170 CEMENTS, LIMES, AND PLASTERS. 



tatively interchangeable, i.e., that 10 per cent of silica will have exactly 

 the same effect as 10 per cent of alumina. 



These defects have led the writer to abandon the use of the ''hydraulic 

 index " and to substitute therefor the index described in the next 

 section as the " Cementation Index ". 



The Cementation Index. As explained and defined below, the 

 Cementation Index is a natural outgrowth from the formula proposed 

 by Newberry for proportioning Portland-cement mixtures. The index 

 now proposed differs from that formula in assigning values for the 

 magnesia and iron oxide contained in the cement or lime, a change 

 which is necessary in order to adapt it for use with the magnesian 

 natural cements and the puzzolan cements. The proposed index is; 



(2. 8 x percentage silica) +(1.1 x percentage alumina) 



+ (.7 x percentage iron oxide) 



Cementation Index = /I3 . , . 



(Percentage lime)+(l. 4 x percentage magnesia) 



EXAMPLE. As an example of the details of calculating the Cementa- 

 tion Index, the hydraulic lime of Metz, whose analysis is given as No. 2 

 of Table 74, will be used. The essential ingredients of this lime, as 

 given in the quoted analysis, are: 



Silica (Si0 2 ) 18.47 



Alumina ( A1 2 O 3 ) 5 . 73 



Iron oxide (Fe 2 O 3 ) 3 . 29 



Lime (CaO) 68. 19 



^nesia (MgO) 2.66 



Substituting these values in the formula 



2.8 percentage silica +1.1 percentage alumina 



Cementation Index=-^- + .7 percentage iron oxide_ 



Percentage lime +1.4 percentage magnesia 

 we have 



Cementation Index = (2.8X18.47) +(1.1x5.73) +(.7X3.29) 



(68.19) +(1.4X2.66) 

 _51.716+6.303 + 2.303 



68.19+3.724 

 _ 60.322 

 71.914 

 = .839. 



As will be seen later, this is a very typical value for the Cementa- 

 tion Index of a good hydraulic lime. 



