Difference Between Gypsum and Plaster of Paris

Gypsum vs Plaster of Paris

Gypsum is a naturally occurring mineral. Plaster of Paris and gypsum both contains calcium sulfate’s hydrate form, but their water content in a molecule differs from each other.


Gypsum is a hydrated calcium sulfate mineral with the molecular formula CaSO4·2H2O. This is the most common sulfate mineral.  It is a rock forming mineral, which can grow up to very large sizes. Usually, the color of the crystal is white or colorless, but can have other shades of colors as grey, red or yellow too. Crystals, also can be transparent or translucent. Gypsum is a soft crystal, which can even be scratched by a fingernail. Further, it is flexible, and its thermal conductivity is low. Gypsum is slightly soluble in water, and when it is heated, water will evaporate and can achieve the anhydride solid state again. Gypsum can be found in many places all over the world (in UK, Russia, Canada, Africa, Asia, USA and Europe). However, Gypsum is abundant in Colorado and Mexico in USA.

Gypsum is mainly formed from precipitation of marine water. While forming the minerals, water or unwanted material can trap inside the crystal which is the cause of different colored crystals. There are three types of gypsum known as selenite, alabaster, and satin spar. Selenite is crystalline in nature and appears transparent or translucent. Alabaster grows into massive mineral beds. It has a light color or lightly tinted color (due to impurities). Satin spar is fibrous or silky in nature. This is widely used for making plaster of Paris, some cement, fertilizer (ammonium sulfate fertilizer) and as an ornamental stone. Gypsum is also used as manure, and it is a good source of sulfur. Gypsum has the capability of becoming plastic like when it is heated upto 175 oC. This nature of Gypsum is important in producing plaster of Paris. If the content of CaSO4·2H2O in gypsum is high, it is very effective in producing fertilizer, plaster of Paris and cement. Therefore, there is a high demand for pure gypsum, which has at least 80% CaSO4·2H2O content.

Plaster of Paris

Plaster of Paris is made up from gypsum. This has been used from the ancient times. Plaster of Paris got its name because earlier it was used near Paris, to make plaster and cement. They also used it to do ornamental work on ceilings and cornices. Plaster of Paris contains the calcium sulfate hemihydrates (CaSO4·0.5H2O). This is prepared by heating the gypsum which contains calcium sulfate dihydrate (CaSO4·2H2O) to a temperature about 150 oC (120-180 oC). Certain additives are added when heating. Plaster of Paris is a fine, white powder. When it is hydrated it can be used to mould things, and when allowed to dry, it hardens and retains whatever the shape it is set before drying.


What is the difference between Gypsum and Plaster of Paris?

• Plaster of Paris is made from Gypsum.

• Gypsum contains calcium sulfate dihydrate (CaSO4·2H2O) and plaster of Paris contains calcium sulfate hemihydrates (CaSO4·0.5H2O).

• When added water to plaster of Paris, it will re-form into gypsum.

• Plaster of Paris can be mould into different shapes when it is moistened, but gypsum does not have that property. 

• Gypsum is a naturally occurring mineral whereas Plaster of Paris is manufactured. 


  • Jan Parker

    Yet, plaster of Paris can be used as fertilizer on crops.

  • R aj Kumar

    why it is needed to have 0.5 water content in plaster of paris