192 SAND AND CEMENT 



to the center of the outside bolt ot the vertical lug should 

 be 3 in.; the pitch of the bolts, 3 in.; the projection of the 

 bracket beyond the column, 4 in.; the depth of the ver- 

 tical leg of the bracket, 7 1 in. ; etc. 



SAND AND CEMENT 



CEMENTING MATERIALS 



Any substance that becomes plastic under certain treat- 

 ment and subsequently reverts to a tenacious and inelastic 

 condition may, in a broad sense, be termed a cement. How- 

 ever, nearly all the cementing materials employed in build- 

 ing construction are obtained by the heating, or calcination, 

 as it is called, of minerals composed wholly or in part of lime. 

 The different composition of these minerals, as well as the 

 properties of the calcined products, enables the various 

 resulting substances to be classified as limes, hydraulic 

 cements, plasters, and miscellaneous cements. Although all 

 these materials have cementing properties, the term cement 

 is commonly used to apply only to the group made up of 

 hydraulic cements, hydraulic meaning that these substances 

 possess the ability to set, or become hard, under water. 



Limes and hydraulic cements (commonly called simply 

 cements') are composed essentially of oxide of calcium, or 

 lime, generally called quicklime, with which may be com- 

 bined certain argillaceous, or clayey, elements, notably silica 

 and alumina, it being to these elements that the hydraulic 

 properties of certain of these materials are due. The 

 quantity of silica and alumina present in these substances 

 enables them to be classified as common limes, hydraulic limes, 

 and cements. 



The ratio of the quantity of silica and alumina present in 

 these materials to the quantity of lime is called the hydraulic 

 index. In common limes, this index is less than -fifoi in 

 hydraulic limes, it lies between ^& and rift; and in cements, 

 it exceeds jV5r- These limes merge into each other so grad- 

 ually, however, that it is often difficult to distinguish the 

 dividing line between them. 



