PURPOSE: To obtain a zeolitic composition advantageously in terms of resources and energy from a blast furnace slag, by melting an alkali metallic carbonate in the blast furnace slag in a molten state, solidifying the resultant melt, and subjecting the resultant solidified material to the hydrothermal treatment.

CONSTITUTION: An alkali metallic carbonate at 0.1W0.3 molar ratio based on Ca contained in a blast furnace slag and a component supplementary substance are added to he blast furnace slag in a molten state and molten to give a modified melt, which is then cooled at ≥50°C per 1sec temperature reducing rate to afford a solidified material. The resultant solidified material is then subjected to the hydrothermal treatment with water or an aqueous solution of an alkali. If the amount of the alkali metallic carbonate is ≤0.1 molar ratio, a calcium silicate is mostly formed in the hydrothermal treatment after cooling the modified melt containing the molten additive, and the formation of the zeolite structure is inhibited. On the other hand, at ≥0.3 molar ratio, the modified melt is cooled to form a dense composition structure, and the induction into the zeolite structure by the subsequent hydrothermal treatment is actually impossible.