266 



CEMENTS, LIMES, AND PLASTERS. 



These tests would appear to show that the addition of even 1 per 

 cent of plaster has injurious effects on the soundness of the cement, 

 and less markedly on its tensile strength. Unfortunately, the analysis 

 of the cement tested is not given, and even its name is suppressed, so 

 that the results are less instructive than they might have been. 



4000 



7 14 21 28 35 42 49 50 02 70 77 84 



FIG. 51.* Effect of salt on compressive strength. (Tetmajer.) 



14 21 28 35 42 49 50 62 70 77 84 



FIG. 52.* Effect of salt on tensile strength. (Tetmajer.) 



In general it may be said that the effects of gypsum or plaster will 

 be directly proportional to the percentage of alumina contained in 

 the cement. This statement has never before been explicitly made, 



* From Johnson's "Materials of Construction", p. 621. 



