Cement Production Process. CLINKER GRINDING AND CEMENT PACKING. Roller press with ball mill is used to grind the clinker. 3.5 to 4% Gypsum is added to make OPC. To make PPC, up to 35 % fly ash is added to the clinker in addition to Gypsum. Three grades of cement (OPC 43, 53 and PPC) are manufactured and stored in silos.