46 



The limestone used for flux at the furnace which supplies the 

 slag will usually be found to be of suitable composition for use 

 in making up the cement mixture. 



Economics of using slag-limestone -mixtures. The manufac- 

 ture of a true Portland cement from a mixture of slag and 1 lime- 

 stone presents certain undoubted advantages over the use of any 

 other raw materials, while it has also a few disadvantages. 



Probably the most prominent of the advantages lies in the 

 fact that the most important raw material the slag can usu- 

 ally be obtained more cheaply than an equal amount of natural 

 raw material could be quarried or mined. The slag is a waste 

 product, and a troublesome material to dispose of, for which 

 reasons it is obtained at small expense to the cement plant. An- 

 other advantage is due to the occurrence of the lime in the slag 

 as oxide, and not as carbonate. The heat necessary to drivte 

 off the carbon dioxide from an equivalent mass of limestone 

 is therefore saved when slag farms part of the cement mixture, 

 and very low fuel consumption is obtained when slag-limestone 

 mixture is burned, 



Of the disadvantages, the toughness of the slag and the ne- 

 cessity for drying it before grinding are probably the most im- 

 portant. These serve to partly counterbalance the advantages 

 noted above. A third difficulty, which is not always apparent at 

 first, is that of securing a proper supply of suitable slag. Un- 

 less the cement plant is closely connected in ownership with the 

 furnaces from which its slag supply is to be obtained, this diffi- 

 culty maiy become very serious. In a season when a good iron 

 market exists the furnace manager will naturally give little 

 thought to the question of supplying slag to an independent 

 cement plant. 



The advantages of the mixture, however, seem to outweigh 

 its disadvantages, for the manufacture of Portland cement from 

 slag is now a large and growing industry in both Europe and 

 America. Two Portland cement plants using slag and lime- 

 stone as raw materials have been established for some time in 

 this country, several others are in course of construction at pres- 

 ent, and it seems probable that in the near future Alabama will 

 join Illinois and Pennsylvania as am important producer of Port- 

 land cement from slag. 



