Gypsum is commonly used to slow down the setting of cement. It is added during the manufacturing process of cement to control the setting time and prevent rapid hardening. Gypsum reacts with the tricalcium aluminate compound in cement, forming a gelatinous material that slows down the hydration process and extends the setting time. This allows the cement to be workable for a longer period, making it easier to handle and manipulate during construction or other applications.