Interactions between Wastes and Binders - Stabilization and Solidification of Hazardous, Radioactive, and Mixed Wastes - page 155

D. Sodium silicate is a popular additive in waste solidification. Providing both

silicate and a high pH, it may be used as an accelerator or activator for
pozzolanic materials, or to consume excess water. Particularly in the latter
instance, or in the event that excessive amounts are added, formation of silica
gel may lead to a physically unstable matrix, with swelling and shrinkage
caused by changes in the humidity of the surrounding environment.

E. Sulfates also have several possible effects on portland cement hydration.

Possibly leading to acceleration or retardation by reaction with C 3 A and
C 4 AF, they may also lead to false or flash setting, by forming gypsum
instead of ettringite, or matrix destruction through delayed formation of
ettringite, which has a high volume because of its waters of hydration.
Thiosulfate is also reported to be an accelerator. Chloride salts can also
form voluminous chloroaluminates that are destructive to the matrix if
their formation is delayed. Both chlorides and sulfates, as well as carbon-
ates and other anions, can also result in matrix destruction if the solubility
of one of their salts in the pore solution is exceeded and crystallization
results. 6,26,27

F. More than a couple of percent of MgO can destroy the cement matrix by

gradually hydrating to more voluminous Mg(OH) 2 ; MgSO 4 also reacts to
form more voluminous products, gypsum and Mg(OH) 2 , as well as degrad-
ing C-S-H.

G. Matrix destruction may also be caused by evolution of gas; for instance,

from reaction of aluminum metal or ammonia at high pH.

H. Whereas soluble chromium salts accelerate hydration of portland cement,

chromium (III) oxide has little effect on setting. The review conducted
by Mattus and Mattus 3 indicated that chromium substitutes for silicon in
C-S-H (see also Section 7.3.4), but that the final strength of the matrix is
decreased. Chromate, CrO 4 2- is thought to act similarly to sulfate and form
chromoaluminate crystals (C 6 A[CrO 4 ] 3 H y or C 4 A[CrO 4 ]H y , see also Sec-
tion 0) that coat C 3 A grains. Like sulfate, chromate can also act as an
accelerator.

I. The effect of ZnSO 4 illustrates the importance of considering the com-

bined effect of both the anion and cation. Whereas Zn in most compounds
is generally thought of as a retarder of portland cement, ZnSO 4 has been
found to be an accelerator at concentrations less than 2.5% and a retarder
at concentrations between 2.5 and 5.5%; it completely inhibits cement
hydration at higher concentrations.

J. Boric acid is an example of a compound that has a strong accelerating

effect on setting of portland cement, but retards hardening. Hydroxides,
carbonates, silicates, and aluminates accelerate setting, but retard harden-
ing or leave it unaffected; halides and nitrates accelerate both setting and
hardening.

In order to generalize regarding the results of different investigations, separate
out the effects of cations and anions, and look for interaction effects, data from 12
literature studies of pure compound additions to portland cement paste were