Influence of Hydrothermal Pre-Treatment of Coal Fly on the Synthesis of Belite Phase

In this paper, the results of an extensive investigation of hydrothermal pre-treatment for synthesizing belite phase from reactive mixtures （CaO/SiO2 molar ratio of 2） consisting of one waste kinds （bottom ash-BA or fly ash-FA） from fluidised brown coal combustion in Slovakian power plant and CaO （analytical grade reagent） addition are summarized. Changes in structure and phase composition of hydrothermally synthesized belite precursors and subsequent calcinated products were compared with those of starting mixtures. Based on XRD diffraction patterns, the formation of the new profiles corresponding to CSH phases with low degree of ordering as belite precursors after hydrothermal treatment was confirmed. Calcination of hydrotermally treated products at 900℃ led to transformation of CSH phases to wollastonite, belite and gehlenite phase. Differences in phase composition of products before and after calcination depend upon waste quality and precursor＇s synthesis conditions. Bottom ash isn＇t suitable as raw material for synthesizing belite phase because of high CaO content fixed in anhydrite form （44.1%）. Coal fly ash with low CaO content in anhydrite form （4.2%） and its hydrothermal treatment in combination with subsequent heating offer opportunities for the utilization of coal fly ash as raw material for belite production.