PART IV. HYDRAULIC LIMES, SELENITIC LIME, 

 AND GRAPPIER CEMENTS. 



CHAPTER XIII. 

 THE THEORY OF HYDRAULIC LIMES. 



BEFORE taking up the manufacture and properties of the various 

 closely allied products hydraulic limes, selenitic limes, and grappier 

 cements which are to be discussed in this part of the volume, it seems 

 desirable to devote some space to a consideration of the general principles 

 on which the manufacture and use of these products are based. 



The materials heretofore discussed in this volume the plasters, 

 common lime, magnesia, etc. have been simple in both composition 

 and action. With the hydraulic limes, however, we take up the first 

 member of a great class of very complex products. All these products 

 possess hydraulic properties. In composition, they further agree in 

 that they all consist essentially of silica, alumina, and lime, with or 

 without magnesia and iron oxide. This group of complex cementing 

 materials includes the hydraulic limes, the natural cements, the Port- 

 land cements, and the puzzolan cements. These four classes are quite 

 distinct commercially, but it is at times difficult to draw the dividing 

 lines between the classes in words. Before defining the class of 

 "hydraulic limes" it will therefore be well to explain the principal 

 criterion which will be employed in drawing up that definition. This 

 criterion is the "Cementation Index", a more satisfactory form of the 

 older "hydraulic index". 



The "Hydraulic Index". In discussing the classification of cement- 

 ing materials, in the introduction to this volume, the statement is 

 made that the power of setting under water, possessed by the hydraulic 

 limes and cements, is due to the formation of compounds of silica, alu- 

 mina, and lime during the manufacture of the cementing materials 



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