198 



CEMENTS, LIMES, AND PLASTERS. 



certain well-known products cannot be explained satisfactorily without 

 taking account of the magnesia and iron oxide they contain. 



Use of the Cementation Index. If the assumptions on which the 

 Cementation Index is founded are well based, it is evident that the 

 hydraulic properties or, rather, the hydraulic possibilities of a prod- 

 uct are indicated by its index. A product whose index falls below 

 1.00 must necessarily contain free lime or free magnesia, whatever the 

 temperature at which it is burned, and such a product should therefore 

 be strictly classed with the hydraulic limes, which require slaking before 

 use. It will be seen later, however, that if a product contains much mag- 

 nesia (say 20 per cent MgO or over) its Cementation Index may fall below 

 1.00 without demonstrable defects in the cement. This point is taken 

 up on later pages in discussing the actual composition of various 

 natural cements. A product with an index exceeding 1.00 can be burned 

 so as to give complete combination of all its lime and magnesia, leaving 

 none free. As the index increases, the temperature necessary to attain 

 such complete combination decreases, but the hydraulic activity of the 

 product also decreases, until an index exceeding 2.00 indicates a very 

 lightly burned, but also very feeble, cement. 



Cementation Index of natural cements. The term "natural cement" 

 as used in this volume will cover a very large class of cementing prod- 

 ucts. In the United States the name has become fairly well fixed 

 in use, so that there need be little misunderstanding concerning the 

 limits of the groups. In English and European practice, however, 

 the term " natural cement " has never come into extensive use. It 

 may therefore be necessary to state that, as above denned, it includes 

 the lightly burned but often high-limed cements known to the Euro- 

 pean trade as " Roman cements ", " quick-setting cements ", etc., as 

 well as the so-called "natural Portlands ". 



The differences in composition between the various cements included 

 in this heterogeneous class naturally give rise to corresponding differ- 

 ences in their cementation index. It may be said for the group taken 

 as a whole that the Cementation Index of natural cements varies 

 between the limits of 1.00 and 2.00, falling below 1.00 only in the case 

 of certain highly magnesian cements, and that most of the natural 

 cements will fall between the narrower limits of about 1.15 to 1.60. 



This variation of the Cementation Index may be used as a con- 

 venient basis for subdividing the "natural cements " into smaller groups 

 of more homogeneous character. 



A. Cements with an index between 1.00 and 1.15. These products 

 when burned at sufficiently high temperature are rather slow- 



