ASTM standards govern the manufacture of gypsum board and set strict criteria for strength, water resistance, and other characteristics. To produce gypsum board, crushed natural gypsum or synthetic gypsum is is heated or calcined to dehydrate the feedstock. Next the calcined gypsum is mixed with water and additives to form a slurry which is fed between continuous layers of recycled paper on a long board machine.
As the board moves down a conveyer line, the calcium sulfate recrystallizes or rehydrates, reverting to its original rock state. The paper becomes chemically and mechanically bonded to the core. The board is then cut to length and conveyed through dryers to remove any free moisture.