In metallurgy, slag is a by-product of smelting ores and recycled metals. It is a mixture of metal oxides and silicon dioxide, and can also contain metal sulfides and elemental metals. Slag is produced during the smelting process when impurities in the metal ore are separated from the molten metal and can be removed. The major components of slag include the oxides of calcium, magnesium, silicon, iron, and aluminum, with lesser amounts of manganese, phosphorus, and others depending on the specifics of the raw materials used. Slag can be classified based on the abundance of iron among other major components.

Slag has several uses in metallurgy and other industries. It can be used as a coarse aggregate in certain concretes, as a road material and ballast, and as a source of available phosphate fertilizer. Slag can also be used to create fibers used as an insulation material called slag wool. In modern times, slag is often used in concrete in combination with Portland cement as part of a blended cement. Ground granulated slag reacts with a calcium byproduct created during the reaction of Portland cement to produce cementitious properties. Slag is used in the manufacture of high-performance concretes, especially those used in the construction of bridges and coastal features. Its low permeability and greater resistance to chlorides and sulfates can help to reduce corrosive action and deterioration of the structure.