Slag is a by-product of smelting ores and recycled metals. It is formed during processes like smelting or other combustion and metallurgical processes. Slag is mainly a mixture of metal oxides and silicon dioxide, but it can also contain metal sulphides and elemental metals. The gangue present is treated with flux to form the fusible slag. Slag consists mostly of mixed oxides of elements such as silicon, sulfur, phosphorus, and aluminum; ash; and products formed in their reactions with furnace linings and fluxing substances such as limestone. The molten metal is separated from the impurities as slag is formed. It floats on the surface of the metal above, to prevent the metal from coming in contact with the atmosphere. Slag doesn’t allow the metal to get oxidized.

Slag is classified as ferrous (by-products of processing iron and steel), ferroalloy, and non-ferrous (by-products of processing other metals) . Slag from steel mills in ferrous smelting is designed to minimize iron loss, which gives out the significant amount of iron, following by oxides of calcium, silicon, magnesium, and aluminum. Slag forms a coarse aggregate used in certain concretes, and it is used as a road material and ballast and as a source of available phosphate fertilizer.

Examples of slag include alumina and silica.