CHAPTER XXIX. 

 CALCULATION AND CONTROL OF THE MIX. 



IF, as In the present volume, we exclude from consideration the 

 so-called "natural Portlands" (see page 215), Portland cement maybe 

 regarded as being entirely an artificial product, obtained by burning 

 to semi-fusion an intimate mixture of pulverized materials, this mix- 

 ture containing lime, silica, and alumina varying in proportion only 

 within certain narrow limits, and by crushing finely the clinker resulting 

 from this burning. 



If this restricted definition of Portland cement be accepted, four 

 points may be regarded as being of cardinal importance in its manu- 

 facture. These are: 



1. The cement mixture must be of the proper chemical compo- 



sition. 



2. The materials of which it is composed must be carefully ground 



and intimately mixed before burning in order to insure that 

 chemical combination shall take place after calcination. 



3. The burning must be conducted at the proper temperature, 



which varies considerably according to the chemical com- 

 position of the mixture, and the length of time during which 

 it is subjected to the burning process. 



4. After burning, the resulting clinker must be finely ground. 



In this and the succeeding chapters these points will be taken up 

 separately and in some detail. 



The present chapter deals with the calculations and arrangements 

 necessary for insuring the correctness of the cement mixture. It, there- 

 fore, includes discussions of the theoretical and practical considera- 

 tions which determine the proportions of the mixture. Among these 

 considerations are the theoretical composition and constitution of Port- 

 land cement; the influence of various normal constituents on the prop- 

 erties of the mixture; the influence of fuel ash and other accidental 

 impurities; and the methods of calculating and controlling the mix in 

 actual practice. 



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