CLINKER COOLING, GRINDING, AND STORAGE. 531 



Somewhere in the process it is necessary to provide for the addition 

 of a certain comparatively small percentage of gypsum or plaster, in 

 order to bring the setting properties of the cement up to commercial 

 requirements. Though this addition is commonly made during the 

 grinding process, it will be discussed later in the chapter. 



The power allowed and machinery installed for pulverizing the 

 clinker at a Portland-cement plant using the dry process of manu- 

 facture are very closely the same as that required for pulverizing the 

 raw materials for the same output. This may 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 Ibs. of raw materials 

 must be pulverized, while only a scant 400 Ibs. of clinker will be treated, 

 that the large crushers required for some raw materials can be dis- 

 pensed with in crushing clinker, and that the raw side rarely runs full 

 time. The raw material side and the clinker side of a dry-process Port- 

 land-cement plant are, therefore, usually almost or exactly 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 difficulty 

 of grinding, for example, increases with the percentage of lime carried 

 by the clinker, because of the higher burning which has been necessary, 

 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 thor- 

 oughly 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 finely 

 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 means a decrease 

 in the product per hour of the grinding-mills employed, and a conse- 

 quent increase in the cost of cement. At some point in the process, 

 therefore, the gain in strength due to fineness of grinding will be counter- 

 balanced by the increased cost of manufacturing the more finely ground 

 product. 



The increase in the required fineness has been gradual but steady 

 during recent years. Most specifications now require at least 90 per 

 cent to pass a 100-mesh sieve; a number require 92 per cent; while 

 a few important specifications require 95 per cent. Within a few years 

 it is probable that almost all specifications will go as high as this. 



