202 SAND AND CEMENT 



The theory of the composition of a correctly proportioned 

 mortar is that the cement paste will just a little more than 

 fill all the voids between the particles of sand, thus giving 

 an absolutely dense mortar at the least expense. The cor- 

 rect proportion of cement to sand, therefore, is more or less 

 variable, depending on the granulometric composition of the 

 sand. Since, however, Portland-cement paste that has set 

 weighs nearly as much as sand, and since the average sand 

 contains about 30 to 40% of voids, it is evident that 1-3 

 mixtures most nearly approach the best and most economical 

 proportion. 



Mortars, however, are made in proportions varying from 

 1-1 to 1-8. The richer mixtures are used for facing, point- 

 ing, waterproofing, granolithic mixtures, etc., the 1-2 mix- 

 ture being usually made for such purposes. The leaner 

 mixtures are used for rough work, filling, backing, etc., 

 but should never be employed where either much strength 

 or much density is desired. Natural-cement mortars are 

 commonly made 1 part of sand less than Portland-cement 

 mortars intended for the same purpose; that is, where a 

 1-3 Portland-cement mortar would be used, a 1-2 natural 

 mortar would be required, although natural-cement mortars 

 should be decreased by about 2 parts of sand to equal the 

 strength of Portland. In other words, a 1-4 Portland 

 mortar closely equals the strength of a 1-2 natural mortar. 

 Puzzolan cements are usually proportioned the same as 

 Portlands. 



Cements are commonly proportioned by volume, the unit 

 volume of the cement barrel being assumed. If a 1-3 mortar 

 is desired, a box having a capacity of 10.8 cu. ft. is filled 

 with sand and mixed with 4 bags or 1 bbl. of cement. A 

 box 3 ft. 3^ in. square and 1 ft. deep will have a capacity 

 of very nearly 10.8 cu. ft. and, besides, makes a convenient 

 size of box for actual work. 



For general purposes, the mortar should be of a plastic 

 consistency firm enough to stand at a considerable angle 

 yet soft enough to work easily. Wet mortars are easiest to 

 work and are the strongest. However, they are subject to 

 greater shrinkage, are slower setting, and are more easily 



