UTILIZATION OF COAL COMBUSTION ASHES FOR THE SYNTHESIS OF ORDINARY AND SPECIAL CEMENTS

Raw mixes containing pulverised coal fly ash (with limestone and silica sand) or fluidised bed coal combustion ash (fly and bottom, with added limestone, anodization mud and, when necessary, flue gas desulfurization gypsum), aimed at generating Portland or calcium sulfoaluminate clinkers, respectively, were heated in a laboratory electric oven at temperature ranging from 1150° to 1500°C 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 the residual free lime contents, measured after heating at 1350°, 1400°, 1450° and 1500°C. The latter showed very good results in terms of conversion of reactants and selectivity degree towards the main mineralogical constituent, calcium sulfoaluminate (4CaO∙3Al2O3∙SO3), even if the behaviour of a reference mixture consisting of limestone, bauxite and natural gypsum was slightly better. The introduction of a fluidised 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 desulfurisation gypsum, respectively