Hydrated sodium silicate with 25 wt% water contents was synthesized by hydrothermal reaction using anhydrous sodium silicate. The hydrated sodium silicate was expanded at $370^{circ}C$ for 30 min. and then pulverized, classified (- 200 mesh) and press-formed. The samples were heat treated at $400{sim}900^{circ}C$ for 30 min. in order to study the expansion characteristics depending on heat treatment temperature. A porous body with closed pore was formed above $600^{circ}C$. The volume expansion ratio and the pore size were increased and the specific gravity was decreased with increasing heat treatment temperature. However, the volume expansion ratio was decreased and the specific gravity was increased above $850^{circ}C$ due to the softening of the sodium silicate.