

CHAPTER XXVI. 

 ALKALI WASTE: BLAST-FURNACE SLAG. 



THE two raw materials to be discussed in the present chapter agree 

 in being waste products or by-products of other industries which, because 

 of their chemical composition, can be used in Portland-cement manu- 

 facture. In almost every other respect they differ. Alkali waste is 

 a fine-grained, soft and pure form of lime carbonate. Slag is very hard, 

 coarse-grained, and is composed of lime (CaO), silica, and alumina. 



Waste products or by-products can, of course, be usually obtained 

 at a low or nominal cost, and on this account both slag and alkali waste 

 assume an importance entirely out of proportion to their other proper- 

 ties. But it must be recollected that as by-products their production 

 and quality depend entirely upon the condition of the industries of 

 which they are wastes, and that no furnace manager or alkali-works 

 superintendent will run his plant solely in order to turn out a by-product 

 regular in amount and composition. For this reason it is essential that 

 a cement-plant using a waste product must be closely identified in owner- 

 ship with the furnace or works which furnishes this waste product. 

 Common ownership is practically the only way of insuring .a sufficient 

 and regular supply of satisfactory composition. 



Alkali Waste. 



A large amount of waste material results from the processes used 

 at alkali works in the manufacture of caustic soda. This waste mate- 

 rial is largely a precipitated form of calcium carbonate, and if sufficiently 

 free from injurious impurities, furnishes a cheap source of lime for use 

 in Portland-cement manufacture. The value of the waste for this pur- 

 pose depends largely on the process from which it resulted. 



Leblanc-process waste. The waste resulting from the Leblanc 

 process carries a very large percentage of sulphur, mostly in the form 

 of lime sulphides, carried over from the pyrites used in the process. A 



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