xii. a, 3 Witt and Reyes: Calcium Sulphate on Cement 135 



(up to 2.5 per cent) in some cases increased the strength of 

 neat briquettes, it caused considerable expansion. In attempt- 

 ing to compare some of the papers on the subject, considerable 

 confusion arises from the terminology of various writers. 

 Some of them refer to the percentage of gypsum added to cement 

 and others to the percentage of plaster of Paris ; as a rule, they 

 do not state the percentage of sulphuric anhydride. Others 

 mention simply calcium sulphate, and the reader has no means 

 of knowing in which form it was added. Although the effects 

 of the two substances are similar, 12 there is a considerable dif- 

 ference in the amount of sulphuric anhydride in the commercial 

 products. Different cements require varying amounts of calcium 

 sulphate for the purpose of controlling the set. However, there 

 must be some limit specified for the addition of material sub- 

 sequent to calcination to avoid adulteration by unscrupulous 

 manufacturers. The specifications state the maximum amount 

 that can be allowed with safety. It remains to investigate 

 the region between this amount and that definitely known to 

 be injurious. It seems likely that the specifications of the 

 countries that make allowance for the conditions to which con- 

 crete is to be exposed show an advance in the right direction 

 and that in addition to this there should be some relation be- 

 tween the composition of the cement and the sulphuric an- 

 hydride allowed. Until the subject is better known, safety 

 demands that upper limits should be kept well below the amounts 

 known to be harmful. 



The present work was undertaken to determine the effect of 

 various amounts of calcium sulphate on several cements on the 

 local market and to study in some detail the behavior of a 

 cement made from raw materials available to the Bureau of 

 Science for investigation. The finished cements investigated are 

 from five different factories and are herein designated as A, B, 

 C, D, and E. Each contained a certain amount of calcium 

 sulphate placed there by the manufacturer. Various amounts 

 of plaster of Paris were added to each, giving increasing amounts 

 of sulphuric anhydride up to about 10 per cent. Investigation 

 of the other cement, designated by F, started with the clinker, 

 which contained only a trace of sulphuric anhydride. The set- 

 ting time on each sample was determined as a preliminary test. 

 These were used as a guide to determine the percentages of 

 sulphuric anhydride best suited to extended tests. Tables I and 



"Meade, op. cit., 307. 



