Slag
The molten by-product formed when a flux combines with the gangue during smelting.
Unit
Dimensionless
In depth
Being lighter than the molten metal, the slag forms a floating layer that both allows easy separation and shields the metal from atmospheric oxidation. Modern practice recovers rather than dumps it, using granulated blast furnace slag as a cement replacement.
Examples in the real world
Calcium silicate slag tapped from a blast furnace and ground into road aggregate and cement.