What is the difference between gypsum and plaster of Paris?
Gypsum is the natural mineral calcium sulphate dihydrate, CaSO₄·2H₂O. Plaster of Paris (POP) is made by heating gypsum to drive off most of its water, producing calcium sulphate hemihydrate, CaSO₄·½H₂O. When POP is mixed with water, it takes the water back, recrystallises into gypsum and sets hard.
How is plaster of Paris made?
Crushed gypsum is calcined, typically at about 120–180 °C. The chemical reaction is:
Temperature control is critical. Under-calcined material keeps raw gypsum, which accelerates setting unpredictably. Over-calcined material forms anhydrite, which sets slowly or not at all. Good POP has a narrow, consistent phase composition.
What are alpha and beta plaster?
| Type | How it is made | Water demand | Strength | Typical uses |
|---|---|---|---|---|
| Beta hemihydrate | Calcined dry at atmospheric pressure | High | Standard | Building plaster, plasterboard, moulds, ceilings |
| Alpha hemihydrate | Calcined under steam pressure or in solution | Low | High | Dental and medical models, precision moulds, self-levelling floors |
Which specifications matter for plaster of Paris?
Where are gypsum and POP used?
| Material | Main uses |
|---|---|
| Raw gypsum | Cement retarder (added to clinker), agriculture soil conditioner, fillers |
| Beta POP | Wall plaster, plasterboard, ceiling tiles, cornices and decorative mouldings |
| Alpha POP | Ceramic and sanitaryware moulds, dental, orthopaedic, precision casting |
| Gypsum fillers | Paint, paper and plastics fillers where a sulphate filler is acceptable |
In cement production, a few percent of raw gypsum is ground with clinker to control the setting of the cement. This is one of the largest single uses of gypsum worldwide.
How should plaster of Paris be stored?
POP absorbs moisture from the air. Partly hydrated plaster sets faster, loses strength and forms lumps. Store bags off the floor, in a dry, covered area, and use stock in rotation. Most producers state a shelf life of about 6–12 months in sealed bags.