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

The compositional ranges of SFCA-I and other SFCA series and the phase equilibrium relationships were investigated in the iron-rich corner of the CaO–Fe2O3–Al2O3 system at 1240°C in air using powder and single crystal XRD as well as EPMA. To obtain the desired composition, reagent-grade CaCO3, Fe2O3, and Al2O3 powders were weighed, mixed, and equilibrated at 1240°C in air. Each of the obtained samples 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 crystalline structures of the SFCA series for some samples were analyzed by a single XRD. The results revealed that not only SFCA-I but also SFCA-II and SFCA have been found in the compositional area of this study; the Al/(Al+Ca+Fe) concentration ranges are 7.89–20.33 mol% for SFCA-I, 17.62–24.55 mol% for SFCA-II and 20.93–27.60 mol% or even higher for SFCA. The compositions of SFCA are on the line satisfying M14O20, while those of SFCA-I and SFCA-II deviate from the lines satisfying M20O28 and M17O24, respectively, and are closer to the line satisfying M14O20. Since the compositions were derived based on the assumption that all Fe irons are Fe3+, the apparent compositions of SFCA-I and SFCA-II deviate from M20O28 and M17O24, respectively, owing to the presence of trace amounts of Fe2+.