PURPOSE: To improve heat insulating properties by blending flotation slag with gypsum and slaked lime, then adding water into a slurry state, pouring to a mold and hardening by hydration action.

CONSTITUTION: Flotation slag abandoned from a mine is burnt at about 500-900°C. The slag is blended with a given amount of gypsum such as hemihydrate or III type anhydrous gypsum and a given amount of slaked lime. Then the blend is mixed with a proper amount of water into a slurry state, poured into a mold of given shape, hardened by hydration action and released from the mold to take out a precursor. The precursor is dried at air temperature, cured with steam to give a porous material having excellent water resistance and heat insulating properties.