PURPOSE: To solidify waste material such as slag, inorg. sludge, etc. economically advantageously by constituting a material to be solidified of a composition consisting primarily of fly ash, and a small amt. of hydraulic material.

CONSTITUTION: A small amt. of cement, gypsum or slaked lime, or gypsum or cement alone, is mixed with fly ash. Water is added thereto and the mixture is kneaded to botain a blended material and harmful waste material such as slag or inorg. sludge is solidified with said blended material. By this process, a solidified product causing elution within a range not exceeding a standard limit for elution is obtd. inexpensively.