The Clean Air Act, as amended in 1992, spurred the development of flue gas desulfurization (FGD) technologies. Several FGD processes produce a dry, solid byproduct material consisting of excess sorbent, reaction products containing sulfates and sulfites, and coal fly ash. Generally, dry FGD by-products are treated as solid wastes and disposed in landfills. However, landfill sites are becoming scarce and tipping fees are constantly increasing. Provided the environmental impacts are minimal, socially acceptable beneficial uses of dry FGD by-products can provide economic benefits to both the producer and the end user of the FGD by-products.A study was initiated in December, 1990 to demonstrate large volume beneficial uses of FGD by-products. A Phase 1 report provided results of an extensive characterization of chemical, physical, mineralogical and engineering properties of 58 dry FGD by-product samples. The Phase 1 report concluded that high volume beneficial reuses will depend on the economics related to their ability to substitute for existing materials for various types of applications (e.g. as an agricultural liming material, soil borrow for highway embankment construction, and reclamation of active and abandoned surface coal mine lands). Phase 2 objectives were (1) to conduct laboratory and greenhouse studies of FGD and soil (spoil) mixtures for agronomic and engineering applications, (2) to initiate field studies related to high volume agronomic and engineering uses, and (3) to develop the basic methodological framework for estimation of the financial and economic costs and benefits to society of several FGD reuse options and to make some preliminary runs of economic models.Laboratory studies were conducted to investigate time-dependent changes in solution chemistry and solid phase composition resulting from the forced leaching of mine spoil-FGD mixes (FGD applied at 0, I O , 20 and 40 wt %). Water quality of the leachate was primarily a function of FGD by-product type, application rate and reaction time. Spoil type had little effect on pore water quality except at the lowest FGD application rate. FGD by-products containing free lime (CaO or Ca(OH),) initially yielded leachate pH values above 12 but the pH rapidly decreased with time. Leachates were generally dominated by Ca, S and CI. If the sorbent used to scrub the flue gases contained Mg, then Mg concentrations were also found to be greatly increased in the leachate of the treated spoil. In contrast, AI and Fe concentrations, which often reach toxic levels in acid mine spoil, were decreased to near instrument detection limits when FGD was applied at rates of 10% or greater. Arsenic, Cd, Cr, Cu and Ni concentrations were below the maximum contaminate levels (MCL) defined for drinking water The MCL for Se was exceeded in 78% of the samples in the first leachate, but in only 25% of the samples after 26 days of reaction. Geochemical modeling of the leachate chemistry suggested that ettringite (Ca,AI,(S0,)3( OH),2.6H,0)forms as a reaction product in F...