The hydration of calcium sulphate hemihydrate has been measured with temperature, ultrasonic velocity, attenuation and acoustic emission for the water to gypsum, W/G, ratio 0.50-0.80. The rate equation with the factor of W/G ratio has been derived and investigated the reliability with experimental results. In general the ultrasonic methods were more sensitive than the temperature measurements. The velocity for the W/G ratio <0.70 had three stages, while no distinctive stages was observed for the W/G ratio > 0.70. The change of ultrasonic attenuation was noticeable after the completion of hydration detected by temperature and velocity measurements. The rapid increment of acoustic emission was attributed to the highest rate of hydration.
