CHAPTER :J. 



RAW MATERIALS. GENERAL CONSIDERATIONS. 



For the purposes of the present chapter, it will be sufficiently 

 accurate to consider that a Portland cement mixture, when ready 

 for burning, will consist of about 75 per cent, of lime carbonate 

 (Ca CO3) ancl 20 per cent, of silica (SiO2), alumina (A12O3) 

 and iron oxide (Fe2C>3) together, the remaining 5 per cent, in- 

 cluding any magnesium carbomate, sulphur and alkalies that 

 may be present. 



The essential elements which enter into this mixture, lime, 

 silica, alumina and iron, are all abundantly and widely dis- 

 tributed in nature, occurring in different forms in many kinds 

 of rocks. It can, therefore, be readily seen that, theoretically, 

 a satisfactory Portland cement mixture could be prepared by 

 combining, in an almost infinite nurnber of ways and propor- 

 tions, many possible raw materials. Obviously, we, too, might 

 expect to find perfect graduations in the artificialness of the 

 mixture, varying fron\the one extreme where a natural rock of 

 absolutely correct composition was used to the other extreme 

 where two or more materials, in nearly equal amounts, are re- 

 quired to make a mixture of correct composition. 



The almost infinite number of raw materials which are theo- 

 retically available are, however, reduced to a very few in prac- 

 tice under existing commercial conditions. The necessity for 

 making the mixture as cheaply as possible rules out of conside- 

 ration a large number of materials which would be considered 

 available if chemical composition was the only thing to be taken 

 into account. Some materials otherwise suitable are too scarce ; 

 some are too difficult to pulverize. In consequence, a compara- 

 tively few combinations of raw materials are actually used in 

 practice. 



In certain localities deposits of argillaceous (clayey) lime- 

 stone or "cement rock" occur, in which the lime, silica, alumina 

 and iron oxide exist in so nearly the proper proportions that 

 only a relatively small amount (say 10 per cent, or so) of other 

 material is required in order to make a mixture of correct com- 

 position. 



