Hard wall
MATERIALS OF CONSTRUCTION
CHAPTER I
PLASTERS AND NATURAL CEMENTS
A. GYPSUM PLASTERS
1. Definition and Classification.—Gypsum plasters may be defined as those plasters which are produced by the partial or complete dehydration of gypsum. Pure gypsum is a mixture of 1 part of calcium sulphate (CaSO4) and 2 parts of water (2H20). Gypsum plasters may be classified as follows:
1. Those produced by the incomplete dehydration of the gypsum, the
calcination being carried on at a temperature less than 400 degrees Fahrenheit, (a) Plaster of Paris (CaSO, + JiH2O), in which no foreign material
has been added either during or after the calcination. (6) Cement plaster, which is made from an impure gypsum or by adding certain impurities, during the manufacture, to act as a retarder to the plaster. This plaster is often called hard wall or patent plaster.
2. Those plasters produced by the complete dehydration of the gypsum,
the calcination being carried on at temperatures greater than 400
degrees Fahrenheit.
(a) Calcined plaster (flooring plaster), a pure calcined gypsum.
(6) Hard-finish plaster, which is made by calcining gypsum at a red
heat or higher temperature and to which certain substances,
such as alum or borax, have been added.
2. Manufacture of Gypsum Plasters.—The process of manufacture is essentially the same for all of the first class of gypsum plasters, variations being made in the temperature of calcination and the purity of the gypsum used. The raw material is a natural gypsum rock usually containing from 1 to 6 per cent of impurities. The rock is crushed, ground to a powder, and then heated in a large calcining kettle. If a rotary calcincr is used, the fine grinding is done after the calcination. In making plaster of Paris, a pure gypsum is calcined at a temperature of about 220 degrees Fahrenheit thus driving off three-fourths of the water present. Cement plaster is made in the same way, an a retarder (a substance to led after the calcination ing lumps of gypsum in a used for the calcination of ion is usually about 850 squired is about 3 hours. g will burn the plaster and
and hardening. After the F ground.
ddnd of hard-finish plaster. very pure gypsum at a red turn solution, recalcining it,
-All gypsum plasters will proper amount of water. plaster and the water to faries from 5 minutes to 2 and the conditions of the Ire gypsum are the quicker £form the harder substances. wncerning the strength of Sions of testing have never ties according to the quality 9, and the care taken in the ve an idea of the strength
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