THE EFFECT OF CALCIUM SULPHATE ON CEMENT * 



By J. C. Witt and F. D. Reyes 



{From the Laboratory of General, Inorganic, and Physical Chemistry, 

 Bureau of Science, Manila) 



Calcium sulphate in the form of either gypsum or plaster of 

 Paris is almost universally used to control the setting of cement. 

 The practice has led to extensive research in the attempt to 

 determine the reaction between the two substances and the 

 amount of calcium sulphate that can be added without causing 

 harmful effects. As a result of these investigations some points 

 are fairly well established. A number of writers 2 agree that 

 the retardation of set caused by the addition of calcium sulphate 

 is proportional to the amount added, only within certain limits ; 

 that is, the setting time cannot be increased without limit by 

 adding more and more of this substance. On the contrary, after 

 a certain point is reached, further additions cause an acceleration 

 of the set. 



Many believe that the permissible amount of calcium sulphate 

 added to a cement should depend to some extent on the condi- 

 tions to which the concrete made from it is to be exposed. 

 Several countries, among which are France, 3 Japan, 4 and 

 Argentina, 5 specify that a cement, intended for construction 

 exposed to sea water, shall have a lower sulphuric anhydride 

 (S0 3 ) content than one to be used under ordinary conditions. 



The German Portland Cement Manufacturers' Association ° 

 recommended that "a uniform permissible maximum limit of 

 S0 3 , namely 2.5 per cent, be generally adopted in Specifications 

 for Portland Cement, whatever may be the purpose for which 



1 Received for publication November 13, 1916. 



2 Carpenter, R. C, Eng. Digest (1908), 3, 385. Rohland, P., Stahl u. 

 Eisen (1908), 28, 1815. Reibling, W. C., and Reyes, F. D., This Journal, 

 Sec. A (1911), 6, 225. 



"Ciment (1912), 17, 213. 



*Mitt. Zentralstell. Ford. Deut. Port. Zem. Ind. (1912), 1, 167. 



3 Ibid. (1912), 1, 305. 



6 Report of the International Association for testing materials (1912), 

 Sec. 2, article XVIL, 1. (Translated by G. Salter.) 



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