The conversion of phosphogypsum waste (a waste product of phosphoric acid production in Richard Bay, South Africa), using sodium carbonate was tested. Response surface methodology (RSM) was used to investigate the combined effect of relevant process variables to maximize the production of calcium carbonate in a batch reactor. The process variables include time (0, 60, and 120 min); slurry content (5, 10, and 15%); agitation speed (100, 300, and 500 rpm); and sodium carbonate/gypsum molar ratio (0.8, 1.4, and 2). In the optimum conditions of the process (the slurry concentration of 5%, molar ratio sodium carbonate/gypsum of 2, stirring rate of 500 rpm) the conversion of waste gypsum to calcium carbonate after 105 minutes reaches over 98.5%. The calcium carbonate produced in this work compares favorably to the commercial calcium carbonate (laboratory grade) during Acid Mine Drainage (AMD) neutralisation.