The purpose of the blast furnace is to chemically reduce and physically convert iron oxides into a liquid iron called " hot metal ". The raw materials require 6 to 8 hours to descend to the bottom of the furnace where they become the final product of liquid slag and liquid iron. These liquid products are drained from the furnace at regular intervals. Once a blast furnace is started it will continuously run for four to ten years with only short stops to perform planned maintenance.

Blast furnace slag is the co-product of molten iron in the blast furnace. The total basic elements, reported as equivalent calcium oxide and magnesium oxide divided by silica results in slag chemistry such as:

Calcium Oxide (CaO) 38%

Magnesium Oxide (MgO) 10%

Silicon Dioxide (SiO₂) 36%

with minor elements such as:

Iron Oxide (FeO & Fe₂O₃ ) <1%

Sulphur (S) 1%+

Alumina (Al₂O₃) 6-12%

The possible composition of a sample of blast furnace slag

Limestone is one of the raw materials, along with iron ore and coke that is put into the blast furnace. Since the limestone is melted (to become the slag which removes sulphur and impurities) the blast furnace operator may blend the different stones to produce the desired slag chemistry that creates optimum properties, such as having a low melting point and a high fluidity.

The beginning of the production of blast furnace slag is from the calcium oxide, which is formed from the decomposition of calcium carbonate (limestone):

CaCO₃ = CaO + CO₂

This calcium oxide is used to remove sulphur from the iron that is necessary before the hot metal becomes steel. This sulphur removing reaction is:

FeS + CaO + C = CaS + FeO + CO

The Calcium sulphide becomes part of the slag. The slag is also formed from any remaining silica, alumina, magnesium oxide or calcium oxide that entered the iron ore, pellets, sinter or coke. The liquid slag then trickles to the bottom of the furnace. Here, it floats on top of the liquid iron since it is less dense.

In the early stages of mass iron and steel production blast furnace slag was treated as a waste product of this fundamental industrial process. " Slag heaps " were proving to be unsightly eyesores throughout the countryside and the areas where the blast furnace was located.

Yet, from approximately the mid-stages of the 20^th century, blast furnace slag has been found to be relatively useful in various areas of industry. Possible manufactured forms of blast furnace slag, which is then transported elsewhere for various uses, are as follows:

• Air Cooled - Molten slag poured into pits in layers, allowed to solidify, cooled then crushed and screened.

• Expanded or lightweight - Produced by rapidly pouring molten slag into sloping pits, moist at the bottom to allow steam in while still viscous, causing many small air pockets, such as those in polystyrene.

• Granulated - Rapidly cooled by a high velocity mixing of molten slag flow with water jets.

• Pelletised - A variation of granulating in which less water is used.

These four types of manufactured blast furnace slag can be used for the following different areas of industry and construction:

• The production of glass - the main sources required are lime, silica and alumina and bottle makers can use blast furnace slag as a partial source of all three.

• The production of mineral wool - blast furnace slag has many of the mineral ingredients of mineral wool and is a principle raw material for its manufacture.

• Use as aggregate in cement concrete – blast furnace slag is often preferred when maximum durability, higher strength, hardness of aggregate, fire-resistance and lighter weight is required.

• Use as rail ballast - Coarse aggregate sizes of air-cooled blast furnace slag are all types of railroad tracks, from industrial spurs and yards to high-speed main lines. Slag provides better drainage because of its high void space, its cleanliness and its degradation.