COMPOSITION AND PROPERTIES OF NATURAL CEMENTS. 267 



but it is a necessary corollary from recent studies on the behavior of 

 cements with gypsum and in sea-water. 



Effect of salt on strength. In laying masonry in freezing weather 

 it has been customary to specify the use of salt in the water used for 

 the mortar. This lowers the freezing temperature of the water, but 

 does not seem to be of any particular benefit in other respects. It 

 decreases quite markedly the tensile and compressive strength of the 

 mortars, even when only a small percentage of salt is added. 



OJT i 4 6 8 10 



PERCENTAGE OF SALT IN WATER 



FIG. 53.* Effect of salt on tensile strength. 



The effect of salt on the strength of natural-cement mortars is shown 

 in Figs. 51, 52, and 53. Of these Tetmajer's experiments were made 

 on European natural-cement mortars, mixed 1 cement to 3 sand and 

 tested at various ages. Those shown in Fig. 52 were made on Louis- 

 ville and Portland-cement mortars and all tested at six months. 



Tensile strength. In tensile strength the average natural cement 

 ranks considerably lower than the average Portland. This is particu- 

 larly noticeable when the cements are tested with sand. 



This general rule as to the relative strength of natural and Port- 

 land cements is well known, but the exceptions to the rule are not fre- 

 quently discussed. It is a fact, however, that certain brands of natu- 

 ral cements are about as strong, either neat or with sand, as the aver- 

 age imported Portland, and there is no reason why a number of natural 

 cements could not be carried up to this grade. The average results 

 of extensive series of tests on various natural cements are given dia- 

 grammatically in Figs. 54, 55, 56, 57, 58, 59, and 60. 



In Fig. 55 are shown the results of a very large number of tensile 



* From Johnson's "Materials of Construction", p. 618. 



