SAND AND CEMENT 201 



nor the property of resisting water. It cannot be used 

 in damp or wet situations, nor should it ever be laid in cold 

 weather, as it is very susceptible to the action of frost, being 

 much injured thereby. Moreover, since it hardens by the 

 action of dry air, only the exterior of lime mortar ever 

 becomes fully hardened, so that anything like a concrete 

 with lime as a matrix is impossible. However, for second- 

 class brickwork, such as is commonly used in the walls of 

 smaller buildings, lime mortars are economical and suf- 

 ficiently good. 



The strength of lime mortars is extremely variable, depend- 

 ing on the ingredients themselves and on their treatment, 

 environment, etc. Moreover, it is unsafe to figure a lime- 

 mortar joint as possessing much strength, since only a part 

 of the joint is hardened and capable of developing any 

 strength at all. The tensile strength of thoroughly hard- 

 ened 1-3 lime mortars averages from 40 to 70 Ib. per sq. in., 

 and the compressive strength from 150 to 300 Ib. 



Cement Mortars. The sand for all mortars should be 

 clean, of suitable size and granulometric composition. For 

 structures designed to withstand heavy unit stresses, or for 

 those intended to resist either the penetration of moisture 

 or the actual pressure of water, the selection of the sand 

 should be most carefully made. Generally, it is not advi- 

 sable to use a sand containing over 5% of loam by the wash- 

 ing test, nor one that soils the fingers when it is rubbed 

 between them. Very fine sand, such as is found on the sea- 

 shore, should not be employed in mortar unless it is intended 

 simply for pointing or for grouting. 



A simple method of determining the best sand for cement 

 mortar is to prepare mixtures of the cement, sand, and 

 water, using the same quantities in each case, and ftien to 

 place each mixture in a measure; that mixture giving the 

 least volume of mortar may be considered to contain the 

 most desirable sand for use. 



Limestone screenings, brick dust, crushed cinders, etc., 

 are sometimes substituted for sand in making mortars, and, 

 if care is taken in their selection, they may prove economical 

 and entirely suitable for certain purposes. 



