Stability of ettringite in CSA cement at elevated temperatures

Calcium sulfoaluminate cements are promising low carbon dioxide alternatives to Portland cement. Their main hydration products are ettringite and aluminium hydroxide. Ettringite has recently been identified as a potential heat storage material. The reversible dehydration of ettringite to metaettringite at elevated temperatures and under dry conditions involves a relatively high enthalpy, which can be used for (seasonal) heat storage. In this context, the effect of elevated temperatures (up to 110°C) and humidity conditions (steam curing and dry curing) on the stability of ettringite in calcium sulfoaluminate was studied experimentally using thermogravimetric analysis, X-ray diffraction and thermodynamic modelling. The experimental results show that ettringite decomposes under steam curing conditions at temperatures far below 100°C to monosulfate. This may lead to delayed ettringite formation when the temperature is lowered again under humid or wet conditions. Under dry conditions at low water vapour saturation, the expected dehydration of ettringite to metaettringite is found. A thermodynamic model for the stability of hydrated calcium sulfoaluminate cements based on cement composition and calculations of thermodynamic equilibria has been established. The modelled phase development under different hygro-thermal conditions agrees well with the experimental findings. Stable heating and drying conditions for calcium sulfoaluminate based heat storage materials could be identified.