Slags are a by-product of the smelting "Smelting (metallurgy)") of ores "Ore (mining)") used to purify metals. They can be considered a mixture of metal oxides, although they may contain metal sulfides and metal atoms in element form. Although they are often used as a waste removal mechanism in metal casting, they can also serve other purposes, such as assisting in temperature control during casting and minimizing reoxidation of the final liquid metal before entering the mold.[1]​.

In nature, metal minerals, such as iron, copper, aluminum and others, are found in an impure state, often oxidized and mixed with silicates of other metals.

During smelting, when the ore is exposed to high temperatures, these impurities separate from the molten metal and can be removed. The set of compounds that is removed is the slag.

Ferrous and non-ferrous smelting processes produce different slags. For example, non-ferrous copper and lead smelting is designed to remove the iron and silica that typically occur in these minerals and separate it as iron silicate-based slag. On the other hand, the slag from steel mills (called slurry "Slurry (metallurgy)"), in which a ferrous foundry is produced, is designed to minimize the loss of iron and therefore contains mainly calcium, magnesium and aluminum.[2]​.

Granular blast furnace slag is often used in combination with portland cement mortar as part of a cement mixture (the "blast furnace slag" component is added as an additive in cement manufacturing after sintering and grinding of the portland clinker). This type of slag reacts with water to produce cementitious properties. Mortar containing granular blast furnace slag develops great resistance over a long time and offers lower permeability and greater durability. Since the unit volume of portland cement is also reduced, the mortar is less vulnerable to alkali-silica and sulfate attack.

Recently, slag foaming has gained interest. This foaming of the slag is mainly caused by the generation of bubbles of carbon monoxide gas, carbon dioxide, water vapor, sulfur dioxide, oxygen and hydrogen inside the slag, which becomes foamy, like soapy water. In a basic oxygen furnace (HBO), slag foaming is caused by the combustion of carbon from the metal itself and is a drawback of the process; foaming slag can be ejected violently, removing metal from the furnace and creating dense, brown smoke that can cause problems in the smoke removal system or cause a health and safety problem. Foaming can be controlled by injecting gas into the base of the furnace or by introducing fine particles of coke into the furnace. scum.[3]​.