58 CEMENT MATERIALS AND INDUSTRY. [bull. 243. 
addition to the cost of cement manufacture, it should be remembered 
that careful drying- and line pulverizing enable the manufacturer to 
use much poorer, and therefore cheaper, grades of coal than could 
otherwise be utilized. 
CLINKER GRINDING. 
The power and machinery required for pulverizing the clinker at a 
Portland-cement plant using the dry process of manufacture are not 
much more than those needed for pulverizing the raw materials. This 
ma}^ seem at first sight improbable, for Portland-cement clinker is 
much harder to grind than any possible combination of raw materials; 
but it must be remembered that for every barrel of cement produced 
about 600 pounds of raw materials must be pulverized, while onl} T a 
scant 400 pounds of clinker will be treated, and that the large crushers 
required for some raw materials can be dispensed with in crushing 
clinker. With this exception, the machinery for treating the raw 
material and that for treating the clinker of a dry-process Portland 
cement plant are' usually almost duplicates. 
The difficulty, and in consequence the expense, of grinding clinker 
will depend in large part on the chemical composition of the clinker 
and on the temperature at which it has been burned. The difficult! 
of grinding, for example, increases with the percentage of lime carried 
by the clinker, and a clinker containing 64 per cent of lime will be 
very noticeably more resistant to pulverizing than one carrying 62 
per cent of lime. So far as regards burning, it may be said in general 
that the more thoroughly burned the clinker the more difficult it will 
be to grind, assuming that its chemical composition remains the same. 
The tendency among engineers at present is to demand more finebj 
ground cement. While this demand is doubtless justified by the- 
results of comparative tests of finely and coarsely ground cements, it 
must be borne in mind that any increase in fineness of grinding mean! 
a decrease in the product per hour of the grinding mills employed 
and a consequent increase in the cost of cement. At some point ii 1 
the process, therefore, the gain in strength due to fineness of grinding 
will be counterbalanced by the increased cost of manufacturing th< 
more finely ground product. 
The increase in the required fineness has been gradual but stead j 
during recent years. Most specifications now require at least 90 pe 
cent to pass a 100-mesh sieve, a number require 92 per cent, while ; 
few important specifications require 95 per cent. 
ADDITION OF GYPSUM. 
The cement produced by the rotary kiln is invariably naturally s< 
quick-setting as to require the addition of sulphate of lime. This sub 
stance, when added in quantities up to 2-j or 3 per cent, retards th i 
