PORTLAND CEMENT: PRELIMINARY STATEMENTS. 299 



reaches its maximum of efficiency and becomes of great service in every 

 phase of the subject, from the selection of the raw materials and the 

 proportioning of the mix to the valuation of the finished product. In 

 later chapters the basis and determination of the Cementation Index 

 will be found discussed in detail. In the present chapter it is only 

 necessary to state that its value is obtained from the following formula: 



(2. 8X percentage silica) -!- 

 (l.lX percentage alumina)4- 



, , .7 X percent age iron oxide 



Cementation Index = 77 r ' ;-. 



Percentage lime (CaO) + 1.4 percentage magnesia 



This formula is applicable to raw materials as well as to cements, 

 but the user must recollect that the first factor in the divisor is based 

 on the percentage of lime (CaO), not of lime carbonate (CaCOa), and 

 similarly with the magnesia. 



The Cementation Index, determined as above described, is a meas- 

 ure of the degree of basicity of a cement, or the relation of the acid 

 (Si02,Al 2 03,Fe 2 O3) to the basic (CaO,MgO) factors in its composition. 

 A high cementation index means a high-limed and low-clayed cement, 

 while a low index would mean the opposite. In Portland cements 

 as at present made the Cementation Index will commonly fall within 

 the limits of 1.00 and 1.20, 1.00 being the ideal index for a Portland. 



Silica-alumina ratio. The ratio between the silica and the alumina 

 + iron oxide gives the second important index to the character of a 

 cement. For convenience of reference this may be termed the silica- 

 alumina ratio. This ratio, properly speaking, should take into account 

 the different combining weights of the three compounds concerned, 

 and would, therefore, theoretically be found from the formula 



. . ,., T , 2. 8 X percentage silica 



Acidity Index = 7- ; f r 7 : rr-c 



( 1 . 1 X percentage alumina) -f ( .7 X percentage iron oxide) 



To the value determined by this formula the term "Acidity Index" 

 might be very properly applied. But in ordinary practice the per- 

 centage of iron oxide present is so small that the ratio between the 

 silica and the alumina + iron is given correctly enough by simple 

 division, i.e., 



Percentage silica 

 Percentage alumina + percentage iron oxide* 



The value thus obtained will be called briefly the silica-alumina ratio 

 (though it considers the iron oxide also) . It may be said that the per- 

 centage of lime being constant, the clinkering temperature decreases 



