Why does false set occur?
A. In the manufacture of cement, gypsum is added to control the set. The gypsum is normally added in the final milling process. If in the final milling the temperature of the cement gets too hot, the gypsum will dehydrate forming anhydrate or hemihydrate (Plaster of Paris). When water is added the Plaster of Paris hardens very fast and gives the appearance of a false set.