A method for treating aqueous liquid and semi-liquid wastes by solidification comprising the steps of mixing with the waste a dry water-reactive solidification agent comprising cement, a dry water absorbent material, and a powdered alkali metal silicate in a quantity sufficient to convert the mixture into a chemically and physically stable solid end product substantially insoluble in water and which contains no substantially free-standing water, and allowing the mixture to set to a sedentary mass. Additive agents such as surfactants, fixatives, waterproofing agents, coloring agents, and the like are also disclosed as assisting in the solidification reaction by reacting with certain constituents in the waste.