Slag, also called cinder because of its dark and crumbly composition, is a by-product created from smelting metal. Slag is comprised of the undesired impurities in the metal being smelted. It is comprised of a number of components, including oxides of sulfur, silicon, aluminum, and magnesium.
Sulfur is always present in slag. It also contains phosphorous, calcium, ash, and remnants of other materials like limestone and residual materials as a result of chemical reactions between the metal and the lining of the furnace. There can be other compounds found in slag depending on the type of metal being smelted.
Slag is formed by smelting metals in various ways. During the smelting process the metals oxidize. Slag forms a crust of oxides on top of the metal.The desired liquid metal underneath the slag is protected by this slag crust. When the metal is totally melted, the slag is skimmed off the top of the metal and disposed of in a slag heap to age. Exposure to the weather over a long duration enables the slag to cool and age until it is broken down and used for industrial purposes.
Slag can be used as an ingredient in concrete and cement. It can also be ground up to a fine consistency and used as a loose gravel or phosphate fertilizer. It also often mixed with other materials to be used on roadways or as a ballast on trains and trucks.