The feasibility of using relatively low-purity silica as the starting material for hydrothermal solidification was investigated from the consideration of effective utilization of natural resources. Hydrothermal solidification was performed at 140 degrees -180 degreesC for periods ranging from 2 to 40 h using a mixture of 80% silica sand (containing 67% quartz, 22% clay minerals (mica, clinochlore, and kaolinite), and 7% feldspar) and 20% Ca(OH),. Within the range of experimental conditions, the flexural strength of the solidified bodies increased with increased treatment temperature and treatment time, reaching values of up to 20 MPa. The flexural strength was proportional to the amount of, but independent of the type of, reaction products. Hydrogarnet and C-S-H were formed in the initial stages of the reaction; the amount of these phases tended to decrease, and 1.1 am tobermorite formed as the reaction further progressed. These results indicated that hydrogarnet and C-S-H were precursors of 1.1 nm tobermorite.