Mechanisms of early hydration of calcium sulfoaluminate cement in aqueous and alkaline media

The early hydration of calcium sulfoaluminate (CSA) cement in water (CSAH) and in 2 M sodium hydroxide (CSA2M), both with a liquid/solids ratio of 0.5, was investigated. Hydration kinetics were assessed using in situ X-ray diffraction, isothermal conduction calorimetry, ultrasonic pulse velocity (UPV) and mechanical strength measurements. The maximum heat release occurred earlier in the CSA2M system and UPV measurements revealed a more rapid increase in mechanical stiffness. The presence of alkalis accelerated the dissolution of ye’elimite and anhydrite, leading to a shorter induction period and faster precipitation of ettringite. The phase refinement confirmed a higher dissolution rate of anhydrite in the alkaline environment, while the formation of ettringite stabilised within 10 h. At age 25 h, the ettringite content was 58.3% in CSAH and 48.2% in CSA2M. This finding confirms that the UPV measurements provided a reliable indication of the onset of setting and early structural development in CSA cements and contribute to a deeper understanding of the early hydration mechanisms of CSA cements and the impact of alkalis on phase evolution.