THE THEORY OF HYDRAULIC LIMES. 171 



The use of the Cementation Index, as here stated, involves certain 

 assumptions as to the constitution of hydraulic cementing materials. 

 These are, in order of importance: 



(1) That in hydraulic limes and cements the hydraulic activity is 

 due to the formation during manufacture of certain compounds of 

 lime and magnesia with silica, alumina, and iron. 



(2) That the silica combines normally with the lime in such molec- 

 ular proportions as to form the tricalcic silicate, 3CaO.SiO 2 . 



(3) That the alumina combines with the lime as the dicalcic alu- 

 minate, 2CaO.Al 2 3 . 



(4) That magnesia is, molecule for molecule, equivalent to lime 

 in its action. 



(5) That iron oxide is, molecule for molecule, equivalent to alumina. 

 Of these five assumptions, the first is simply a general statement 



of conditions which are recognized by everybody as probably existing. 

 The second assumption, likewise, is generally accepted, since it agrees 

 with the views of both Le Chatelier and Newberry. The third, based 

 on Newberry 's experiments and confirmed by those of Richardson, is 

 practically accepted by all American cement chemists, though not by 

 those who follow Le Chatelier. 



The fourth and fifth assumptions, however, are open to question, 

 and the writer realizes that serious objections may be urged against 

 them. But he also realizes that magnesia and iron must be accounted 

 for in some way, that the assumptions above made are inherently prob- 

 able, and that the resulting " Cementation Index " works out very 

 well in practice. For the present, therefore, the " Cementation Index " 

 will be accepted as a guide in discussing the composition and the char- 

 acteristics of the hydraulic limes. 



Use of the Cementation Index in classification. The Cementation 

 Index will be used in classifying the various hydraulic products, for 

 it gives information of value concerning the properties of the various 

 products. But it cannot be the sole basis for classification, because 

 the properties of a hydraulic cementing material will be later seen to 

 depend not only on its composition, but on the conditions of its manu- 

 facture. A material having a Cementation Index of 1.05 might be, 

 for example, a hydraulic lime, a natural cement, or a cement of the 

 Portland type, depending chiefly on the temperature at which the raw 

 material was burned. In general, however, the possible variation is 

 by no means so wide as this. A material with a Cementation Index 

 of 0.40, for example, could under no possible temperature conditions 

 yield anything but a somewhat weak hydraulic lime. 



