Various kinds of amorphous silicates were used as raw materials to synthesize building materials based on calcium silicate hydrates. Relationships between the reactivities of silicates and castabilities of the building materials were investigated. In addition, effects of the reactivities of silicates on the mechanical properties of casted specimens were studied by analyzing microstructures and hydrates produced. As the reactivity of silicate increase, the press castability increases and the crystal size of hydrate and pore size also increase. For the mechanical properties, the flexural strength increases with decreasing crystal size and densifing microstructure. The compressive strength is greatly dependent on the den-sification of microstructure rather than crystal size of hydrate. Based on the results, diatomous ma-terials are desirable due to high reactivity and formation of densified microstructure. Slag and fly ash may be partially used as raw materials with amorphous silicates to manufacture building materials based on calcium silicate hydrates.