Building Materials, Third Edition

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wall plaster stucco, and hard finish plaster are extensively used in wall construction. Flooring
plaster, made by calcining gypsum at a high temperature has been considerably used. In all of
these powders, gypsum in a more or less dehydrated state is the essential element. Gypsum-
based items should be used only in dry state and in premises of not more than 60 per cent
relative air humidity.

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The water of crystallization in the gypsum (CaSO 4
2H 2 O) is not held firmly by the mineral.
Therefore, when it is heated to about 160°C it loses a part of water of crystallization and is
known as half-hydrate gypsum.

160°C

4 2422

Gypsum Plaster of paris

1

2 CaSO 2H O 2CaSo H O + 3HO

2



At still higher temperatures (About 200°C), gypsum loses all its water of crystallization and
turns out into an hydrate gypsum.

200°C

CaSO 4 2H O CaSO 2  4

The lost water of crystallization can be regained under favourable damp or moist conditions.

Hydration

42 2 42

2CaSO^1 H O + 2H O 2CaSO 2H O

2

 

CaSO 4 + 2H 2 O  CaSO 4 
 2H 2 O

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The setting and strengthening of gypsum are due to intergrowth of very fine and poorly
soluble crystals of dehydrated gypsum as they precipitate from a solution which remains
oversaturated as long as the hydration of gypsum proceeds. Following are the two theories of
setting of gypsum.
According to the crystallization theory proposed by Le-chatelier when water is added to
gypsum, the latter dissolves forming a saturated solution of dehydrate gypsum. Since the
solubility of semihydrate gypsum is about 3.5 times more than of dehydrated gypsum, the
soloution that is saturated with respect to the semihydrate gypsum causes dehydrated gypsum
to crystallize. In this process the concentration of semihydrate gypsum is reduced causing
more of it to dissolve until again the solution is oversaturated and consequently again yielding
crystals of dehydrate gypsum. The process continues until all the semihydrate gypsum is
hydrated and crystallized.
According to colloidal theory when water is added to gypsum, the semihydrate gypsum
goes into solution until the latter is saturated. In an oversaturated solution, the interaction of
water with the solid semihydrate continues on their surface due to high mutual chemical
affinity. The resultant dehydrated gypsum fails to dissolve further and precipitates as an
unstable disperse colloid mass in the form of gel, the process being accompanied by the setting
of the mass. The resultant crystals grow both in number and size, while orienting randomly
and interwining, convert the jelly like mass into a crystalline growth. The resultant CaSO 4
2H 2 O
crystals grow into a single crystalline concretion which on drying becomes very strong.