PURPOSE: To effectively utilize coal ash and dust generated in the production of a lightweight aggregate by adding cement, dust generated in the production of a lightweight aggregate, natural rock powder, etc., to coal ash, granulating the mixture while sprinkling water, aging and then drying the granulated material and using the product as the roadbed material, etc.

CONSTITUTION: One or ≥2 kinds among cement, the dust collected in the production of an artificial lightweight aggregate and perlite, the dust collected in a quarry, the dust collected in a waste building material treating plant and the natural rock powder are added to coal ash, and the mixture is granulated while sprinkling water. The granulated material is aged by wet air or steam and then dried to obtain the water retaining material, oil absorbent, roadbed material, aggregate of a concrete product and heat-insulating aggregate. Namely, a product similar to pumice as the natural lightweight aggregate is obtained from the coal ash as the industrial waste which is produced from a coal-fired power plant in large quantities and has mostly been discarded as the reclamation material and from the dust collected in the production of the lightweight aggregate and natural aggregate, and the coal ash is effectively utilized as the oil absorbent, aggregate, water retaining material, roadbed sand, etc.