44 



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 accomplished in ball and tube 1 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 

 limestone in proper proportions. This mixture is run through 

 a rotary calciner, heated by waste kiln gases, in which the tem- 

 perature is sufficient not only to dry the mixture, but also to 

 partly po'wder it, and to reduce most of the limestone to quick- 

 lime. 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 adapt- 

 ed to the use of the rotary kiln. The third seems to be the 

 most economical, and has given remarkably low fuel consump- 

 tion in practice, but so far has not been taken up in the United 

 States. 



Certain points oif manufacture peculiar to the use of mix- 

 tures of slag and limestone will now be described. 



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

 land 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 they are for this use. The slags 

 utilized will generally runi from 30 to 40 per cent. lime. The 

 presence of over 3 peir cent, or so of magnesia in a slag is of 

 course enough to render its use as a Portland cement material 

 inadvisable ; and on this account slags from furnaces using do- 

 lomite (magnesian limestone) as a flux, are unsuited for ce- 

 ment manufacture. The presence of any notable percentage of 

 sulphur is also a drawback, though, as will be later noted, part 

 of the sulphur in the slag will be removed during the process 

 of manufacture. 



Granulation of slag. If slag be allowed to cool slowly it 

 solidifies into a dense, tough material, which is not readily re- 

 duced to the requisite fineness for a cement mixture. If it be 

 cooled suddenly, however, ais by bringing the stream of molten 

 slag into contact with cold water, the slag is "granulated," i. e., 

 it breaks up inito small porous particles. This granulated slag 

 or "slag sand" is much more readily pulverized than a slowly 



