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Air-cooled blast furnace slag (ABS)

What is ABS?

ABS is a crystalline by-product produced by the solidification of molten blastfurnace slag.

Production and Processing

Iron production is via a continuous batch process with the iron and slag produced in the blastfurnace at a temperature around 1500ºC. The furnace is fed repeatedly with materials in a sequence of a 'coke dump', followed by a 'ferrous dump'. The 'ferrous dump' typically consists of a pre-blend of mainly sinter, but also graded lump iron ore and iron ore pellets, also known as acid pellets. Sinter typically consists of 10% lime derived from calcined dolomitic limestone and crushed fine ores which consist of 60% Fe ( 90% Fe2O3 ), silica and alumina depending on the source. Approximately 10-20% of sinter is recycled materials from the iron and steel works. The acid pellets consist of metal iron ore balls, approximately 1cm in diameter which have been fired with bentonite as a binder. The 'ferrous dump' pre-blend, also known as the 'burden', is introduced into the top of the blastfurnace via a hopper with an airlock to prevent gas escape. The burden uniformity controls the BFS composition.

The combined oxides form blastfurnace slag, which has a lower density than molten iron and hence floats as a separate layer on top of the iron. The molten slag and iron collect at the 'hearth' at the bottom of the furnace. The iron and slag are 'tapped' (removed) together at regular intervals, held in a trough to allow separation, then the slag is allowed to run into open air pits where it cools in layers. The cooling is facilitated by water spraying which causes the slag to crystallise and crack, aiding excavation from the pit. The cooling conditions affect gas liberation (i.e. escaping gases become trapped in the slag), which in turn affects the slag porosity and density.

Typical Chemical Properties   


Typical value











Typical Physical Properties


Typical value

Relative Density (SSD)


Water Absorption


Magnesium Sulfate Soundness


Los Angeles


Aggregate Abrasion Value


Polished Stone Value



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Standards and Specifications Applicable to the Use of Metalliferous Slag Aggregates

For further information concerning the standards and specifications applicable to Slag Aggregates refer to:

For case studies applicable to the use of slag aggregates refer to:

For more information contact Martin Bridges

Please note this website is maintained to provide information and guidance on UK issues, products and applications of those products.


For more information on aggregates, please contact ( or phone us on 020 7963 8000

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