LONG-TERM STABILITY OF LANDFILLED COAL COMBUSTION BY-PRODUCTS

Many of the by-products from coal combustion and flue gas desulfurizat ion are cementitious, which suggests utilization in some settings repl acing conventional cements. Information on the long-term stability of these materials in Nature can be obtained from studies of landfilled b y-products. Five coal combustion by-products (CCBs) landfilled at four sites in the USA were characterized by X-ray diffraction. Where moist ure was available during or after disposal, the materials were changed chemically, physically and mineralogically over time, in processes an alogous to diagenesis of buried sediments. At three of the sites, init ially formed crystalline-phase assemblages were transformed into an as semblage dominated by ettringite and thaumasite. At one site where a h igh-sodium by-product was landfilled, an assemblage of Na-rich phases, including a zeolite and a zeolite-related nosean-hauyne phase, as wel l as tobermorite, was observed. This assemblage is unique and previous ly unobserved in CCBs. Diagenesis associated with many of these CCBs r educed strength and increased permeability after only a few years in t he natural environment. The characteristics of the altered by-products resemble those of soils more than concrete. Initially promising 7 or 28 day laboratory tests of strength and permeability may not be charac teristic of these materials on exposure to the environment. Blending o f CCBs with fly ash to increase the proportion of cementitious C-S-H, and controlling subsequent moisture additions, could minimize deleteri ous by-product diagenesis. (C) 1997 Elsevier Science Ltd.