32 CEMENTS, LIMES, AND PLASTERS. 



burned gypsum products will be considered in Chapter IV, under the 

 head of Flooring and Hard-finish Plasters. 



Recurring to plasters burned at temperatures lower than 400 F., 

 it may be said that if the gypsum is pure, the resulting plaster will 

 harden or set very rapidly when mixed with water, reabsorbing sufficient 

 water to regain its original composition of CaSO4 + 2H 2 0. Such quick- 

 setting pure plasters are conveniently grouped as plaster of Paris. If, 

 however, the crude gypsum carried a large percentage of impurities, 

 or if certain materials are added to the plaster after burning, the product 

 will set much more slowly. Such slow-setting plasters are of value in 

 structural work, and are marketed under the somewhat misleading 

 name of "cement plasters". The term is unfortunate, because such 

 "cement plasters" are in no way related to the much better known 

 "hydraulic cements" discussed later in this volume. 



Using the properties above noted as a basis for classification, the 

 group of plasters may be subdivided as follows: 



CLASSIFICATION OF PLASTERS. 



A. Produced by the incomplete dehydration of gypsum, the calcination being 



carried on at a temperature not exceeding 400 F. 



1. Produced by the calcination of a pure gypsum, no foreign materials 



being added either during or after calcination . . PLASTER OF PARIS. 



2. Produced by the calcination of a gypsum containing certain natural 



impurities, or by the addition to a calcined pure gypsum of cer- 

 tain materials which serve to retard the set of the product. 



CEMENT PLASTER. 



B. Produced by the complete dehydration of gypsum, the calcination being 



carried on at temperatures exceeding 400 F. 



3. Produced by the calcination of a pure gypsum. . . FLOORING-PLASTER. 



4. Produced by the calcination, at a red heat or over, of gypsum to 



which certain substances (usually alum or borax) have been 

 added HARD-FINISH PLASTER. 



Commercial classification of plasters. In the trade the names above 

 suggested are used quite extensively, but at times in a careless and 

 indefinite fashion. 



Calcined plaster commonly means a burned plaster to which no 

 retarder has been added. If the gypsum from which it was made was 

 pure, the resulting calcined plaster will be a plaster of Paris, as defined 

 above. If the gypsum used was impure, however, the resulting calcined 

 plaster would be a cement plaster, as defined above. 



Stucco is almost a synonym for plaster of Paris, as it contains no 

 retarder and is made from fairly pure gypsum: but the product handled 



