Conversion of hazardous waste into calcium sulfoaluminate cement: effect of alkalinity modulus on mineral composition and Fe₂O₃ incorporation in Ye'elimite

The production of calcium sulfoaluminate (CSA) cement using hazardous wastes can enhance its sustainability and economic benefits. Incorporating Fe₂O₃ into ye'elimite can reduce the Al₂O₃ demand in waste-based CSA cement, while the influence of alkalinity modulus (Cm) on this incorporation remains unclear. This study synthesised CSA cement clinker from hazardous wastes with varying Cm. Results show that forming inert phases like C₂AS and Fe₂SiO₄ hinders the synthesis of hydraulically active minerals, requiring a higher Cm of 1.10 to attain the highest 28-day compressive strength of 76.4 MPa. Additionally, increasing Cm reduces the Fe/(Al + Fe) ratio in ye'elimite, particularly in low-aluminium clinkers. This contributes to the transformation of C₄A₃S¯-o to C₄A₃S¯-c and the slowed hydration. However, facilitated ye'elimite formation under higher Cm and increased Al₂O₃ content in feedstock leads to more Fe₂O₃ being incorporated into ye'elimite. This study provides guidance on regulating Fe₂O₃ incorporation in the production of waste-based CSA cement clinker.