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Plaster of Paris is formulated as hemihydrate of calcium sulphate 2CaSO4.H2O (or CaSO4.0.5H2O) and is believed to consist of a series of hydrates ranging from 3CaSO4.2H2O to anhydrous CaSO4. The commercially available plaster of Paris is a mixture of calcium, gypsum and sulphate . POP forms a quick bond, that’s why it is widely used in making decorative items for houses and for other purposes too.


It can be prepared from gypsum by following unit operations-

(1)Crushing and grinding of gypsum-Gypsum is basically a soft sulfate mineral composed of calcium sulphate dihydrate, which has a unique chemical formula represented as CaSO4•2H2O. It is usually used as a fertilizer, and also as one of the main constituent in many forms of plaster, blackboard. A massive fine-grained white or a lightly tinted variety of gypsum, called alabaster, has been used for a sculpture by many cultures including Ancient Egypt, Mesopotamia, and Ancient Rome. Mohars scale of mineral hardness is usually based on scratch hardness comparison, defines hardness value as gypsum. It forms as an evaporite mineral and as a hydration product of anhydrite.

(2)Calcination of ground gypsum in kilns by heating to about 155℃-Calcination reactions usually takes place mostly at or above the thermal decomposition temperature or the transition temperature. This temperature is usually defined as the temperature at which the standard Gibbs free energy for a particular calcinations reaction is equal to zero. For example, in limestone calcinations, as a decomposition process, the chemical reaction is basically represented as –

CaCO3 → CaO + CO2.

The standard Gibbs free energy of reaction can be estimated as ΔG°r = 177,200  − 160T. The standard free energy of reaction is zero , in this case as  the temperature is equal to 1122 K, or 848 °C(approx). Calcination equilibrium of calcium carbonateCalcination can be used in carbon negative electricity generation.

(3)Pulverizing the calcined product-The pulverized gypsum contains many components like calcium, Sulphur, Moisture and many more in the following ratio respectively.

Calcium (Ca): 20%,Sulphur (S): 13%,Moisture: 5%,Calcium Sulphate Dihydrate: 85%,etc.

The PH of gypsum is 7.25, and also the density per cubic foot is 35.The required information is given below-

  • pH: 7.25
  • Density per cubic foot: 35 (15.86 kg)

Particle size varies from 1/4″ to dust

Pulverized gypsum has following uses–Agricultural Crops,Lawns & Gardens,Compost Additive,Animal Bedding Additive,Poultry Litter Amendment,Soil Stabilization & Remediation,As a Bulking, Solidification or Stabilization Agent,Water Clarifier,Industrial Products,and many more .

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