ly contains certain impurities, it is called "cement plaster." 

 When either plaster of Paris or cement plaster are mixed with 

 water, the percentage of water which was driven of! during 

 calcination is reabsorbed, and the mixture hardens, having 

 again becomes a hydrous sulphate o<f lime. The processes involv- 

 ed in the manufacture and setting of the dead-burned plasters 

 and hard-finish plasters are slightly more complicated, but the 

 reactions involved are of the same general type. 



Sub-group Ib. Carbonate Cements. 



The cementing materials falling in the present sub-group are 

 oxides, derived from natural carbonates by the application of 

 heat. On exposure, under proper conditions, to any source of 

 carbon dioxide, the cementing material recorbonates and ''sets.'' 

 In practice the carbon dioxide required for setting is obtained 

 simply by exposure of the mortar to the air. In consequence 

 the set of these carbonate cements, as commonly used, is very 

 slow (owing to the small amount of carbon dioxide which can 

 be taken up from ordinary air) ; and, what is more important 

 from an engineering point of view, none of the mortar in the 

 interior of a wall ever acquires hardness, as only the exposed 

 portions have an opportunity to absorb carbon dioxide. From 

 the examination of old mortars it has been thought probable 

 that a certain amount of chemical action takes place between the 

 sand and the lime, resulting in the formation of lime silicates ; 

 but this effect is slight and of little engineering importance com- 

 pared with the hardening which occurs in consequence of the 

 reabsorption of carbon dioxide from the air. 



Limestone is the natural raw material whose calcination fur- 

 nishes the cementing materials of this group. If the limestone 

 be an almost pure calcium carbonate it will, on calcination, yield 

 calcium oxide or "quicklime." If, however, the limestone 

 should contain any appreciable percentage of magnesium car- 

 bonate, the product will be a mixture of the oxides of calcium 

 and magnesium commercially known as magnesian lime. A brief 

 sketch of the mineralogic relationships of the various kinds of 

 limestone, in connection with the chemistry of lime-burning, 

 will be of service at this point of the discussion. 



Pure limestone has the composition of the mineral calcite, 

 whose formula is CaCO3, corresponding to the composition 



