Mineralogy and microstructure of sintered lignite coal fly ash☆

Lignite coal fly ash from the ‘Nikola Tesla’ power plant in Yugoslavia has been characterised, milled, compacted and sintered to form monolithic ceramic materials. The effect of firing at temperatures between 1130 and 1190 °C on the density, water accessible porosity, mineralogy and microstructure of sintered samples is reported. This class C fly ash has an initial average particle size of 82 μm and contains siliceous glass together with the crystalline phases quartz, anorthite, gehlenite, hematite and mullite. Milling the ash to an average particle size of 5.6 μm, compacting and firing at 1170 °C for 1 h produces materials with densities similar to clay-based ceramics that exhibit low water absorption. Sintering reduces the amount of glass, quartz, gehlenite and anhydrite, but increases formation of anorthite, mullite, hematite and cristobalite. SEM confirms the formation of a dense ceramic at 1170 °C and indicates that pyroplastic effects cause pore formation and bloating at 1190 °C.