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Chemical Papers. 



61 



ON THE DURABILITY OF CEMENT PLASTER. 



By E. H. S. Bailey and W. G. Stromquist, University of Kansas. Lawrence. 



TT^ORMERLY the only material used for inside plastering was 



r 



lime mortar, and upon this was spread a skim coat made from 



plaster of Paris. Since the discovery of extensive beds of gypsite 

 or gypsum dirt, in various localities in Texas, Indian Territory, 

 Kansas, and Wyoming, this material has been heated to drive off 

 nearly all of the water, and is put upon the market under the names 

 of Agatite, Acme, Alabastine, etc., as a substitute for lime mortar. 

 As this crude gypsum occurs in beds where it can be handled by a 

 grader, loaded on wagons, and hauled directly to the mill, the pro- 

 duction of this kind of plaster has become an important industry. 

 While the ordinary gypsum rock must be ground before it can be 

 heated in the kettles, this material needs no preliminary treatment. 

 The composition of the raw gypsite, the finished product and 

 the set plaster is as follows : * 



From an examination of the analyses, it will be seen that the 

 crude material loses all but about five per cent, of its water when 

 "boiled," and when water is added to the "plaster," before spread- 

 ing on the wall, it retains practically the same amount of water as 

 it lost when heated; and in fact it again becomes gypsum, or 

 CaS04+2H20. 



For practical work, the mason mixes with the cement plaster 

 from three to five parts of sand, and a little "retarder" is used to 

 prevent the too rapid setting. This material has been used for 

 plastering walls for the past fifteen years, and, as far as observed, 

 it seems to last as well as lime mortar. 



An opportunity to study the disintegration of this plaster under 



* University Geological Survey of Kansas, vol. V, p. 163. . 



