Evolution of microstructure in flyash-containing porcelain body on heating at different temperatures

15 wt% flyash (a calcined byproduct of thermal power plant) was incorporated in a normal triaxial kaolin-quartz-feldspar system by replacing equivalent amount of quartz. The differences in microstructural evolution on heating the compact mass of both normal and flyash-containing porcelain at different temperatures (1150-1300 degrees C) were examined using scanning electron microscopy (SEM) operating in secondary electron image (SEI) mode. Microstructure of normal porcelain did not show the presence of mullite and quartz grains at 1200 degrees C and the viscosity of silica-rich glass restricted the growth of mullite crystals at 1250 degrees C. Flyash porcelain, on the other hand, shows the presence of primary mullite aggregates in the clay relict and a significant growth of mullite crystals in a low viscosity glassy matrix at 1200 C itself. At 1300 C, both the bodies show a larger region of more elongated (> 1 mu m) secondary mullite along with clusters of smaller sized primary mullite (< 1 mu m). Small primary mullite crystals in the clay relict can be distinguished from elongated secondary mullite crystals in the feldspar relict in their size. Primary mullite aggregates remain stable also at higher temperatures. XRD studies were carried out for quantitative estimation of quartz, mullite and glass, which supported the SEM observations. An attempt was also made to correlate their mechanical strength with the constituent phases