Crystalline phase control of glass ceramics obtained from sewage sludge fly ash

Different types of glasses and glass-ceramics were made using fly ash from sewage sludge incinerators. The optimum nucleation condition was heating at 760 °C for 1 h. Crystallization of the nucleated specimen in the region of 1050–1200 °C resulted in the formation of two crystalline phases, i.e. anorthite and diopside. The relative fractions of these two phases changed with crystallization temperature. Specimens heat-treated for 2 h at 1050 °C consisted mainly of diopside crystals with a minor proportion of anorthite. On the other hand, those heat-treated at 1150 °C were primarily composed of anorthite. Glass-ceramics containing large amounts of diopside (1050 °C/2 h) generally displayed better physical and chemical properties than their anorthite counterparts (1150 °C/3 h) due to the interlocking microstructure of diopside crystals.