640 CEMENTS, LIMES, AND PLASTERS. 



be slightly modified by the presence of the elements taken up from the 

 fuel. The slags or " cinders" obtained in refining the metals differ 

 from the normal slags in that they may contain a very appreciable 

 percentage of metal, sufficient in many cases to justify further treat- 

 ment of the slag in order to recover its metallic contents. As this utili- 

 zation of such slags is entirely a metallurgical operation, they will not 

 be further discussed in the present volume. 



While many elements may occur in slags, those which are of universal 

 or even common occurrence are relatively few. The slags most com- 

 monly formed are silicates, consisting essentially of silica, oxides of the 

 alkaline elements, and certain metallic oxides, these last, with the ex- 

 ception of alumina, being usually present in small quantity only. In 

 certain metallurgical operations, however, the percentage of metallic 

 oxides may rise so as to make them important ingredients in the slag. 

 According to the processes, ores or fluxes used, slags may also contain 

 more or less phosphoric anhydride, sulphur and fluorine. 



The particular use, or uses to which the slag from any given furnace 

 may be most profitably put, will depend upon several factors. When 

 considering possible utilizations, the most important factor will gen- 

 erally be found to be the chemical composition of the slag. It is true 

 that, for certain uses, as for example highway macadam and railroad 

 ballast, the physical condition of the slag is of rather more importance 

 than its chemical composition; but the two utilizations named are 

 among the less profitable, and are only to be considered when the slag 

 cannot be disposed of more profitably. Local conditions, under which 

 head may be grouped questions of furnace management, possible markets, 

 and transportation routes and charges, will be found to be of great 

 economic importance. These factors are, however, too variable to be dis- 

 cussed in the present volume, with one exception. The exception noted 

 is the effect of slag utilization upon the general furnace management. 

 The furnace manager who is endeavoring to profitably utilize his slag 

 will often find it necessary to consider how far he may economically 

 go in changing details of his main process in order to increase the value 

 of his by-product. This is particularly the case where the slag is used 

 for cement. 



Blast-furnace slags of certain types have been used extensively 

 in Europe, and to a less extent in the United States, in the manufacture 

 of slag cement. The following chapters will therefore be devoted to a 

 discussion of the materials, manufacture and properties of slag cements. 



