360 ANNUAL REPORT 
off and the vessel, together with the cement, weighed. A sketch of the 
apparatus is shown in Figure 77. 
FINENESS OF THE GRAIN. 
All cement specifications include the point of fineness of grinding, as 
it is now generally recognized that the finer the cement is the more com- 
plete will be its hydration and the more sand will it be able to cement 
tagether. A number of well known investigations have brought this 
out so prominently that no further discussion is necessary. 
The fineness of the cement is commonly measured by means of 
sieves, the 100 mesh screen being taken as the standard. Not more than 
5 per cent. of the weight of the cement should be retained by. the sieve, 
the wire being No. 40 of Stubbs’ wire gauge. Finer sieves than the 150 
mesh are not satisfactory for dry sieving, as they invariably tend 
to clog and give unreliable results. If they are to be used the cement 
should be put through wet, that 1s, made up into a thin paste with 
alcohol. In the nature of the case sieves cannot afford an accurate 
means of separating the really fine dust in a cement. This must be 
accomplished by means of the so-called mechanical analysis described at 
length in previous paragraphs. 
Other means have also been suggested, like the separation by a cur- 
rent of air blown through a conical vessel, at a definite pressure, the 
current of air being conducted through a central pipe nearly to the bot- 
tom of the vessel. The fine particles are caught in a large cylinder. The 
air pressure is equal to a water column 32 inches high. The fine flour 
thus removed from commercial cements was found to be about 50 per 
cent. By increasing or reducing the pressure various sizes can be sep- 
arated. An apparatus of this kind was proposed by W. F. Goreham. 
SOME PHYSICAL AND CHEMICAL CONSTANTS. 
The coefficient of expansion of pure Portland Cement 1s not of as 
great practical importance as that of the cement mortar or concrete. It 
is evident, of course, that the coefficient 1s somewhat influenced by the 
character of the aggregate in concrete. The coefficient of expansion of 
Portland cement, between 8° and 70° C. was found to be, by C. H. Au, 
©.0000115 per degree C. 
3ouniceau. found the coefficient of expansion of concrete to be 
0.0000137, that of wrought iron 0.00001235. ‘This explains why rein- 
forced concrete does not give rise to difficulties dué to the differences in 
the coefficients of expansion, the expansion values of the concrete and 
iron being nearly the same. It might be mentioned here, also, that in 
reinforced concrete, according to the work of Considere, the tensile 
strength obtained approaches very closely that of the iron employed and 
cracking appears only when the tensile strains approach the elastic limit 
of the iron. | 
