At an integrated steelworks, the first step in the production of steel is to manufacture iron from iron ore. Australian iron ores are amongst the purest in the world with around 60% iron content.

The iron ore, together with coke as a fuel and limestone as a fluxing agent, is fed into a blast furnace, which consists of a large vertical chamber, into the lower section of which is blasted large volumes of hot air.

This process operates continuously with the column of raw materials slowly settling as the iron is melted from the ore in a reducing atmosphere, to molten pool around 1600 oC at the bottom of the furnace.

Slag, being comprising the lime from the limestone and the silicates and aluminates from the iron ore and coke, forms as the iron is reduced and the coke is consumed as energy.

Being lighter, the slag floats on the molten “liquids” pool at the bottom of the furnace. Every two to three hours, the molten iron and slag are drained from the furnace.

Approximately 300kg of slag are produced for each tonne of iron. The furnace operates on a continuous basis, 365 days a year, for a furnace life of approximately 10 years.

After separating from the heavier iron, slag can be allowed to run into or to be transported to ground bays to air cool.  There it solidifies and becomes rock, or crystalline blast furnace slag.

Rock slag is typically a grey vesicular material that is lighter than most basalts. It is both pozzolanic with some self-cementing properties, which increases in proportion to the amount of fines present.

Although called air-cooled slag, after initial solidification occurs in the ground bays, the slag is sprayed with water to reduce temperatures and to induce fracturing of the slag mass. Therefore, when dug, usually with a wheel loader, the slag readily breaks up with an appropriate maximum lump size of 300mm, and graded down to fine material.

As dug, the rock slag is known as uncrushed blast furnace slag. It is processed with conventional crushing and screening machinery as used in hard rock quarries to produce a similar range of products from aggregates form concrete to road bases and manufactured sands.

When crushed slag fractures to an excellent cubical shape with relatively few elongated pieces.