Smelting waste, commonly called slag, is the byproduct of metal extraction from natural ores. It is usually composed of metal oxides and a significant amount of silicon dioxide. Slag can be used to modulate the chemistry inside a smelter, but it must eventually be disposed of.
Slag can sometimes be recovered and sold for new applications after the useful ores have been extracted from it. Slag works as a filler for cement masonry, and it can be laid down as ballast for road beds. It can also be ground up and suspended in roofing tar to form an aggregate roof for homes and industrial buildings.