Effects of ferric oxide on the calcination and phase evolution of low-carbon calcium sulfoaluminate cement

To achieve a low-carbon transition in the cement industry, Calcium sulfoaluminate cement (CSA) has garnered significant attention, especially when raw materials are substituted with solid waste. Many solid wastes contain ferric oxide, which significantly effects the properties of cementitious materials. However, there is a lack of comprehensive research on the evolution of mineral phases during the calcination process of CSA with and without ferric oxide, limiting theoretical guidance for optimizing cement mineral phases based on calcination process. To address this problem, X-ray diffraction (XRD), Rietveld quantitative analysis, and Scanning electron microscopy (SEM) were used to investigate the evolution of key mineral phases in CSA clinkers with and without Fe₂O₃ over the temperature range of 900 ~ 1250/1300 °C, as well as the influence of CaO content on these processes. The study revealed that the introduction of Fe₂O₃ enhances the reactivity of the mineral phase system at lower temperatures, accelerates the transformation of intermediate phases into target phases, and shifting the rapid reaction stage of C₄A₃$ from 1250 °C–1300 °C to 1150 °C–1250 °C. This shift allows for more C₄A₃$ formation through an intermediate phase (CA and CA₂), optimizing its generation pathway. While Fe₂O₃ accelerates the rapid reaction of C₄A₃$, it slightly reduces the selectivity of Ca²⁺ for C₂S, affecting its reaction process but having minimal impact on its final content. Increasing CaO content does not alter the original mineral phase system of the clinker but accelerates the chemical reaction rate of minerals, with this effect further enhanced in systems containing iron. Higher CaO content inhibits the substitution of Fe³⁺ for Al³⁺ in C₄A₃$, thereby reducing the overall amount of C₄A₃$ in clinker.