How POP works?

Plaster of Paris is calcium sulphate hemihydrate. The formula is given as, CaSO4 . 1/2H2O or (2 CaSO4) . H2O. The powder is called plaster of Paris, because the gypsum which was used to get the powder was mainly found in Paris. POP is prepared by heating gypsum at 373K in rotary kilns, where it gets partially dehydrated.


If the temperature is not maintained carefully, further dehydration will take place at higher temperature and setting property of the plaster will be partially reduced.


How POP works?

Plaster of Paris is a white powder. When it is mixed with water (1/3rd of its mass), gypsum is obtained back. It initially forms a plastic mass with the evolution of heat and then sets to a hard solid mass within 5 to 15 minutes. Setting of plaster of Paris is accompanied by a slight expansion (about 1%) in volume which makes it suitable for making casts for statues, toys, etc. The setting of plaster of Paris can be catalysed by adding sodium chloride to it.


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