Abstract: A novel method for permanently disposing of liquid or semi-liquid wastes containing soluble toxic materials to prevent the toxic materials from being leached into the surrounding environment is disclosed. The process comprises admixing the liquid or semi-liquid waste, such as aqueous sludge from the manufacture of phosphoric acid containing soluble and insoluble arsenic, sulfur and like toxic compounds, with a solidifying agent consisting essentially of a hydraulic cement in amounts sufficient to provide a fluid mass that will set to a contiguous rock-like solid upon standing, and then allowing the admixture to set to a contiguous rock-like solid mass which is insoluble in water. By the inventive process, the soluble and insoluble toxic materials of the waste are wholly entrapped in the contiguous rock-like solid mass which thereby prevents them from being leached into the surrounding environment when exposed to ambient moisture.