Hydraulic Cement Systems for Stabilization/Solidification - Stabilization and Solidification of Hazardous, Radioactive, and Mixed Wastes - page 58

existing drinking water standards. The liberation of Na + and K + during the hydration
of fly ashes will compete with the potential sorption sites for other metals. For this
reason, the fly ashes used for cation sorption should have a low alkali content. Also,
the pozzolanic reaction between pozzolans and Ca(OH) 2 released from the hydration
of cement or high-calcium silicate hydrate or both may reduce the usefulness of the
adsorption capability of pozzolanic materials. 25
In a slag-based waste form for S/S of low-level radioactive alkaline salt solution
at the Savannah River Plant, 17 Class F fly ash was used to reduce the heat of hydration
and the permeability of the waste form. Both portland cement and portland fly ash
cement were used to solidify ion-exchange resin wastes, loaded with radionuclides,
transition metals, and organic chelating agents. 36 The solidified waste forms were
tested following the ANSI/ANS-16.1-2003 procedure for release rates, effective
diffusivities, and leachability indexes. Portland fly ash cement-solidified waste forms
exhibited lower leachability than portland cement-solidified waste forms. The author
attributed this to the decreased permeability by introducing fly ash. This is confirmed
by another study. 37
For over 20 years, the Oak Ridge National Laboratory has disposed of low-level
radioactive waste by mixing it with portland cement, Class F fly ash, and clay
minerals and injecting the grout into an impermeable shale formation at a depth of
approximately 1000 feet. 38 In Florida, a mixture of portland cement and fly ash was
used to treat 62,000 cubic yards of polychlorinated biphenyl (PCB)-contaminated
soil in a flowable fill mixture. 39 The contaminated soil was excavated, crushed,
mixed, and replaced in the trenches from where it had been excavated.
However, some publications also report that the use of fly ash is not helpful
during S/S of waste materials. Akhter et al. 19 used various combinations of Type I
portland cement, Class F fly ash, blast furnace slag, lime, and silica fume as additives
to immobilize soils containing heavy metals at concentrations in the range of 10,000
to 12,200 ppm. They concluded that fly ash, used together with lime, blast furnace
slag, or portland cement, does not serve to improve performance when used as an
additive.

4.2.3.5 Portland Silica Fume Cement

Condensed silica fume, also known as volatilized silica, microsilica, or simply silica
fume, is a by-product of the manufacture of silicon or of various silicon alloys by
reducing quartz to silicon in an induction arc furnace at temperatures up to 2000°C.
Gasified SiO at high temperatures condenses in the low-temperature zone to tiny
spherical particles consisting of noncrystalline silica.
The chemical composition of silica fume depends not only upon the raw mate-
rials used, but also upon the quality of the electrodes and the purity of the silicon
product. Generally speaking, the impurities in condensed silica fume decrease as
the amount of silicon increases in the final products. The by-products from the silicon
metal and the ferrosilicon alloy industries, producing alloys with 75% or higher
silicon content, contain 85 to 95% noncrystalline silica; the by-product from the
production of ferrosilicon alloy with 50% silicon contains a much lower silica
content. Minor components in silica fume are 0.1 to 0.5% Al 2 O 3 , 0.1 to 5% Fe 2 O 3 ,