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Slag is a waste product of various ores after going through metal smelting (pyrometallurgical) processes. It can be classified into two types of slag, which are ferrous and non-ferrous. Many cases were studied on slags along with their applications.

Ore smelting
In nature, iron, copper, lead, nickel, and other metals are found in impure states called ores, often oxidized and mixed in with silicates of other metals. During smelting, when the ore is exposed to high temperatures, these impurities are separated from the molten metal and can be removed. Slag is the collection of compounds that are removed. In many smelting processes, oxides are introduced to control the slag chemistry, assisting in the removal of impurities and protecting the furnace refractory lining from excessive wear. In this case, the slag is termed synthetic. A good example is steelmaking slag: quicklime (CaO) and magnesite (MgCO3) are introduced for refractory protection, neutralizing the alumina and silica separated from the metal, and assisting in the removal of sulfur and phosphorus from the steel.

As a co-product of steelmaking, slag is typically produced either through the blast furnace - oxygen converter route or the electric arc furnace - ladle furnace route. To flux the silica produced during steelmaking, limestone and/or dolomite are added, as well as other types of slag conditioners such as calcium aluminate or fluorspar.

Compositions
Slag is usually a mixture of metal oxides and silicon dioxide. However, slags can contain metal sulfides and elemental metals.

The major components of these slags 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. Furthermore, slag can be classified based on the abundance of iron among other major components.

Classifications
There are two types of slag: ferrous and non-ferrous slags, which are produced through different smelting processes.

Ferrous slag
During the process of smelting iron, ferrous slag is created, but dominated by calcium and silicon compositions. Through this process, ferrous slag can be broken down into blast furnace slag (produced from iron oxides of molten iron), then steel slag (forms when steel scrap and molten iron combined). The major phases of ferrous slag contain calcium-rich olivine-group silicates and melilite-group silicates.

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 aluminium. As the slag is cooled down by water, a several chemical reactions from a temperature of around 2600 F (like oxidization) taken place within the slag.

Non-ferrous slag
Non-ferrous slag is produced from non-ferrous metals of natural ores. Non-ferrous slag can be characterized into copper, lead, and zinc slags due to the ores' compositions, and they have more potential to impact the environment negatively than ferrous slag.

The smelting of copper, lead and bauxite in non-ferrous smelting, for instance, is designed to remove the iron and silica that often occurs with those ores, and separates them as iron-silicate-based slags.

pH influences
One ground water case study in Lake Calumet, Illinois shows that steel slag can form extremely alkaline ground water by rising the pH consistently up to 12.8. Slag contains calcium silicates (CaSiO4) which react with water and produce Ca-OH ions that leads to a higher concentration of hydroxide (OH-) in ground water. When ground water discharges and is in contact with CO2 in the atmosphere, calcite precipitation forms and accumulates as thick as 20cm. As the pH increases, many other metals also from slag such as iron (Fe), manganese (Mg), nickel (Ni), and molybdenum (Mo) become more insoluble in water and mobile as particulate matters. The most effective method to detoxicate alkaline ground water discharge is air sparging.

Trace elements
Based on a case study at the Hopewell National Historical Site in Berks and Chester counties, Pennsylvania, USA, ferrous slag usually contains lower concentration of various types of trace elements than non-ferrous slag. However, some of them, like arsenic (As), Fe, and Mn, were accumulated up in the groundwater and surface water and exceeded the environmental guidelines standards.

Copper slag
Copper (Cu) slag is the waste product of smelting copper ores, which was found in an abandoned Penn Mine in California, USA. For 6 - 8 months per year, this region is flooded and become a reservoir for drinking water and irrigation. Samples collected from the reservoir showed the higher concentration of Cadmium (Cd) and Lead (Pb) that exceeded regulatory guidelines.

Applications
Slags can serve other purposes, such as assisting in the temperature control of the smelting, and minimizing any re-oxidation of the final liquid metal product before the molten metal is removed from the furnace and used to make solid metal. In some smelting processes, such as ilmenite smelting to produce titanium dioxide, the slag can be the valuable product.

Ancient uses
During the Bronze Age of the Mediterranean there were a vast number of differential metallurgical processes in use. A slag by-product of such workings was a colorful, glassy, vitreous material found on the surfaces of slag from ancient copper foundries. It was primarily blue or green and was formerly chipped away and melted down to make glassware products and jewelry. It was also ground into powder to add to glazes for use in ceramics. Some of the earliest such uses for the by-products of slag have been found in ancient Egypt.

Historically, the re-smelting of iron ore slag was common practice, as improved smelting techniques permitted greater iron yields—in some cases exceeding that which was originally achieved. During the early 20th century, iron ore slag was also ground to a powder and used to make agate glass, also known as slag glass.

Modern uses
Ground granulated 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. Concrete containing ground granulated slag develops strength over a longer period, leading to reduced permeability and better durability. Since the unit volume of Portland cement is reduced, this concrete is less vulnerable to alkali-silica and sulfate attack.

Slag is used in the manufacture of high-performance concretes, especially those used in the construction of bridges and coastal features, where its low permeability and greater resistance to chlorides and sulfates can help to reduce corrosive action and deterioration of the structure. The slag can also be used to create fibers used as an insulation material called slag wool.

Because of the slowly released phosphate content in phosphorus-containing slag, and because of its liming effect, it is valued as fertilizer in gardens and farms in steel making areas. However, the most important application is construction.