Gypsum found in cement is a hydrated calcium sulfate. The chemical name of gypsum, calcium sulfate, contains calcium, sulfur and oxygen to produce the molecular formula CaSO4. Hydrated calcium sulfate is combined with two water molecules (H2O), which is also referred to as "dihydrated." When gypsum is "hemihydrated" the mixture contains half of one H2 molecule, and this is the chemical composition of Plaster of Paris.
Plaster of Paris, also known as cement plaster, is similar to cement and mortar but remains soft after it sets because it is entirely made from gypsum. To produce Plaster of Paris, gypsum is heated to 150 degrees Celsius (300 degrees Fahrenheit) to release water vapor (steam). The resulting powder is rehydrated with water to form gypsum, sort of like a gypsum concentrate. This use of gypsum is common in the medical industry to form casts in support of a broken bone; cementing the bandages to protect the area while the body heals. This cement plaster is also used in dentistry to produce models of teeth, such as during orthodontic treatment.
Gypsum is a key component to cement. When making cement, clinker is first created from a powder of limestone, alumino-silicate and other raw materials that are heated to a point just below melting, so they remain solid. When hot, the materials begin to bond together forming lumps between 3 mm and 5 mm in diameter. Once the clinker has cooled, gypsum is added to the mixture up to 5 percent, but often less than 5 percent. The entire mixture containing the clinker and the gypsum is further ground to produce a very fine powder, which is known as Portland cement or standard cement.
Gypsum serves a primary role in the setting qualities of cements. By adding gypsum to the clinker, the cement will set (become solid) at a slower rate, thus allowing construction workers to smooth the surface, fill a mold, or trim or shape the cement mixture for different purposes. Without gypsum, cement would set instantly, similar to the portrayal of cement in classic cartoons where it sets immediately. This would create problems for cement trucks and mixers, as well as create problems for anyone working with cement, possibly damaging machinery or injuring people. The addition of gypsum slows the drying time by hours, with the drying time longer when larger quantities of gypsum are added.