 ~ ~ __ _ ~ _ _ ~ ~ 1 ~ ~
~'ERMIX~ was born, and the base tormulahon was set at one part attapulglte to ~ pans OI Slag.
At the present time, it is possible to target 10~9 cm/s. Any increase in stag content is matched to a
certain increase in clay to assure stability and chemical balance. Any stag proportioning above 15
percent involves some structural design consideration.
Once permeability to water was assured, compatibility with common pollutants was
addressed. Organic chemicals whether aqueous or nonaqueous, heavy metals, strong alkalis and
~.
. ~ ~ . · . T T ~ ~ 1% ~ 1 1 _ _ 1 1 ~ ~ _ _ _ 1 ~ 1 ~ ~ ~ ~ ~ 4- 1^ ~ ~
acid solutions to a pti level or a, all caused a ~ong-~em1 aet;rt;~ 1[~ By we ~
degradation. Addition of fly ash, silica fume had no positive effect on the permeability. Addition of
ground lime appeared to provide a positive decrease in permeability on well-cured samples,
although initially retarding the set. Addition of filtrate reduction polymers proved very effective
from a technical standpoint, although not always cost effective.
Applications
With a natural setting time of 70 hours (at room temperature, shorter in warm weather), the
working hme of an IMPERMIX~ slurry is unusually long, allowing for the construction of
practically jointless barriers. The long setting time coupled with high strength (for a self-hardening
slurry, 150-250 psi) has allowed the construction of support of excavations where structural
elements are placed and suspended inside the fluid trench until the slurry is hardened. A number of
shoring systems can take advantage of this technique.
The high fluidity of ~'ERMIX~ mixes, which have proven to be non-sh~ink, opens a
market for backfilling abandoned utility ducts or sewers where only cellular concrete could do the
job at a much higher cost. A means of accelerating the setting of I~'ERMIX~ has opened a new
field of application for light-weight backfill material. IMI'ER=X~ is now being used to pretrench
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APPENDIX~PAPERS PRESENTED
D-129
along the alignments of the slurry walls supporting the excavation for the Central Artery In Boston.
In this application, the IMPERMIX~ slurry is used as the trenching support slurry to minimize the
size of the excavation required to remove obstacles such as granite blocks or Amber cribbings,
inherited from the rich history of this New England capital city. The elimination of dewatering and
shoring is a major simplification and a much safer procedure. In lieu of backfilling with a
conventional lean mix, the modified IMPERMIX@~ sets in place overnight and hardens enough in 2
days to permit trenching of the slots for the guide wall construction ahead of the slurry wall panels
excavation proper.
~ERMIX~'s fluidity is also an asset when using jet-grouting techniques to install a
barrier under utilities and connecting to conventional trenched cutoffs or soil-displacement narrow
walls.
The potential for soil mixing in situ exists and has been demonstrated in the laboratory. The
potential for sludge fixation also exists with the smallest increase in volume. One must bear in mind
that THERMS may represent only 13 percent of solids; 87 percent is water (water content > 600
percent). It is perfectly conceivable to mix the contents of a leaky tank almost hill of contaminated
water (e.g., low-radiation water) into an IMPERMIX~ grout that will not leach and to consider the
problem solved satisfactorily over the long term, as long as the tank remains buned. IMPERM1X~
will only release water by drying. In a submerged condition, despite the already high water content,
the water content of IMPERMIX~ tends to increase.
Research Needs
There is still much that needs to be learned about ~ERMIX~ before we start to understand why
the performance of this combination of materials is so good. No electron microscope observation
has been made, and no mercury permeation has been attempted to determine the system's porosity
structure. An answer needs to be found to why IMPERMLX~ is capable of combining, after set,
more water than its already extremely high water content. A low-gradient permeability test had
water going in for a month with no water coming out at all and no change In volume. From a
practical standpoint, when performing a compatibility test, the practice with materials having a
substantial solids content is to percolate a multiple of the pore volume, the latter being defined by
the ratio of dry weight and the initial sample weight. In the case of self-hardening slurries, this
definition results in the pore volume being equal to the water content. With the very low
permeability exhibited, it is clear that most of the water content is part of the structure and not of
the pore space; therefore, the need arises to redefine "the pore volume" for realistic compatibility
testing.
A review of compatibility testing procedures in a flexible wall biaxial cell permeameter
should be made to compare the typical method using unrealistically high gradients (50-200)
through fairly thick samples (4 inch) and methods using low gradients through a large diameter and
thin sample in an oversized biaxial permeameter cell (Haug, 19801. A new test for very low-
pe~meability materials should be developed to establish threshold gradients below which the
matenal can be considered truly impermeable.
Permeability is not the only concern when dealing with hazardous waste. Sorbtive
characteristics and electric diffusion are areas that deserve investigation. Sorbtive properties of
attapulgite clays are well known, and slag cement/lime combinations have been tested successfully
for chromium. No evaluation of ~ER~X~ sorbtive properties has been undertaken yet, nor has
diffusion been studied.
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BARRIER TECHNOLOGIES FOR ENVIRONMENTAL MANAGEMENT
A better understanding of the material will permit the introduction of specific ingredients to
improve chemical retention. This writer has started investigating the benefits of adding activated
carbon to the mix. The chemical activation of the slag itself has caused a reduction in permeability
for low solids IMPER~X~ formulations, an important improvement when trying to minimize the
volume increase in fixation projects. Despite the unglamorous sulfur odor of fresh IMPERMIX~
samples, sulfur contained in slag may be a factor in explaining the exceptional strength and should
be investigated.
CONCLUSIONS
Cement-bentonite slurries and grouts have opened many avenues in geotechnical
engineering and construction. Lack of choice in better local ingredients and chemical and
regulatory thresholds have limited the full use of these products in the hazardous waste remediation
business despite their "eng~neenng" values. With contamination eventually migrating from soils
into the rock basement, barriers must address both media, and self-hardening slurries are a good
tool for dealing with both. A new pair of clay-cement ingredients, bearing the flag of IMPER~X~,
is a good candidate to take over the self-hardening slurry technology to carry it to the hazardous
waste environment. In times when remediation funding is getting tighter, the return to permanent
vertical and horizontal enclosures is in order, where possible. The laxity in slurry trench battier
technology (soil bentonite) that has prevailed up to now because of the planned temporary nature of
the battier or because of the creation of negative gradient baIriers should give way to baITiers that
are engineered and built to be barriers that act like baIriers. The bureaucratic observance of
performance criteria is often limited to a coefficient of permeability threshold without even
defining the laboratory-test~ng criteria. This attitude does not seem to factor in the constructability
versus end-product quality elements that are essential to achieving one's objectives and has led to
risky situations (not to mention law suits) that would not be tolerated in a permanent enclosure
project. Some engineering and construction ngor will have to be reintroduced in the process and, if
this is possible from an overall bus~ness/legal consideration, then long-term in-situ bioremediation
(Nature's work) will take place safely at a fraction of the cost of the present approaches in areas
where land can be furrowed for an extensive penod oftime.
BIBLlOGRAPlIY
D'Appolonia, D. I. 1980. Soil bentonite cutoffs. ASCE Journal. 106:(GT41339-417.
Dupeuble, E. P., and P. Habib. 1978. Coupures Stanches en beton plastique, 13th ICOLD congress,
Q. 4S, New Delhi.
Evans, I. C., H.-Y. Fang, and T. I. Kugelman. 1985. Containment of hazardous materials with soil
bentonite slurry walls. Paper presented at HMCRI, Washington, D.C. November 4-6.
Haug, M. D. 1980. Optimization of slurry trench cutoff design. PhD. disertation. University of
California, Berkeley.
Jefffies, S. A. 1985. Clay slag cement grouts for pollution control. Workshop on Blast Furnace Slag
Cements, Kings College, London.
Johnson, A. I., R. K. Frobel, and N. S. Cavalli eds. 1985. Hydraulic barriers in soil and rock.
Proceedings of the ASTM/ASCE Symposium, Denver, Colo., June 1984. ASTM STP 874.
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APPENDIX~PAPERS PRESENTED
D-131
Kahl, T. W. I. L. Kauschinger, and E. B. Perry, 1991. Plastic Concrete Cutoff Waits for Earth
Dams, U.S. Army Corps of Engineers report REMG-GT-! 5.
Millet, R. A., and I. Y. Perez. 1981. Current USA practice, slurry wall specifications. ASCE
Journal. 107(GT81:1041-1056.
Noguera, G. 1985. Diaphragm wall for Colbun main dam. Paper presented at the 15th [COLD
Congress, Q.5S, Lausanne.
Pare, I. I., P. I. GIover, and G. Dussault. 1985. Construction control of remedial work at LG 3 south
dikes. Paper presented at the ~ 5th {COLD Congress, Q.5S, Lausanne.
Ryan, C. R. 1977. Slurry Cutoff Wails. Interim report, technical course at Resource Management
Tnc., February.
Schmednecht, F. 1976. Slurry injection. Construction Digest. January 8.
Tallard, G. R. 1984. Slurry trenches for containing hazardous wastes. ASCE C.E. Magazine,
February.
Tallard, G. R. 1990. New trenching method using synthetic big-polymers. ASTM STP 1129. June.
TalIard, G. R. 1992. Hazwaste hydraulic barriers. Paper presented at Update for the 90's 45th CGS
conference, Toronto, October.
U.S. Environmental Protection Agency (USEPA). 1984. Slurry trench construction for pollution
migration control. EPA.540/2-84-00l, Cincinnati, Ohio.
Representative terms from entire chapter:
stable grouts
