What is the Difference Between Gypsum and Plaster of Paris?

Gypsum and Plaster of Paris are both calcium sulfate-based materials, but they have different properties and uses. The main differences between them are:

1. Chemical Composition: Gypsum is a naturally occurring soft sulfate mineral with the chemical formula CaSO₄.2H₂O, containing two moles of crystallization water. Plaster of Paris, on the other hand, has the chemical formula CaSO₄.1/2H₂O and contains calcium sulfate hemihydrate.
2. Mode of Production: Gypsum is a naturally occurring mineral, while Plaster of Paris is produced by heating gypsum to a high temperature, which evaporates three-quarters of the water content, turning it into a white powder.
3. Water Content: Gypsum contains 2 moles of crystallization water, whereas Plaster of Paris contains only 1.5 moles of crystallization water.
4. Uses: Gypsum is used as an ingredient in cement and as a soil conditioner. Plaster of Paris is used for protective or decorative purposes, such as in false ceilings, wall surfaces, and as a fireproof agent. It is also used as a covering over sand-cement plaster to provide a smooth surface.
5. Surface Appearance: Gypsum plastering the wall produces an even, smooth, and crack-less surface, while the surface post application of sand-cement plaster looks rough, uneven, and may have cracks.

In summary, gypsum and Plaster of Paris have different chemical compositions, water contents, and uses, making them distinct materials with specific applications.

Comparative Table: Gypsum vs Plaster of Paris

Here is a table comparing the differences between gypsum and Plaster of Paris:

Gypsum is a naturally occurring soft sulfate mineral, while Plaster of Paris is a building material produced by heating gypsum to a high temperature. Both materials contain calcium sulfate as the major constituent, but their water content in a molecule differs from each other. This difference in water content results in variations in their compressive strength and other properties.