412 CEMENTS, LIMES, AND PLASTERS. 



tional lime must be added, usually in the form of limestone, the slag 

 and limestone must be well mixed and the mixture properly burned. 

 The general methods for accomplishing the proper mixture of the mate- 

 rials vary in details. It seems probable that the first method used 

 in attempting to make a true Portland cement from slag was to dump 

 the proper proportion of limestone, broken into small lumps, into molten 

 slag. The idea was that both mixing and calcination could thus be 

 accomplished in one stage; but in practice it was found that the result- 

 ing cement was variable in composition and always low in grade. This 

 method has accordingly fallen into disuse, and at present three different 

 general processes of preparing the mixture are practiced at different 

 European and American plants. 



1. The slag is granulated, dried, and ground, while the limestone 

 is dried and ground separately. The two materials are then mixed 

 in proper proportions, the mixture is finely pulverized in tube mills, 

 and the product is fed in a powdered state to rotary kilns. 



2. The slag is granulated, dried, and mixed with slightly less than 

 the calculated proper amount of limestone, which has been previously 

 dried and powdered. To this mixture is added sufficient powdered 

 slaked lime (say 2 to 6 per cent) to bring the mixture up to correct 

 composition. The intimate mixture and final reduction are then accom- 

 plished in ball and tube mills. About 8 per cent of water is then added, 

 and the slurry is made into bricks, which are dried and burned in a 

 dome or chamber kiln. 



3. Slag is granulated and mixed, while still wet, with crushed lime- 

 stone in prpper proportions. This mixture is run through a rotary 

 calciner, heated by waste kiln gases, in which the temperature is suffi- 

 cient not only to dry the mixture but also to partly powder it and 

 to reduce most of the limestone to quicklime. The mixture is then 

 pulverized and fed into rotary kilns. 



Of the three general processes above described the second is unsuited 

 to American conditions. The first and third are adapted to the use 

 of the rotary kiln. The third seems to be the most economical, and 

 has given remarkably low fuel consumption in practice, but so far has 

 not been taken up in the United States. 



Certain points of manufacture peculiar to the use of mixtures of 

 slag and limestone will now be described. 



Composition of the slag. The slags available for use in Portland- 

 cement manufacture are of quite common occurrence in iron-producing 

 districts. Those best suited for such use are the more basic blast- 

 furnace slags, and the higher such slags run in lime the more available 



