Effect of Alumina on the Phase Equilibria of the Iron-rich Corner of the CaO–SiO<sub>2</sub>–Fe<sub>2</sub>O<sub>3</sub> System at 1240°C in Air

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–Fe2O3–5 mass%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–Fe2O3–5 mass%Al2O3 system, thus indicating that the addition of Al2O3 contributes to the stability of SFCA-I.