A method and apparatus for producing SO.sub.2 -containing gas and cement clinker by splitting waste gypsum and anhydrite, using paper fibers as the reducing agent, and firing the splitting residue with additives. The cement forming additives are formed into a mixture with the anhydrite and the mixture is compacted before the final drying and calcining, whereupon the anhydrite containing mixture is subjected to the splitting reaction and firing to form the clinker.