COMPOSITION AND PROPERTIES OF NATURAL CEMENTS. 265 



From this table it will be seen that the maximum retardation of 

 the initial set took place with both brands when 2 per cent of plaster 

 was used. The final set, however, experienced its greatest retardation 

 in both cases when 3 per cent of plaster was employed. 



PERCENTAGE OF PLASTER PARIS 



FIG. 50.* Effect of blaster on setting time. (Sabin.) 



Sabin also tested t the effects of plaster on the tensile strength 

 of both neat and mortar briquettes. The results of these tests are 

 shown in the following table (129) : 



TABLE 129. 

 EFFECT OF PLASTER ON TENSILE STRENGTH OF NATURAL CEMENT. (SABIN.) 



1 Surf ace cracks. 2 Swelled and nearly disintegrated. 3 Badly cracked and swelled. 



* From Johnson's "Materials of Construction ", p. 187. 

 t Report Chief of Engineers, U. S. A., 1896, p. 2857. 



