Environmental Engineering Reference
In-Depth Information
8.6.4
V ISCOSITY C ONTROL A GENTS
Small amounts of clays or polymers are sometimes employed to control grout
viscosity and reduce available water as well as to sorb organics and metals. 72 Another
technique is to use sorbents such as diatomaceous earth, vermiculite, rice hull ash,
zeolites, and various expanded mineral absorbents to control free water content. 73
Kiln dust (lime and cement) also increases grout viscosity and reduces the available
water. One reason for increasing viscosity is to prevent phase separation.
Gel clays have been used for decades in geotechnical applications, e.g., con-
struction (slurry walls and clay caps) and drilling (drilling muds and cement mixes),
to resist solids segregation (suspension aid), prevent bleed water, and act as an
engineered hydraulic barrier to water penetration (into a construction zone, waste
disposal site, and so on). The most commonly used clay for these purposes is
bentonite, sodium montmorillonite, “ … a colloidal clay mined in Wyoming and
South Dakota. It imparts viscosity and thixotropic properties to fresh water by
swelling to about 10 times its original volume. Bentonite (or gel) was one of the
earliest additives in oilwell cements to decrease slurry weight and to increase slurry
volume.” 74,75 The individual clay particles of bentonite are plate-shaped. The particle
faces are positively charged, while the edges are negatively charged. When mixed
with water the platelets separate and disperse throughout the fluid. When mixing
ceases, the clay particles form a multilayered colloidal gel structure due to the
attraction of opposite charges. However, the electrostatic double-layer forces are
lessened with increasing ionic strength. 76 Consequently, high-salt solutions (notably
chloride, sulfate, and phosphate salts as well as acids and bases) collapse these gels,
lessening their dispersive effectiveness and releasing the large volume of water
collected around the clay particles (i.e., free water can form if salt solutions are
grouted). 77
This susceptibility compromised the use of bentonite in off-shore oil drilling in
salty waters. For this reason, attapulgite was adapted as the gel clay used in such
salty applications. Attapulgite clay particles carry no charge and are not affected by
high salt content. The individual attapulgite particles resemble needles, rather than
platelets. When mixed with water, these needles are dispersed throughout the fluid
and become aligned along shear planes. When mixing ceases, the random entangle-
ment of these particles, referred to as a “brush-heap effect,” forms a gel structure.
Attapulgite is commercially available from northern Florida and southern Georgia.
Attapulgite has been adopted as the gel clay of choice for salty wastes. Note that
although several forms of attapulgite have been tested for DOE salty wastes, only
attapulgite 150 (Attagel® 150) proved effective. 78
8.6.5
A DDITIVES THAT I NCREASE S TRENGTH OR I MPROVE O THER
P HYSICAL P ROPERTIES
Using flyash as part of a grout recipe typically results in a solidified product that
has increased strength and reduced permeability (see detailed description below). In
one study the leaching of chromium decreased as more portland cement was sub-
stituted with silica fume. The incorporation of silica fume into the cement matrix
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