xii. a, 3 Witt and Reyes: Calcium Sulphate on Cement 137 



Table II. — Physical tests of cements as received — Continued. 



Brand." 



Ten9ile strength— pounds per square inch. 





Neat cement. 



Mortar 1 : 3. 



1 day. 



7 days. 



28 days. 



60 days. 



1 day. 



7 days. 



28 days. 



60 days. 





353 

 337 

 330 

 348 

 388 



695 

 617 

 586 

 633 

 628 



685 

 651 

 679 

 722 

 731 



673 

 649 

 644 

 677 

 720 



100 

 100 

 133 

 124 

 154 



238 

 248 

 282 

 294 

 331 



333 

 308 

 380 

 409 

 407 



360 

 825 

 428 

 441 

 414 



B 



C... 



D 



E 





* Brand F, without any addition of calcium sulphate, was quick-setting ; consequently no 

 test was made on the original material. 



The plaster of Paris contained 55.88 per cent sulphuric an- 

 hydride. Each cement was analyzed, and the approximate 

 amount of plaster necessary to give a certain percentage of 

 sulphuric anhydride was calculated. The two substances were 

 then placed in a ball mill and thoroughly mixed. In the case 

 of sample F the clinker was crushed to pass a 20-mesh sieve, 

 mixed with the plaster, and ground to the fineness indicated in. 

 Table III. This table shows the effect of sulphuric anhydride 

 content on the setting time and on the tensile strength for va- 

 rious periods up to ninety days. 



The results in Table III are not entirely uniform; however, 

 as the results were obtained from more than 1,000 briquettes, 

 general conclusions can be based on averages as follows : 



Setting time. — In conformity with the results, already men- 

 tioned, of other investigations with calcium sulphate, most of 

 the cements show a maximum retardation with 2 per cent or 

 less of sulphuric anhydride and a shorter setting time with a 

 higher or lower percentage. 13 In general, the initial-set curve 

 obtained by plotting the percentage of sulphuric anhydride 

 against the time is parallel to the final-set curve for the same 

 cement. 



" Calcium chloride, sodium sulphide, and several other electrolytes have 

 somewhat the same effect. [See Carpenter, R. C, Eng. Rec. (1904), 50, 

 769. Witt, J. C, This Journal, Sec. A (1916), 11, 273.] 



