Influence of free calcium sulfate levels on the early hydration of sulfate-rich belite sulfoaluminate clinkers: A comparative study with anhydrite and gypsum

In the present study, belite sulfoaluminate (BSA) clinkers containing varying levels of free calcium sulfate (f-CaSO4)—specifically 0 %, 5 %, and 10 %—were synthesized. These clinkers were subsequently mixed with anhydrite and gypsum for comparative analysis. Utilizing a range of analytical techniques such as isothermal calorimetry, X-ray diffraction (XRD), and thermogravimetric analysis (TGA), the early hydration process and the constitution of hydration products were systematically investigated. The experimental data reveal that sulfate-rich BSA clinkers demonstrate enhanced hydration activity, as well as superior early compressive strength. As the f-CaSO4 content escalates, there is a corresponding increase in the cubic crystal sulfoaluminate within the BSA clinkers. This compositional shift has a beneficial impact on both the initial and final setting times, thereby enhancing the 3-day compressive strength. Contrary to ordinary BSA cement clinkers, those rich in sulfate retard the dissolution rate of external gypsum. It is noteworthy that anhydrite, possessing a solubility greater than gypsum, exerts a more pronounced acceleration effect on the early hydration process of sulfate-rich BSA clinkers.