424 



CEMENTS, LIMES, AND PLASTERS. 



Actual fineness attained. After its final reduction, and when ready 

 for burning, the mixture will usually run from 90 to 95 per cent through 

 a 100-mesh sieve. In the plants of the Lehigh district the mixture 

 is rarely crushed as fine as when limestone and clay are used. New- 

 berry * has pointed out in explanation for this that an argillaceous 

 limestone (cement rock) mixed with a comparatively small quantity 

 of purer limestone, as in the Lehigh plants, requires less thorough mix- 

 ing and less fine grinding than when a mixture of limestone and clay 

 (or marl and clay) is used, for even the coarser particles of the argilla- 

 ceous limestone will vary so little in chemical composition from the 

 proper mixture as to affect the quality of the resulting cement but 

 little should either mixing or grinding be incompletely accomplished. 



A very good example of typical Lehigh Valley grinding of raw mate- 

 rial is afforded by a specimen examined f by Prof. E. D. Campbell. This 

 specimen of raw mix ready for burning was furnished by one of the 

 best of the eastern Pennsylvania cement-plants. A mechanical analysis 

 of it showed the following results. 



Mesh of sieve 50 100 200 



Per cent passing 96.9 85.6 72.4 



Per cent residue. . 3.1 14.4 27.6 



The material, therefore, is so coarsely ground that only a trifle over 

 85 per cent passes a 100-mesh sieve. 



Bleininger has recently published the results of a series of tests 

 for fineness, made on the raw mixtures used by various plants. The 

 results are given in Table 175. 



Gradual vs. one-stage reduction. This question is now of little 

 more than theoretical interest, as almost all cement-plants seem to 

 have given the same decision in regard to it Until within the past 

 few years, examination of a number of cement-plants would have 

 developed the fact that two radically different systems of reduction 

 were in use. Reference is made to the "gradual" and "one-stage" 

 methods. 



In those earlier days many plants, after preliminary treatment in 

 a coarse crusher, completed the entire process of further reduction in 

 a Griffin mill or ball mill, for it must be recollected that the latter mill 

 was introduced earlier than its companion, the tube mill, and was at 

 first expected to do the work now done by both. 



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

 t Jour. Amer. Chem. Soc., vol. 25, p. 39. 



