300 CEMENTS, LIMES, AND PLASTERS. 



with the silica-alumina ratio; while the setting - time and ultimate 

 strength of the cement are in inverse proportion to the values of the 

 ratio. 



Kinds of material used. Before taking up the detailed discussion 

 of the various raw materials used in the manufacture of Portland cement, 

 some general statements on the kinds and combinations of raw mate- 

 rials actually in use will probably be found serviceable. 



In order that the value and availability of different raw materials 

 may be estimated, it will be convenient to assume a certain ideal com- 

 position for a cement rock. For the purposes of the present chapter 

 this can be done in a sufficiently accurate way by considering that 

 a Portland-cement mixture, when ready for burning, should contain 

 about 75 per cent of lime carbonate (CaC0 3 ), and about 20 per cent of 

 silica (Si02), alumina (A^Os), and iron oxide (Fe20s) together, the 

 remaining 5 per cent or so containing any magnesia, sulphur, and alkalies 

 that may be present. More exact information on these points will 

 be found in Chapter XXIX, where a somewhat detailed discussion of 

 the calculation and composition of Portland-cement mixtures, together 

 with a number of analyses of actual mixtures and cements, will be 

 given. 



The essential elements which enter into this mixture lime, silica, 

 alumina, and iron are all abundantly and widely distributed in nature, 

 occurring in different forms in many kinds of rocks; and it can read- 

 ily be seen that, theoretically, a satisfactory Portland-cement mixture 

 could be prepared by combining, in an almost infinite number of ways 

 and proportions, many possible raw materials. Obviously, too, we 

 might expect to find perfect gradations in the degree of artificialness 

 of such a mixture, varying from the one extreme where a natural rock 

 of almost absolutely correct composition was used to the other extreme 

 where two or more materials in nearly equal amounts were required 

 to produce a mixture of correct composition. 



The almost infinite number of raw materials which are theoretically 

 available are, however, reduced to a very few in practice under existing 

 commercial conditions. The necessity for producing the mixture as 

 cheaply as possible rules out of consideration a large number of mate- 

 rials which would be considered available if chemical composition was 

 the only thing to be taken into account. Some materials otherwise 

 suitable are too scarce and consequently too expensive for such use; 

 some are too difficult to pulverize finely and bring into combination. 

 In consequence comparatively few combinations of raw materials are 

 actually in use. 



