Quality characteristics and phase evolution of clinker in direct solar calcination under fluctuating irradiance

Cement manufacturing is an energy-intensive high-temperature industrial process operated at roughly 1500 °C, accompanied by substantial CO 2 emissions originating from both raw meal decomposition and fossil fuel combustion. While the integration of solar energy as an alternative heat source for direct and indirect cement calcination offers a pathway to reduce carbon emissions by over 35%. The cement calcination is a continuous process involving coupled physical and chemical reactions that depend on reaction temperature and duration. For solar-driven cement calcination, the inherent intermittency and instability of solar irradiance induce temperature fluctuations inside the calciner, potentially compromising production stability and crystal development, create substantial challenges for maintaining target reaction temperatures, which further affects clinker quality and phase evolution. Few existing studies have focused on this critical issue. In this study, a direct solar-driven cement sintering experimental platform was established utilizing a concentrated solar furnace to investigate the performance evolution of cement clinker. A temperature controller was used to simulate the fluctuating direct normal irradiance (DNI) while regulating the reaction temperature. The results indicate that, despite temperature variations induced by irradiance instability during solar calcination, the mineralogical and chemical phase composition of solar cement clinker are consistent with conventional standards. The primary clinker phases, including C 3 S, C 2 S, C 3 A, and C 4 AF, are well-formed. The CaO/SiO 2 ratio, as well as the total content of dicalcium silicate (C 2 S) and tricalcium silicate (C 3 S) satisfies the national standard value (≥66%). Prolonging the decomposition duration reduces the C 3 S content from 59.5% to 56.2%, indicating a decrease in CaO activity that slows down the reaction between CaO and C 2 S. This study clarifies the dynamic response of clinker performance to solar radiation fluctuations, providing critical theoretical support for the industrial application of solar-driven calcination in ensuring reliable clinker production.