8 



j. Natural Cements : Produced by the calcination, at a tem- 

 perature between those of decarbonation and clinkering, of a 

 siliceous limestone (which may also carry notable amounts of 

 'alumina and of magnesium carbonate) in which the lime car- 

 bonate is so low, relatively to the silica and alumina, that little 

 or no free lime appears in the cement. 



4. Portland Cements: Produced by the calcination, at the 

 temperature of semi-vitref action ("clinkering") of an artificial 

 mixture of calcareous with silico-aluminous materials, in the 

 proportion of about three parts of lime carbonate to one part of 

 clayey material. 



NATURAL CEMENTS. 



Natural cements are produced by burning a naturally impure 

 limestone, containing from 15 to 40 per cent, of silica, alumina, 

 and iron oxide. This burning takes place at a comparatively 

 low temperature, about that of ordinary lim<e burning. The 

 operation can therefore be carried on in a kiln closely resemb- 

 ling an ordinary lime kiln. During the burning the carbon di- 

 oxide of the limestone is almost entirely driven off, and the 

 lime combines with the silica, alumina, and iron nxide. forming 

 a mass containing silicates, aluminates, and ferrites of lime. In 

 case the original limestone contained much magnesium carbon- 

 ate, the burned rock will also contain a corresponding amount 

 of magnesia and magnesian compounds. 



After burning, the burned mass will not slack if water be 

 added. It is necessary, therefore, to giind it quite finely. After 

 grinding, if the resulting powder (-natural cement) be mixed 

 with water it will harden Vapidly. This hardening or setting 

 will also take place under water. The natural cements differ 

 from ordinary limes in two noticeable ways: 



1 i ) The burned mass does not slack on the addition of water. 



(2) After grinding, the powder has hydraulic properties, 

 i. e., if properly prepared, it will set under water. 



Natural cements are quite closely related to both hydraulic 

 limes on the one hand, and Portland cement on the other, agree- 

 ing with both in the possession of hydraulic properties. They 

 differ from hydraulic limes, however, in that the burned natural 

 cement rock will not slake when water is poured on it. 



