196 CEMENTS, LIMES, AND PLASTERS. 



and finely ground artificial mixtures, but by burning masses of natural 

 rock. 



(2) Natural cements, after burning and grinding, are usually yellow 

 to brown in color and light in weight, their specific gravity being about 

 2.7 to 3.10, while Portland cement is commonly blue to gray in color 

 and heavier, its specific gravity ranging from 3.0 to 3.2. 



(3) Natural cements are always burned at a lower temperature 

 than Portland, and commonly at a much lower temperature, the mass 

 of rock in the kiln rarely being heated high enough to even approach 

 the fusing- or clinkering-point. 



(4) In use natural cements set more rapidly than Portland cement, 

 but do not attain such a high ultimate strength. 



(5) In composition, while Portland cement is a definite product 

 whose percentages of lime, silica, alumina, and iron oxide vary only 

 between narrow limits, various brands of natural cements will show very 

 great differences in composition; while even the same brand, analyzed 

 at different times, will show considerable differences in composition, 

 due to variations in the natural limestones used. 



Cementation Index. In discussing the hydraulic limes (Chapter 

 XIII) attention was called to the desirability of devising some method of 

 general applicability for comparing the hydraulic activity of various 

 cementing materials. The defects of the old "hydraulic index" were 

 pointed out, and a new and more satisfactory index the Cementation 

 Index was suggested as a substitute. The value of this innovation will 

 appear in the present section, for in dealing with the natural cements 

 such great variations in composition are found that it is absolutely 

 necessary to have some means of comparing such different products. 



The Cementation Index of any limestone or cement is found by 

 applying the following formula: 



(2.8 X percentage silica) + 

 (1.1X percentage alumina) 4- 



Cementation Index -,= (.^percentage iron oxide) 



(Percentage lime) +(1. 4 X percentage magnesia) 



When this formula is applied to an unburned limestone it must be 

 recollected that the percentages used in the divisor are those of lime 

 (CaO) and magnesia (MgO) respectively, not those of lime carbonate 

 (CaCO 3 ) and magnesium carbonate (MgCOs). 



Example of calculation. The methods of calculating the Cementa- 

 tion Index of any product may be shown by an example, the Utica natural 

 cement whose analysis appears as No. 1, Table 110, p. 253, being selected 



