Effect of Alumina on the Phase Equilibria of the Iron-rich Corner of the CaO-SiO₂-Fe₂O₃ System at 1240°C in Air

Abstract

The effect of Al₂O₃ on the compositional region of silico-ferrite of calcium and aluminum (SFCA) and the liquid phase and the phase equilibria, including SFCA, was investigated in a CaO-SiO₂-Fe₂O₃-5mass%Al₂O₃ system at 1240 ^°C in air. To obtain the desired composition, reagent-grade CaCO₃, SiO₂, Fe₂O₃, and Al₂O₃ powders were weighed, mixed, and equilibrated at 1240 ^°C in air. Each obtained sample was divided into two parts: one was pulverized into a powder and analyzed by XRD, and the other was subjected to microstructural observation and compositional analysis using EPMA. The results revealed that the compositional region of SFCA lies on the CF₃-CA₃-C₄S₃ plane and is C/S = 2.77–7.60 for 5 mass% Al₂O₃. Compared with the SFC composition region for 0 mass% Al₂O₃, the compositional range of SFCA extended in the CF₃-C₄S₃ direction, suggesting that the addition of Al₂O₃ contributes to the stability of SFCA. Furthermore, the liquid-phase region was divided into a ferrite melt with a high Fe₂O₃ concentration and a silicate melt with a high SiO₂ concentration, both of which shifted to the lower Fe₂O₃ side compared to the liquidus isotherm in the CaO-SiO₂-Fe₂O₃ system. Unlike CaO-SiO₂-Fe₂O₃, SFCA-I (SFC-I) was observed in the CaO-SiO₂-Fe₂O₃-5mass%Al₂O₃ system, thus indicating that the addition of Al₂O₃ contributes to the stability of SFCA-I.