Hydration of calcium sulfoaluminate cement at less than 24 h

Calorimetry, X-ray diffraction, scanning electron microscopy, microprobe and porosimetry were used to investigate hydration processes occurring at 25°C, 55°C and 85°C for commercial calcium sulfoaluminate (CSA) cement during the first 24 h. CSA cement pastes harden through the formation of an initial ettringite skeleton and its subsequent infilling by mixtures of ettringite, AFm and C-S-H. At low water/cement ratios these yield a dense, rather featureless paste. The formation rate of ettringite is significantly increased at elevated temperatures. A ‘shrinking sphere’ model is proposed to explain the competitive kinetics of hydration processes with formation of inner and outer products which are shown to differ significantly in mineralogy.