CALCULATION AND CONTROL OF THE MIX. 391 



fluxing mania persists, and in a few rare cases it may be entirely justi- 

 fiable. 



Fluorspar, sodium carbonate, and other alkali salts are the favorite 

 materials for use as fluxes. It is certainly true that the addition of 

 a very small percentage of some of these salts will decrease materially 

 the difficulty of clinkering a cement mixture. Any other effect they 

 may have on the cement, however, is either negatively or positively 

 harmful; and in all cases their use can be avoided and equally good 

 burning results obtained by a slightly increased fineness of grinding 

 of the raw materials. 



The direct addition of iron oxide as a flux, a practice which is followed 

 by at least one large American plant, is somewhat different from the 

 use of fluorspar or alkalies. The iron oxide decreases the clinkering 

 temperature very materially and gives a slower setting product than 

 would an equal percentage of alumina. Adding it separately to the 

 mixture is, however, a difficult matter to arrange. The more natural 

 course to pursue would be to look for another source of clay supply, 

 attempting to find a clay sufficiently high in iron to obviate the necessity 

 for adding iron oxide separately. 



Calculating Mixtures of Untried Materials. 



When absolutely untried raw materials are being tested for the 

 first time, the experimental mixture must be solely on the basis of their 

 analyses, as developed in the formula given below or in some similar 

 device. After the plant has once started, more empirical methods of 

 calculating the mix are used, as set forth in a later section (pp. 393-394). 



Cementation Index. Recalling the discussion on page 383 of the 

 theoretical constitution of Portland cement, it is evident that the ideal 

 cement (and therefore the cement mixture) should contain its various 

 ingredients in such percentages that the following compounds can be 

 formed: 3CaO, Si0 2 , 2CaO.Al 2 3 , 2CaO.Fe 2 O 3 , 3MgO.Si0 2 , 2MgO.Al 2 3 , 

 2MgO.Fe 2 O 3 . These conditions are satisfied if the formula below, called 

 for convenience the Cementation Index, gives a value of unity. In 

 this formula the chemical equivalents above noted have been changed 

 into percentages. 



(2.8 X percentage silica (Si0 2 ) ) + (l.lX percentage alumina, A1 2 3 ) 



+ (.7 X percentage iron oxide, Fe 2 O 3 ) __ 



(Percentage lime, CaO)-f(l. 4 X percentage magnesia, MgO) 



When the value given by this formula falls below 1.0, the cement 

 must necessarily contain free lime or free magnesia; when it rises 

 above 1.0, the cement must necessarily be lower in lime than is theo- 



