414 



CEMENTS, LIMES, AND PLASTERS. 



Grinding the slag. Slag can be crushed with considerable ease to 

 about 50-mesh, but notwithstanding its apparent brittleness it is diffi- 

 cult to grind it finer. Until the introduction of the tube mill, in fact, 

 it was almost impossible to reduce this material to the fineness neces- 

 sary for a cement mixture, and the proper grinding of the slag is still 

 an expensive part of the process, as compared with the grinding of 

 limestone, shales, or clay. 



Composition of the limestone. As the slag carries all the silica and 

 alumina necessary for the cement mixture, the limestone to be added to 

 it should be simply a pure lime carbonate. 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 manufacture of 

 a true Portland cement from a mixture of slag and limestone 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 usually be ob- 

 tained more cheaply than an equal amount of natural raw material 

 could be quarried or mined. The slag is a waste product, and a trouble- 

 some material to dispose of, for which reason it is obtained at small 

 expense to the cement-plant. Another advantage is due to the occur- 

 rence of the lime in the slag as oxide, and not as carbonate. The heat 

 necessary to drive off the carbon dioxide from an equivalent mass of 

 limestone is therefore saved when slag forms part of the cement mixture^ 

 and very low consumption is obtained when slag-limestone mixture 

 is burned. 



Of the disadvantages, the toughness of the slag and the necessity 

 for drying it before grinding are probably the most important. 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. Unless the cement-plant is closely 

 connected in ownership with the furnaces from which its slag supply 

 is to be obtained, this difficulty may 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 dis- 

 advantages, for the manufacture of Portland cement from slag is now 

 a large and growing industry in both Europe and America. Two Port- 

 land-cement plants using slag and limestone as raw materials have been 



