Environmental Engineering Reference
In-Depth Information
TABLE 4.2
Coal Combustion By-Product Used in
Waste S/S (short tons)
30
Year
Fly Ash
Bottom Ash
FDG* Material
1999
1,930,000
61,000
15,600
2000
2,000,000
35,400
20,900
2001
1,439,407
68,930
47,258
2002
3,187,773
19,091
67,053
* Flue gas desulfurization gypsum.
Fly ashes react with lime much more slowly than the hydration of portland
cement. Rod-like ettringite grows on the surface of fly ash at the age of about 1 day
and the surface of fly ash is thickly coated with hydration products at 3 days.
29
The
presence of fly ash decreases or eliminates the free lime content in hardened cement
pastes. The products resulting from the pozzolanic reactions between lime and fly
ash refine the pore structure of hardened pastes and reduce the permeability of
hardened pastes.
Coal fly ash and other coal combustion by-products have been used extensively
for waste stabilization. Table 4.2 shows the amounts of these materials used for
waste S/S.
30
Fly ashes are used in three ways: (1) fly ash alone (mainly Class C fly
ash), (2) lime-fly ash mixture, and (3) portland cement-fly ash mixture. A previous
report reviewed the use of coal fly ash for waste S/S.
31
Many laboratory research projects and commercial applications have used port-
land fly ash cement to stabilize/solidify hazardous, radioactive, and mixed wastes.
31,32
A portland cement/Class F fly ash binder was used to solidify a heavy metal sludge
containing cadmium, chromium, mercury, and nickel.
31
Results indicate a wide
variability in the composition of partially dewatered sludges. Such variability can
lead to localized differences in the chemical composition of the solidified material.
Microanalyses of the cement and fly ash mixtures indicate that fly ash spheres reacted
with the portland cement component to form a variety of reaction products, including
ettringite. To a minor degree, fly ash was involved in the chemical fixation of the
waste elements. Lead nitrate up to a concentration of 10% (by the mass of the binder)
was solidified with Type I portland cement and a cement-fly ash (equal proportion)
mixture.
33
The quantity of divalent lead leached depended on the initial lead nitrate
concentration and the binder systems adopted. Lead solidified with a cement-fly ash
mixture showed slightly less leaching compared to the cement binder.
The use of fly ash can enhance the sorption of Cs in a cement-based system.
34
Weng and Huang
35
employed fly ash to adsorb metals in industrial water, then used
cement to fix the metal-contaminated adsorbent. It was found that fly ash could
provide an acceptable level of metal adsorption for zinc and cadmium in dilute
wastewater streams, with adsorption capacities of 0.27 and 0.05 milligrams per gram,
respectively. Tests of leachates derived from the cement-fixed metal-laden fly ash
indicated that concentrations of the metals in the leachates were lower than the
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