PREPARING THE MIXTURE FOR THE KILN. 423 



Necessity for fine grinding. The necessity for very fine grinding 

 of the raw mixture, if a sound and volume-constant cement is to be 

 obtained, was early stated by Newberry,* and the value of such fine 

 grinding has been recently expressed in the quantitative form by the 

 experiments of Professor Campbell, f 



To secure a sound and volume-constant cement it is necessary that 

 the raw mixture be very finely ground. Other things being equal, the 

 finer the grinding of the raw mixture the better will be the resulting 

 cement. The degree of fineness necessary to secure a given grade of 

 cement will depend upon: 



(a) The percentage of lime in the mixture. The higher the percentage 

 of lime in the mixture, the finer the raw mixture must be ground, because 

 the chances of getting an unsound or expensive cement will increase 

 as the percentage of lime rises, and this tendency will have to be coun- 

 teracted by greater fineness of grinding. 



(6) The carefulness with which the materials have been mixed. 

 The more careful and thorough the mixture, the less care need be be- 

 stowed upon the grinding, and vice versa. 



(c) The character of the raw materials. This point, which has been 

 emphasized by Newberry,* is of great importance. When a very 

 pure limestone or marl is mixed with a clay or shale, the grinding 

 must be much finer than in the plants (such as those in the Lehigh 

 district of Pennsylvania) where a highly argillaceous limestone (" ce- 

 ment rock") is mixed with a comparatively small quantity of purer 

 limestone. In the latter case the coarser particles of the argillaceous 

 limestone will be so near in chemical composition to the proper mixture 

 as to do little harm to the resulting cement, even if both the grinding 

 and the mixing should be incompletely accomplished, while in the 

 former case, where a pure limestone or marl is mixed with clay or shale, 

 both of the constituents are very different in composition from the 

 proper mixture, and coarse particles will therefore be highly injurious 

 to the cement. 



(d) The duration of the burning. In the old-fashioned dome kilns, 

 where the mixture was exposed to the action of heat for a week or more, 

 the duration of the burning compensated in some degree for the lack 

 of thoroughness in grinding or mixing. In modern rotary kilns, how- 

 ever, in which the mixture is burned for only an hour or so, this aid can- 

 not be counted on, and both grinding and mixing must therefore be 

 done more carefully. 



* 20th Ann. Rept. U. S Geol. Survey, pt. 6, p. 545. 

 f Journ. Am. Chem. Soc., vol. 25, p. 40 et seq. 



