Utilization of Coal Combustion Ashes for the Synthesis of Ordinary and Special Cements

Raw mixes containing pulverized coal fly ash (with limestone and silica sand) or fluidized bed coal combustion ash (fly and bottom, with added limestone, anodization mud, and, when necessary, flue gas desulfurization gypsum), aimed at generating ordinary Portland or calcium sulfoaluminate clinkers, respectively, were heated in a laboratory electric oven at temperatures ranging from 1150 to 1500C and submitted to X-ray diffraction analysis. The former had the same qualitative phase composition as that of a reference mixture, composed by limestone and clay; furthermore, they exhibited an excellent burnability on the basis of their residual free lime contents, measured after heating at 1350, 1400, 1450, and 1500C. The latter showed very good results in terms of conversion of reactants and selectivity degree toward the main mineralogical constituent, calcium sulfoaluminate (4CaO 3Al2O3 SO3), even if the behavior of a reference mixture consisting of limestone, bauxite, and natural gypsum was slightly better. The introduction of a fluidized bed coal combustion ash in the raw mix generating calcium sulfoaluminate clinker implies a saving of bauxite and natural gypsum, which can be fully replaced through the addition of anodization mud and flue gas desulfurization gypsum, respectively.