31 



inalyses of alkali waste. 



Of the analyses quoted in the preceding table, those in the 

 first and third columns represent materials which are actually 

 used in Portland cement manufacture in England and the 

 United States. The alkali wastes whose analyses are given in 

 the second and fourth columns are notably too high in mag- 

 nesia to be advisable for such use. 



Blast furnace slag. 



True Portland cements, which must be sharply distinguished 

 from the slag (or puzzolan) cements can be made from mix- 

 tures which contain blast furnace slag as one ingredient. In 

 this case the slag is intimately mixed with limestone and the 

 mixture is finely powdered. It is then burned in kilns and the 

 resulting clinker pulverized. 



The slags from iron furnaces consist essentially of lime 

 (CaO), silica (Si()2), and alumina (A\2O^) ; though small 

 percentages of iron oxide (FeO), magnesia (MgO), and sul- 

 phur (S), are commonly present. Slag may therefore be re- 

 garded as a very impure limestone or a very calcareous clay, 



The slag used at a German Portland cement plant has the 

 following range in composition. 



