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Plaster of Paris

Plaster of Paris
Jay Sampathkumar, 2005

  • Plaster of Paris is a derivative of Gypsum
  • Gypsum is a very soft mineral composed of   Calcium sulphate dihydrate
  • Chemical formula for Gypsum CaSO 4 · 2H 2 O.
  • Because the gypsum from the quarries of the Montmartre district of Paris has long furnished burnt gypsum used for various purposes, this material has been called plaster of Paris.

How is Plaster of Paris formed?

Heating gypsum above approximately 150 °C   partially dehydrates the mineral by driving off exactly 75% of the water contained in its chemical structure.

CaSO 4 ·2H 2 O + heat → CaSO 4 · ½H 2 O + 1 ½H 2 O (steam)

The partially dehydrated mineral is called calcium sulfate hemihydrate or commonly known as plaster of Paris (CaSO 4 · ½H 2 O).

·         The dehydration (specifically known as calcination ) begins at approximately 80 °C (176 °F) and the heat energy delivered to the gypsum at this time   tends to go into driving off water (as water vapor) rather than increasing the temperature of the mineral, which rises slowly until the water is gone, then increases more rapidly.

·         This is an endothermic reaction.

·         calcium sulfate hemihydrate has an unusual property: when mixed with water at normal (ambient) temperatures, it quickly reverts chemically to the preferred dihydrate form, while physically "setting" to form a rigid and relatively strong gypsum crystal lattice:

CaSO 4 · ½H 2 O + 1 ½H 2 O → CaSO 4 ·2H 2 O This reaction is exothermic .

·         This phenomenon is responsible for the ease with which gypsum can be cast into various shapes including sheets (for drywall), sticks (for blackboard chalk), and molds (to immobilize broken bones, or for metal casting).

(CaSO 4 , 2 H 2 O) + heat  = (CaSO 4 , ½ H 2 O) + 1.5 H 2 O

Plaster of Paris is a calcium sulfate hemi-hydrate : (CaSO 4 , ½ H 2 O) derived from gypsum, a calcium sulfate dihydrate (CaSO 4 , 2 H 2 O), by firing this mineral at relatively low temperature and then reducing it to powder. Calcination of the gypsum at

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