Hydraulic Cement Systems for Stabilization/Solidification - Stabilization and Solidification of Hazardous, Radioactive, and Mixed Wastes - page 59

2 to 5%, carbon, 0.1 to 0.2% S, less than 0.12% CaO, less than 0.1% TiO 2 , less than
0.07% P 2 O 5 , and less than 1% alkalis. 40
It is well known that the use of silica fume in concrete decreases or eliminates
the free lime content, increases the strength, and decreases the permeability of
concrete. Silica fume showed an effect on the released lime 8 hours after the hydration
of portland cement, 41 which is significantly faster than slag or fly ashes due to its small
particle size.
Laboratory results 37,42 have indicated that the use of silica fume decreases the
diffusion coefficient of contaminants very significantly. Shin and Jun 43 used silica fume
as an admixture to solidify a waste containing a high concentration of organic
contaminants and chromium in a monolithic mass with high strength and very low
leachability for these contaminants. They noticed that silica fume was highly effec-
tive in achieving high compressive strength and low permeability. In another study,
both silica fume and fly ash were blended with portland cement to solidify K061
hazardous waste (electric arc furnace dust [AFD]). 32 Some short-term results indi-
cated that the use of silica fume is more effective than fly ash based on TCLP results.

4.2.4 P ORTLAND C EMENT -S ODIUM S ILICATE S YSTEM

Sodium silicate is the generic name for a series of compounds with a formula
Na 2 O.nSiO 2 . Theoretically, the ratio n can be any number. Commercial liquid sodium
silicates have a ratio from 1.60 to 3.85. Based on 29 Si-NMR (nuclear magnetic
resonance) testing, the SiO 4 structural units can be classified into seven types. Q 0 ,
Q 1 , Q 2 cy-3 , Q 2 , Q 3 cy-3 , Q 3 , and Q 4 . The superscript on the Q represents the number of
linkages between the given Si atom and neighboring Si atoms by = Si-O-Si = bonds.
The symbols Q 2 cy-3 and Q 3 cy-3 designate intermediate or branched SiO 4 structural
units in cyclo-tristructure (six-membered rings). 44 The percentages of structural units
in a sodium silicate solution depend on the Na 2 O/SiO 2 ratio, concentration, temper-
ature, and age. Sodium silicate was widely used as an accelerator of concrete. Today,
it is still a widely used accelerator for shotcrete. Sodium silicate is also a very
effective activator for ground GBFS. The Na 2 O/SiO 2 ratio and concentration have
great effect on the properties of concrete.
Conner 1 first discovered using liquid sodium silicate as an additive for cement-
based S/S in the late 1960s. It was demonstrated in 1970 and applied on full-scale
S/S in 1971 by Chemfix Technologies, Inc. under the trade name of Chemfix ® . The
use of sodium silicate adsorbs extra water in liquid wastes, accelerates the setting
and hardening of the system, and decreases the leachability of heavy metals. The
adsorption of water is attributed to the gelation of sodium silicate in the presence
of portland cement. Thus, a portland cement-sodium silicate system is especially
suitable for liquid wastes from both technical and economical aspects. 1,45 Later on,
a number of vendors have marketed the S/S process using sodium silicates in
different ways and forms, which include Fujimasu Process, Lopat process, SolidTek,
and enviroGuard/ProTek/ProFix. Conner 1 gives more details about the sodium sili-
cate technologies.
It is reported that many investigators have difficulty reproducing in the laboratory
the results that are routinely obtained in the field. 1 The most important reason is that