Influence of calcium hydroxide and calcium sulfate on early-age properties of non-expansive calcium sulfoaluminate belite cement

Calcium sulfoaluminate belite (CSAB) cements are currently being promoted as alternative low CO₂ cements. CSAB cements have ye'elimite as their primary phase. Ye'elimite hydrates rapidly to form ettringite which contributes towards high early-age strength. The hydration characteristics of ye'elimite are influenced by the presence of calcium sulfate and calcium hydroxide, which can also extend to the mechanical properties. Further, this change in phase composition can modify a non-expansive binder into an expansive binder. In this study, the influence of phase composition on hydration characteristics, expansion, pore structure, and mechanical properties was investigated using CSAB binders with varying gypsum and calcium hydroxide levels. The expansion was directly affected by the amount of calcium sulfate and calcium hydroxide. The differentiating factor between expansive and non-expansive systems was the critical pore size of CSAB binders. Expansive CSAB binders exhibited lower critical pore size than non-expansive binders, highlighting the role of supersaturation. The addition of calcium hydroxide accelerated the kinetics of ye'elimite hydration and increased the expansion. Moreover, the change from a non-expansive to expansive system did not lead to the deterioration of the CSAB binder matrix.