THE WOMBAT. 



9 1 



2,500 holes to the square inch, and that not more than 10% 

 will remain on passing it through a sieve having 3,600 holes 

 to the square inch. As a rule manufacturers grind to a far 

 greater degree of fineness than this standard. The fineness 

 of grinding largely determines its usefulness as a constructive 

 materiaJ, for it is well known that the finer a cement is 

 ground, the belter will be the result obtained when mixed 

 with sand. There is a growing demand for finer cement, but 

 it must be remembered that finer grinding means extra wear and 

 tear on the machinery, and consequently an increased cost. 



The chemical test is often of corroborative value in 

 estimating the properties of a sample ; at the same time it is 

 a test which is of more use to the manufacturer than to the 

 consumer. The chief constituents of a cement are about — 



Lime, 60 parts. 

 Silica, 24 parts. 

 Alumina and iron, 12 parts. 



The remaining four parts are composed of alkalies, magnesia, 

 and carbonic acid. The alkalies will not average more than 

 0-75%, and although present in so small a quantity they play 

 a very important part, as they form a flux for the lime and 

 the silica. 



You often hear the term " set " applied to cement. It is 

 a very ambiguous one. " Set " usually means the ultimate 

 hardness due to its nature attained by a substance originally 

 in a liquid state. Cement, however, does not attain its 

 ultimate strength for years, so the word in this case has a 

 different signification. Cement sets in a few minutes, or 

 hours ; after that it hardens. A very common test to apply 

 to cement to see if it is set is to press the pat with the thumb 

 nail. The proper method is to use the Vicat needle, which is a 

 weighted steel rod of aboutaquarter of aninchin diameter. The 

 rod is suspended over the pat and allowed to fall on it like the 

 monkey of a pile-driving machine, penetrating the sample 

 until such time as it is sufficiently set to withstand any im- 

 pression. With regard to a quick or a slow setting cement, 

 this must be determined by the nature of the work for which 

 i4; is intended. The characteristics of each are that a quick 

 cement will probably attain a fair strength in a short time, 

 but will not improve much afterwards, while the slower setting 

 sample will not do so well as the quick one at an early date,, 

 but will eventually attain much greater strength. Cement 

 may be rendered slow in setting by exposing it to the air, but 

 such treatment does not give to a quick-setting sample the 

 characteristics and the ultimate strength of a naturally slow 

 one. 



