Disclosed herein is a method for producing a recycled hardened material using waste gypsum. The method comprises treating acidic wastewater generated in the production of copper, zinc or titanium or the production of sodium hydroxide (caustic soda) and chlorine using saline water, with calcium hydroxide (slaked lime), so as to precipitate sludge, and mixing the precipitated sludge with pulp sludge ashes having pozzolanic properties.