8 CEMENTS, LIMES, AND PLASTERS. 



and properties, the silicate cements agree in two prominent 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 tricalcic silicate (3CaO,Si02). Other silicates of lime, as well as 

 silico aluminates, may also be formed; but they are relatively unim- 

 portant, 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 last named of these classes (the puzzolan 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 manu- 

 facture and in the properties of the finished cements. 



CLASSES OF HYDRAULIC CEMENTS. 



1. Hydraulic limes are produced by burning, at relatively low tem- 

 peratures, a natural siliceous limestone which carries so much lime car- 

 bonate, compared to its content of silica and alumina, that the burned 

 product will contain a considerable amount of free lime (CaO) in addi- 

 tion to the silicates and aluminates of lime that have been formed. 

 In consequence of the relatively large percentage of free lime, the burned 

 masses will slake if water be poured on them, but because much of 

 them is composed of silicates and aluminates of lime, they will slake 

 very slowly and their final product will have hydraulic properties. 



The hydraulic limes are thus intermediate in composition, methods 

 of manufacture, and properties between the true limes and the cements 

 proper. 



2. Natural cements are produced by burning a natural clayey lime- 

 stone, containing from 15 to 40 per cent of silica, alumina, and iron 

 oxide. This burning takes place at a temperature that is usually little 

 if any above that of an ordinary lime-kiln. During the burning the 

 carbon dioxide of the limestone is almost entirely driven off, and the 

 lime combines with the silica, alumina, and iron oxide, forming a mass 

 containing silicates, aluminates, and ferrites of lime. In case the original 

 limestone contained much magnesium carbonate, the burned rock will 

 contain a corresponding amount of magnesia and magnesian compounds. 



After burning, the burned mass will not slake if water be poured 

 on it. It is necessary, therefore, to grind it quite fine, after which, if 



