Effects of thermal treatment on the mechanical properties, microstructure and phase composition of an Ettringite rich cement

Recently calcium sulfoaluminate cements gain increasing attention due to their significant potential to reduce the carbon footprint of cement production compared to Portland cement. However, the conditions applied during its processing play a crucial role for the stability and longevity of the material. Thereby, the temperature has a decisive influence, as it is already known from numerous studies that ettringite structurally changes significantly upon thermal induced dehydration. Within this background, the present study subjects a holistic view of the mechanical, morphological, phase and structural changes of a commercial calcium sulfoaluminate cement related to the dehydration of the contained ettringite upon treatment at drying temperatures from 23 °C to 100 °C for 7 and 28 days. By complementary methods it is shown that with increasing curing temperature, the mechanical stability decreases, the total pore area and porosity increase, while the permeability of the microstructure is lower for samples stored at 100 °C. Removal of water increases the intercolumnar distance within the ettringite lattice, thereby inducing strain which is released upon rehydration. Although during storing at a temperature of 100 °C ettringite is transformed into an X-ray amorphous product, the initial morphology of the crystals embedded in the cementitious matrix is retained.