Control of hexagonal plate-like microstructure of in-situ calcium hexaluminate in monolithic refractories

The self-toughening property of calcium hexaluminate (CaO•6Al2O3, CA6) in engineering ceramics causes a remarkable interest in the in-situ CA6 in monolithic refractory. The determination of significant factors which controls the formation and microstructure of CA6 in monolithic refractory is mainly discussed in this study. Samples were prepared by using calcium aluminate cement and sintered alumina and the chemical composition was fixed at the molar ratio of CaO: Al2O3 = 1:6. In order to evaluate the suitable sintering condition and SiO2 content for CA6 phase formation, the variation of firing temperature and holding time was first modified at 1400–1500°C for 1–5 h. The second factor was done by adding 2–10 mass% of SiO2 into the primary mixture. In comparison, the results were confirmed that high firing temperature at 1500°C provided the highest quantity of CA6 phase and plate-like microstructure. In addition, longer proceeding time contributed to the grain growth of CA6, especially, within 5 h of holding time. SiO2 importantly helped to enhance hexagonal plate-like microstructure due to the ability of ion mobility in low-melting phase of gehlenite but the suitable content of SiO2 should not exceed 2 mass% for the better control ability of CA6 formation and microstructure.