The hydration reaction temperature and the hydrated product were studied using three types of calcium aluminate clinker minerals (CA, C 3 A, CSA) and employing hydration temperatures of 20°C and 5°C. The main hydration product of these rapid hardening clinkers is a highly sulphated form of calcium trisulphoaluminate hydrate (ettringite) whose characteristics are largely affected by the curing temperature. The microscopic structure of cement hydrate cured at 20°C showed close-packed long needle-like ettringite crystlal. On the other hand, the ettringite crystal formed at 5 °C was relatively short compared with that cured at 20°C. The change of crystal form of ettingite at low temperature is presumably due to the decrease of Al 3+ solubility and the increase of SO 4 2- solubility.