MPA - essential materials, sustainable solutions
Quarry Products

Air-cooled basic oxygen steel slag (BOS)

What is BOS?

Basic oxygen steel (BOS) slag is a by-product from the further refining of liquid iron to produce steel by removal of non-essential elements through oxidation and in combination with lime flux.

BOS slag is produced by the basic oxygen steelmaking (BOS) process – also referred to as the basic oxygen furnace (BOF) process, as air cooled steel slag (ASS) or as LD-converter slag.

Production and Processing

BOS slag is formed during the conversion of hot liquid iron from the blast furnace into steel in a basic oxygen furnace.

In this process the hot liquid iron is treated by blowing oxygen to remove carbon and other elements that have a high affinity to oxygen.

Lime and dolomite are added (as fluxes) that combine with silicates and oxides to form liquid slag. Some amounts of scrap are also added in order to control the temperature of the exothermal reactions. The formed slag which in turn absorbs the oxidized impurities (Si, P, S and Mn) from the molten iron.

The BOS slag when removed is tipped into pits and air cooled with the assistance of water. Once cooled the excavated slag is processed through a metal recovery facility where entrapped steel is removed.

Following this process the slag is treated very similarly to virgin aggregates and is crushed and screened into size fractions and stockpiled.

Due to the presence of free lime (calcium oxide) and depending on the final end use of the BOS slag, the stockpiles may be weathered for a designated time period in controlled areas. Weathered slag is tested to check the expansion properties of the material as identified in BS EN 1744-1 to meet the requirements of its end use.

Typical Chemical Properties


% Value











Typical Physical Properties



Relative Density (SSD)


Water Absorption %


Magnesium Sulfate Soundness


Los Angeles


Aggregate Abrasion Value


Polished Stone Value



Applications to include:

  • Granular
  • General fills and capping
  • Pipe bedding
  • Aggregates for asphalt
  • Aggregates for surfacing dressing
  • Hydraulic Bound Mixtures
  • Agricultural
  • Filter media
  • Land remediation

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: URL link to Aggregates page:

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

For more information contact Mike Wharton

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 Mike Wharton ( or phone us on 020 7963 8000

Quarry Picture
You are in: Products - Slag - Air-cooled basic oxygen steel slag
Air-cooled blast furnace slag
Granulated blast furnace slag
Ground granulated blast furnace slag
EAF carbon steel slag
Air-cooled high alloy EAF and stainless steel slag
Print PagePrintable page
Top of page I Disclaimer I Sitemap I Valid HTML 4.01 and CSS