MATERIALS. 127 



cement should be ground, and on this point opinions differ 

 considerably, Apart from pecuniary considerations, however, 

 there can be little doubt that the finer it is ground the better. 



Cement "core" 1 has sometimes been collected and mixed 

 with water without its exhibiting any setting qualities or any 

 inclination to slake. It has, therefore, been erroneously con- 

 sidered inert, and looked upon as so much sand. 



Hard-burnt clinkers of lime, when treated singly, are often 

 very difficult to slake ; but when associated in a heap with less 

 refractory lumps, and there brought under the influence of the 

 general heat and moisture of the mass, they ultimately yield. 

 So it is with the core in cement, or at least with a large per- 

 centage of it, which therefore tends to disintegrate work in 

 which it may have been used. The finer particles of cement 

 absorb water and set more quickly than the coarser ones ; but 

 the latter continue to absorb water, though sometimes very 

 slowly, and in doing so they expand and break up the mass in 

 exactly the same way that small particles of unslaked lime 

 would do ; in fact, it is the free or caustic lime in the cement 

 which causes this expansion to take place. The core ought not, 

 therefore, to be disregarded, but, on the contrary, it should be 

 looked upon as an element of danger. 



By thoroughly air-slaking cement before using it, until no 

 tendency to expand remains, the dangerous properties of the 

 core will to a great extent be removed, and the cement generally 

 will be rendered safer for use. 



In the process of air-slaking cement increases considerably in 

 bulk, which indicates how necessary it is that it should be sub- 

 jected to this treatment ; it also loses weight, becomes slower in 

 setting, and, in short-time tests of strength, it gives lower results 

 than when it was fresh. In the course of a few months, however, 

 the strength of briquettes made with slaked cement will often 

 be found to have overtaken that of briquettes made with the 

 cement when fresh. 



I make it a practice not to use Portland cement until a 

 briquette of the same, seven days old, will stand being boiled in 

 water for six consecutive hours without turning soft or showing 

 indications of swelling. A good cement air-slaked until it will 

 stand this test may be used with confidence, because it can never 

 swell in the work; and if concrete made with it be properly 



1 The coarse particles of cement which will not pass through the testing-sieve. 



