In this study, calcium-doped silica was prepared by modifying sodium silicate-based silica gel in an aqueous solution of Ca(OH)2. Silica gels with less than 1 mm in size with different pH values were treated in Ca(OH)2 solution at concentrations ranging from 2 to 20 g/L and aged at 25, 60, and 90°C for 40 min. The BET surface area of the silica gel decreased while its pore size increased considerably after modification in Ca(OH)2 solution. The pH of 5% calcium-doped silica in water ranged from about 8.5 to 10, depending on the pH of silica gel, the aging temperature, and the concentration of the Ca(OH)2. When the concentration of the Ca(OH)2 solution was 20%, needle-like crystals of calcium silicate hydrate formed over the surface of silica gel.