of waterproofing is, at least in a measure, solved. 

 Efflorescence is also practically overcome by making 

 really dense and reasonably impervious blocks." 



Generally speaking, the greater the proportion of 

 Portland cement used, the less will be the porosity 

 of the concrete block. For it is the minute particles 

 of the finely powdered cement that act in filling up 

 the voids between the larger particles in the aggregate. 

 To get a clear idea as to the amount of space left be- 

 tween the individual particles in a heap of gravel 

 whose units approach the spherical in shape, a pile of 

 perfectly spherical cannon-balls of the same size may 

 be considered. In such a pile the spaces left amount 

 to some twenty-six per cent, of the entire mass. If 

 these spaces were fitted with smaller balls just large 

 enough to touch snugly all points of contact without 

 displacing the larger balls, the voids would be reduced 

 to about twenty per cent. Smaller and smaller balls 

 could be added (theoretically, at least) until all the 

 air spaces had been filled to such an extent that 

 the mass would be impermeable to water. To do 

 this the smallest particles would necessarily be of a 

 fineness corresponding to those of an " impalpable" 

 powder. 



In comparing this mass of perfectly spherical balls to 

 the substances composing the mass of concrete, the 

 Portland cement represents the finest particles, and 

 the ones that give the mass its adhesive quality; the 

 intermediate- sized balls are represented by the sand; 

 and the largest balls by the particles of gravel or crushed 

 stone. The comparison holds only in the matter of 



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