during manufacture or by chemical interaction, in use, of mater- 

 ials which have merely been mechanically mixed during manu- 

 facture. 



In the class of silicate cements are included all the materials 

 commonly known as cements by the engineer (natural cements, 

 Portland cement, pozzuolanic cements), together with the hy- 

 draulic limes. 



Though differing widely in raw material, methods of manu- 

 facture and properties, the silicate cements agree in two promi- 

 nent features: they are all hydraulic (though in very different 

 degrees) ; and this property of hydraulicity is, in all, due largely 

 or entirely to the formation of tri-calcic silicate (3 CaO SiO2). 

 Other silicates of lime, as well as silico-aluminates, may also be 

 formed ; but they are relatively unimportant, except in certain 

 of the natural cements and hydraulic limes where the lime- 

 aluminates may be of greater importance than is here indicated. 

 This will be recurred to in discussing the groups named. 



The silicate cements are divisible, on technologic grounds, 

 into four distinct classes. The. basis for this division is given 

 below. It will be seen that the first named of these classes (the 

 pozzuolanic cements) differs from the other three very markedly 

 inasmuch as its raw materials are not calcined after mixture; 

 while in the last three classes the raw materials are invariably 

 calcined after mixture. The four classes differ somewhat in 

 composition, but more markedly in methods of manufacture and 

 in the properties of the finished cements. 



Classes of Silicate Cements. 



/. Pozswolcenic* Cements : Produced by the mechanical 

 mixture, without calcination, of slaked lime and a silico-alumin- 



ous- material (the latter being usually a, volcanic ash or blast- 

 furnace slag.) 



2. Hydraulic Limes : Produced by the calcination, at a tem- 

 perature not much higher than that of decarbonation, of a silice- 

 ous limestone so high in lime carbonate that a considerable 

 amount of free lime appears in the finished product. 



*Also written Puzzolan. 



