Calcium silicate hydrate was prepared by hydrothermal reaction between calcium oxide and silica (C/S = 2.0) at a temperature of 205degrees-215degreesC and a pressure of 17-19 bar. This reaction with decomposition at 900degreesC produced highly reactive beta-dicalcium silicate (specific surface area 4.55 m(2)/g) contaminated with small amounts of wollastonite as an impurity. Infrared spectral studies have shown that beta-dicalcium silicate prepared at 900degreesC is less symmetric compared with the control prepared at 1450degreesC using boron trioxide as a stabilizer. The specific surface area of beta-dicalcium silicate decreased with temperature. The hydration studies were done by determining the nonevaporable water, calcium hydroxide (CH) contents, and specific surface area of the hydrated samples. X-ray diffraction studies were also done. The results showed that prepared beta-dicalcium silicate is highly reactive. Calcium chloride (1.0 wt%) and gypsum retard the hydration. Possible causes of high reactivity have been discussed.