Influence of sulfur trioxide/aluminium oxide ratio on the properties of calcium aluminate cement–gypsum binder

The calcium aluminate cement (CAC)–gypsum system is a binder with ettringite as the main hydration product. This study investigates the influence of the molar ratio of sulfur trioxide (SO₃) to aluminium oxide (Al₂O₃) (S–/A ratio) on the fluidity, setting time and compressive strength of CAC–gypsum binder. The iso-calorimetry, X-ray diffraction, thermogravimetry–differential thermogravimetry, scanning electron microscopy and mercury intrusion porosimetry tests are conducted to clarify the influence of $S¯$/A ratio on the hydration mechanism, phase assemblage, thermal behaviour, microstructure and pore structure, respectively. The findings suggest that replacing CAC with gypsum accelerates the hydration reactions and reduces the cumulative heat release, while a moderate $S¯$/A ratio delays the hydration process. At lower $S¯$/A ratios, the early formation of ettringite reduces the setting time without significantly affecting fluidity. An $S¯$/A ratio of 0.3 is most conducive to the compactness and compressive strength of CAC–gypsum hardened paste. The increase of $S¯$/A ratio from 0.3 to 1 promotes the formation of ettringite. However, the expansion damage caused by ettringite leads to a decrease in compressive strength with increasing curing age. In addition, when the $S¯$/A ratio is greater than 0.5, the deficiency of Ca²⁺ inhibits the formation of hydration products, resulting in a rapid decrease in compressive strength as the $S¯$/A ratio increases.