Sustainable Synthesis of Refractory Calcium Aluminate Binders Using Aluminum Slag Waste and Limestone

This study investigates the valorization of aluminum slag waste and natural limestone as alternative raw materials for the synthesis of calcium aluminate binders with enhanced refractory potential. The aluminum slag, containing 72.9 wt.% Al2O3, and limestone, containing 56.1 wt.% CaO, were used to formulate four mixtures with different Al2O3/CaO mass ratios ranging from 1.05 to 4.85. The mixtures were sintered at temperatures between 1200 and 1600 °C to investigate the combined effects of composition and temperature on phase evolution and microstructural development. X-ray diffraction (XRD) revealed a systematic evolution of the calcium aluminate phase assemblage, including C3A, C12A7, C5A3, C3A5, CA, CA2, and CA6, together with α-Al2O3, MgAl2O4, C2AS, and calcium ferrite phases. Increasing the Al2O3/CaO ratio promoted the development of alumina-rich phases, particularly CA2 and CA6, while C12A7 and C5A3 were identified in the low-ratio formulation. Scanning electron microscopy coupled with energy-dispersive spectroscopy (SEM–EDS) revealed progressive microstructural densification and phase development with increasing sintering temperature. The formulation with an Al2O3/CaO ratio of 1.05 (M1) exhibited a phase assemblage broadly consistent with calcium aluminate cement formulation reported in the literature and satisfied the measured requirements considered in this study. Overall, the results establish a clear relationship between the Al2O3/CaO ratio, temperature, and phase assemblage, demonstrating the potential of aluminum slag as a secondary raw material for the sustainable production of calcium aluminate binders with promising refractory characteristics.