170 



EEIBLINOt and SALINGER. 



obtained with a mortar containing 10 to 13.5 joer cent of water, and 

 suggest the use of 12.5 per cent.. This is contrary to best practice and 

 results. The correct amount of water for sand, as for neat briquettes, 

 depends upon the nature of tlie cement, and the amount of water necessary 

 to wet the surface of the sand. We find that 12.5 per cent is too much 

 for mortars the neat cement of wliich worked with 20 per cent makes 

 a fairly wet paste; and that 10 per cent for such a cement gives better 

 results. The reason for this is a physical one, as in tamping a very 

 wet mortar into place, much of the cement is unavoidably lost to the 

 briquettes. During the tamping operation the water is forced to both 

 surfaces, and carries with it the finest (most valuable) cement particles. 

 In finishing the briquette, this top surface, especially rich in cement, 

 is struck ofE and the resulting briquette is weakened by the reduction 

 of the 1 to 3 ratio as well as by the loss of a portion of its most valuable 

 constituent. 



We give this explanation as the reason why many briquette machines 

 fail -- and why under certain conditions a slight finger jjressure will 

 make a stronger briquette than powerful mechanical force. The con- 

 siderable pressure exerted on the briquettes by such machines forces 

 the water to the surface and this carries cement with it, while the sand 

 is left in tlie mold. 



As the addition or subtraction of as little as 1 per cent of water may 

 effect the resulting strength of a mortar briquette sufficiently to cause 

 the accei^tanee or rejection of the material, the American Society intro- 

 duces a good feature in cement testing to cover this efl'ect, for in tlieir 

 specifications the amount of water necessary for any mortar is given 

 according to the percentage of water required to reduce the neat cement 

 to the normal consistency paste. This is shown by the following table : 



Table XXIII. — Percentage of water required for standard sand mortars. 



Normal con- 

 sistency, neat. 



1 part cement 

 to 3 parts 



standard Ot- 

 tawa sand. 



Per caiL 



Per cent. 



22 



9.7 



23 



9.8 



24 



10.0 



2.5 



10.2 



26 



10.3 



27 



10.5 



28 



10.7 



29 



10.8 



30 



11.0 



~Eng. News (1902), 48, 130. 



