654 



CEMENTS, LIMES, AND PLASTERS. 



The following analyses are of limes used at different slag-cement 

 plants in the United States: 



TABLE 246. 

 ANALYSES OF LIMES USED IN AMERICAN SLAG-CEMENT PLANTS. 



Of the analyses above tabulated, it will be seen that Nos. 1 to 4 

 inclusive are of the semi-hydraulic type whose value has been noted. 

 Analyses 5 and 6, on the other hand, are representative of the very 

 pure limes used at most slag-cement plants. 



Burning the lime. As a matter of convenience, and also to reduce 

 freight charges, the limestone is burned near the quarry. The subject 

 of burning the lime requires only brief mention here, as it involves no 

 points of particular interest or novelty. Only two minor details demand 

 notice, as affecting the value of the cement. The first is, that the lime 

 should be burned as thoroughly as possible, for unburned lumps of lime- 

 stone are absolutely valueless to the cement-manufacturer, and must 

 be removed before mixing with the slag. The second point to be noticed 

 is, that the lime should be shipped to the cement-plant as soon as pos- 

 sible after it is burned, in order to prevent any considerable proportion 

 of it from air-slaking. Air-slaked particles, while not absolutely inert, 

 are still of little value to the cement. 



Slaking the lime. The granulated slag as it comes to the mill from 

 the tanks to which it is carried in granulating it carries a very large 

 percentage of water. The amount of water carried will vary in prac- 

 tice at different plants between 25 and 50 per cent as limits. Early 

 in the history of slag-cement manufacture attempts were made to 

 utilize this surplus water. To this end the wet slag was mixed with 

 dry unslaked lime, the expectation being that the water in the slag 

 would serve to slake the lime. In practice, however, it was soon found 

 that this plan was not successful. The lime was only partially and 

 very irregularly slaked, and the mixture was not left in such a con- 

 dition as to be economically handled by the pulverizing machinery. 

 In present-day practice, therefore, the lime is slaked previous to being 

 mixed with the slag. 



