Design of self-desiccating binders using CSA cement: influence of the cement composition and sulfate source

Calcium sulfoaluminate (CSA) cements with high ye'elimite content are of interest for the production of self-desiccating binders. In this work, the influence of cement composition and calcium sulfate source on the content of water bound by hydration was investigated, with the future goal of designing non-expansive materials with a dry internal environment. A thermodynamic approach was used first. Ten cement compositions were prepared by blending a CSA clinker comprising 54·3% ye'elimite and 29·1% belite with anhydrite and calcium oxide, respectively, within the ranges 80–95%, 5–20% and 0–15%. Cement suspensions (water/cement ratio (w/c) = 6) were maintained under stirring at 20°C for 1 month. The cement comprising 80% clinker and 20% anhydrite led to the highest contents of ettringite and bound water, which was simulated well by thermodynamic calculations. A kinetic study was then performed on cement pastes (w/c = 0·5 or 0·6), with the cement comprising 80% clinker and 20% calcium sulfate (introduced as anhydrite or gypsum). The addition of gypsum, which dissolves faster than anhydrite, led to the rapid formation of a dense microstructure, which tended to limit the progress of hydration at later age. With anhydrite, higher hydration degrees were reached and specimens cured in a wet or dry environment exhibited smaller volume changes.