Calcite and silica gel composites prepared by sol gel technique were hydrothermally solidified under a saturated steam pressure. The composites were fabricated by calcite and tetraethoxysilane (TEOS) without or with AlCl 3·6H2O. The silica gel particles with several hundreds nm in diameter were observed around the surface of the calcite particles before the hydrothermal processing. The bending strengths of the hydrothermally solidified materials without Al element were relatively low even after the hydrothermal processing. In contrast, the bending strengths of the hydrothermally solidified materials with Al element increased up to 6.0 MPa after 10 h of hydrothermal processing at 200 °C. The SEM photographs of the hydrothermally solidified samples with Al element showed that silica gel particles are converted to platy-like deposit during the hydrothermal processing. The platy-like deposit formed around the calcite particles are estimated to act as a binder between the calcite particles, resulting in the development of the strength.