Effect of calcium compounds on lightweight aggregates prepared by firing a mixture of coal fly ash and waste glass

Abstract Mixtures of coal fly ash and waste glass are amended with three calcium compounds, pressed into green pellets, and fired to prepare lightweight aggregates at 1050–1175 °C. The aggregate particle density, being associated with microstructure, depends on both firing temperature and calcium species. Calcite and gypsum are found to be good flux promoters. In contrast, calcium chloride is an ineffective flux promoter due to the poor solubility between calcium chloride and coal fly ash/glass mixture, thus the glassy layer formed is too insufficient to effectively envelope the gases/vapor released from pellet interior. Further, the calcium chloride-containing lightweight aggregates prepared at 1050 °C and 1100 °C sorb much more water than those amended with the other two calcium compounds. Such higher water sorption capacity results from the poor solubility of major phases (present in coal fly ash/glass mixture) in calcium chloride liquid phase during firing, as evidenced by the prominent presence of sinjarite crystalline phase in the fired pellets. In fact, the 1050 °C and 1100 °C calcium chloride-containing lightweight aggregates were observed to slightly dissolve in the 24-h water sorption capacity test, thereby forming more open pores. Firing the calcium compound-amended green pellets leads to disappearance of corundum and wustite phases, while with crystobalite, diopside and wollastonite emerging as new phases.