To provide a method for solidifying coal ash which assures a specific amt. or more of coal ash which is an industrial waste and a waste water generated at the time of washing transporting cars, mixers or the like in a ready mixed concrete factory and effectively utilizes such coal ash.

The method for solidifying the coal ash, cement, aggregate or the like to be subjected to waste disposal without using consists in kneading the coal ash having a latent hydraulic property and kneading water contg. a large amt. of calcium, packing the mixture into forms or the like and curing the mixture. The calcium is further added 20 to 30% to the kneading water according to the combination of the coal ash and the kneading water.