MASONRY 



311 



because of their greater density. Mixtures of sand and screen- 

 ings often produce stronger mortar than either material alone. 

 With the same aggregate, the strongest and most impermeable 

 mortar is that containing the largest percentage of cement in a 

 given volume of the mortar. With the same percentage of 

 cement in a given volume of mortar, the strongest, and usually 

 the most impermeable, mortar is that which has the greatest 

 density, that is, which in a unit volume has the largest percen- 

 tage of solid materials. 



In the accompanying table is given a fair average of the 

 tensile strength that may be expected from mortars of Portland 

 and natural cements that are made in the field and with a sand 

 of fair quality but not especially prepared. 



The strength of Portland-cement mortar increases up to 

 about 3 mo.; after that period, it remains practically con- 

 stant for an indefinite time. Natural-cement mortar, on the 



TENSILE STRENGTH OF CEMENT MORTARS 



other hand, continues to increase in strength for 2 or 3 yr., its 

 greatest strength being about 25% in excess of that attained 

 in 3 mo. The strength of slag-cement mortar averages about 

 three-quarters of that of Portland-cement mortar. 



