Early-age hydration of ye'elimite (calcium sulfoaluminate phase) in presence of alkalis: role of calcium sulfate

Calcium sulfoaluminate-belite (CSAB) cement is considered a viable and environmentally friendly alternative to Portland cement. Ye'elimite or calcium sulfoaluminate which is the primary phase of CSAB cement undergoes rapid hydration, leading to the formation of ettringite or monosulfate as the major hydration product. The reactivity of ye'elimite can be influenced by other phases (e.g., calcium sulfate, calcium hydroxide, and alkalis) present in CSAB cement, affecting the evolution of hydrated phase assemblage. CSAB cements are known for their useful attributes such as quick setting, early-stage strength development, and the ability to compensate for shrinkage. These properties are directly linked to the early-age hydration of CSAB cement. This study is focused on the influence of alkali and calcium sulfate on the hydration of ye'elimite. Early-age hydration (up to 3 days) of laboratory-synthesized ye'elimite was studied under different alkali concentrations in the mix with or without calcium sulfate. The addition of gypsum was found to accelerate the early-age hydration of ye'elimite, and the alkalis contributed to further acceleration of the hydration in the initial hours. Alkalis were found to stabilize the hydration beyond a few hours, leading to a lower degree of hydration. Furthermore, alkalis influenced the formation as well as morphology of the hydration products, and the influence was more predominant in the absence of gypsum.