(3) One problem in using sodium silicate in a grout is the prevention of
instantaneous gelling prior to injection in the soil mass. T h i s is overcome
by either diluting the silicate and producing a soft gel or by injecting the
silicate and the reactive compound separately in the ground. A third methol
consists of mixing an organic ester with the silicate prior to injection. The
ester, by saponification , is slowly transformed into acetic acid, which neu-
tralizes the s odium silicate, and ethyl alchol. The addition of an organic
ester to a chemical grout results in sufficient setting time to permit ade-
quate grout injection and a high- strength grout.
(4) Another form of precipitation utilizes a combination of lignosulfite
and bichromate (chrome lignin). Lignosulfite (or lignosulfonate) is a by-
product of the manufacture of cellulose from pulpwood. When lignosulfites
are mixed with a bichromate, a firm gelatinous mass will form. By varying
the concentration of bichromate, the setting time may be controlled through
a range from 10 min to 10 hr. The resulting gel strength will vary depending
upon the nature of the lignosulfite, the concentration of lignosulfite and
chrome, and the pH of the mixture. T h e viscosity increases with time. The
hexavalent chromium is toxic and requires special precaution when mixing.
After gelling, the product is not toxic, but under some conditions water will
leach highly toxic hexavalent chromium from the gel. Possible contamina-
tion of water supplies should, be carefully considered.
b . Polymerized Grouts. Polymerization is a chemical reaction in which
single organic molecules (monomers) combine together to form long chain-
l i k e molecules. There is also cross linking of the molecules, resulting in
rigidity of the product. In this process the soluble monomers, mixed with
suitable catalysts to produce and control polymerization, are injected into
the voids to be filled. The mixture generally has a viscosity near that of
water and retains it for a fixed period of time, after which polymerization
o c c u r s rapidly. Because of the low viscosity, polymer grouts can be used in
soils having a permeability as low as 10 cm/sec, which would include
sandy silt and silty sand. The resulting product is very stable with time.
The monomers may be toxic until polymerization occurs after which there
is no danger. Some of the more common polymer- type grouts utilize the
following chemicals as the basic material.
( 1 ) Acrylamide. There are available, under several different trade
n a m e s , chemical grouts that use acrylamide and one of its derivatives as
a base. One of these consists of a mixture of acrylamide and methylene -
bisacrylamide, which produces a polymerization crosslinking gel when prop-
erly catalyzed, that traps the added water in the gel. These grouts are ex-
pensive, but because of the low viscosity, ease of handling with recommended
equipment, and excellent setting time control, they are suitable for certain
a p p l i c a t i o n s . The ingredients are toxic and must be handled with care, but
the final product is nontoxic and insoluble in water..