Slag is a nonmetallic by-product of smelting. Smelting is the process by which a metal is separated from its ore (a mineral compound that is mined for one of the elements it contains, usually a metal element) in a blast furnace (a furnace in which combustion is made more intense by a forced stream of air). Slag is formed in the smelting of iron, copper, lead, and other metals.
Slag consists primarily of silica (a white or colorless crystalline compound) and lime. It is used as an ingredient in cements, concrete, and roofing materials. It is also placed under roads and railways to lend stability.