THE THEORY OF HYDRAULIC LIMES. 169 



in question. This being the case, it is a fair assumption * that the 

 degree of hydraulic activity and the strength of any given cementing 

 material will be related, in some way, to the proportions in which it 

 contains these ingredients (silica, alumina, lime, etc.), and to the man- 

 ner in which they are combined. 



It is obvious that it would be of great value to both manufacturer 

 .and engineer if we could devise some method for quantitatively express- 

 ing this relation between the composition and the hydraulic value of 

 any given sample of cementing material. Several methods of doing 

 this have been suggested and used by various authorities. 



Of these methods of expression, the one that has come into most 

 general use is based upon the calculation of the " hydraulic index ". 

 The " hydraulic index ", as usually defined, is the ratio between the 

 percentage of silica plus alumina to the percentage of lime (CaO). A 

 hydraulic lime, for example, such as that from Metz (Analysis No. 2, 

 Table 74), containing 18.47 per cent silica, 5.73 per cent alumina, and 

 68.19 per cent lime would therefore have for its hydraulic index 



The " hydraulic index ", calculated in this manner, is then used 

 as a basis for classifying cementing materials according to their hydraulic 

 activity. The following grouping, which is substantially that given 

 by Spalding,| is an example of this: 



Hydraulic Index. Product. 



Less than 0.10 ...... Common lime, quicklime 



0.10 to 0.20 ........ Feebly hydraulic limes 



0.20 " 0.40 ........ Eminently hydraulic limes 



0.40 " 0.60 ........ Portland cement (if burned at high temperature) 



0.60 " 1.50 ........ Natural cements 



1.50 " 3.00 ........ Weak natural cements 



3.00 ............... Puzzolanas, etc. 



The " hydraulic index " calculated and used in this fashion is 

 certainly better than nothing, but it possesses defects which render 

 it valueless in dealing with certain classes of cements. These defects 

 arise chiefly* from the facts that in calculating the " hydraulic index " 

 (1) no allowance is made for the action of either magnesia or iron oxide, 

 and (2) the assumption is made that silica and alumina are quanti- 



* Strictly speaking, this statement is based on more than a mere assumption ; 

 but as a matter of convenience discussion of the reasons for it will be deferred 

 to later chapters. 



t Spalding, F. P. "Hydraulic Cement", pp. 8, 31, 38. 



