Conversion of molten iron to steel at an integrated steelworks takes place at the Basic Oxygen steelmaking shop. The BOS vessel is an open topped container with a removable cover into which is placed approximately 50 tonnes of steel scrap plus some 200 tonnes of molten iron brought direct from the blast furnace in brick lined ladles. An oxygen lance is lowered into the vessel and a violent reaction takes place. Lime is added to form a protective slag layer. After the desired carbon level is achieved, components are added to the vessel to produce the required steel specification.

On completion of the reaction, the steel is drained into one ladle and the slag is poured into another. The ladle of molten slag is then poured into ground bays where after initial solidification; the slag is cooled with water sprays.

BOS slag is dark gray in color and very hard and tough compared to blast furnace slag. It has a density about 20/25% greater than basalt or BF slag. It crushes to a cubical shape.

BOS, in common with most steel slags, is subject to slight dimensional expansion until it fully hydrates. Until this occurs, care should be taken not to place it in rigidly confined spaces.

• In areas where there has been a surplus of blast furnace slag over market demand, BOS slags have been mainly used in landfill. However, in more recent times, the advantages offered by its free lime content have been recognized with the development of self cementing, heavy duty pavements. One example is the use of a 300 metre long pavement section leading to a truck arrestor bed on Mt Ousley adjacent to Wollongong, NSW. The mix used comprised 60% crusher run 20mm maximum BOS slag with 40% granulated BF slag. As no binders were necessary, the mix proved to be very economic. UCS strength of 5mpa was achieved at 28 days damp.
   
• As a tough durable rail ballast: The entire 80km rail network in the Port Kembla BHP Steel complex, which is subject to heavy industrial loading, uses BOS slag ballast.
   
• As an aggregate in asphaltic concrete: BOS slag has increased skid resistance compared to basalt and its tough characteristics make it an ideal aggregate for this application. Resistance to rutting makes BOS slag asphalt particularly applicable for heavily trafficked corners and stopping zones.