Effects of AH₃ and AFt on the Hydration-Hardening Properties of the C4A3S¯-CS¯-H₂O System

This study aimed to reveal the effects of the hydration products AH₃ and AFt phases on the hydration and hardening properties of calcium sulfoaluminate (CSA) cement. In addition, the effects of anhydrite (CS¯) and gypsum (CS¯H2) on the properties of CSA cement were compared. Calcium sulfoaluminate (C4A3S¯) was synthesized with analytical reagents, and the C4A3S¯-CS¯-H₂O system with different molar ratios of CS¯ and C4A3S¯ was established. The phase compositions and contents of AFt and AH₃ were determined by X-ray diffraction (XRD), Rietveld quantitative phase analysis, and thermogravimetric analysis (TG). The effects of pore structure and hydration product morphology on mechanical properties were analyzed by mercury intrusion porosity (MIP) and scanning electron microscopy (SEM). The results showed that the compressive strength exhibited a correlation with the AH₃ content. In the case of relatively sufficient anhydrite or gypsum, C4A3S¯ has a high degree of hydration, and the AH₃ content can be considered to contribute more to the strength of the hardened cement paste. When anhydrite was selected, the combined and interlocked AFt crystals were covered or wrapped by a large amount of AH₃. The mechanical properties of the hardened cement paste mixed with anhydrite were better than those of that mixed with gypsum.