Tobermorite and xonotolite are the representative of calcium silicate materials and many studies have been made in the past regarding the methods of synthesis from relatively highly purified starting materials. From the point of preserving the global environment, procurement of pure raw materials for industrial use is likely to become difficult in the future, and the recycled use of various waste materials cannot be avoided. The hydrothermal reaction of slaked lime and silica sand containing feldspar and clay minerals as impurities have investigated. It was found that the flexural strength of the solidified bodies increased with increasing treatment temperature and treatment time, reaching values of up to 20MPa which was enough for building material and was proportional to the amount of reaction products but independent of their type. Hydrogarnet and C-S-H were formed in the initial stages of reaction and the amount of these phases tended to decrease and 1.1 nm tobermorite formed as the reaction progressed further. This indicates that hydrogarnet and C-S-H are precursors of 1.1 nm tobermorite. In this hydrothermal process, it was clarified that the reaction was controlled by the diffusion of Si fraction from kinetic study.