What is Granulated Blast Furnace Slag?

Blast furnace slag is a byproduct of the iron and steel manufacturing process. In the production of steel, iron ore – a mixture of oxides of iron, silica, and alumina – together with a fuel consisting of coke, natural gas, oxygen and pulverized coal and also limestone as a fluxing agent, are fed into a blast furnace, which consists of a large vertical chamber through which large volumes of hot air are blasted. Generally a blast furnace operates on a continuous basis and produces approximately 250 – 300 kg of slag/t of iron produced.

The chemical reaction results in two products: molten iron metal and molten blast furnace slag. At this stage molten blast furnace slag can be converted into two different products; (1) blast furnace slag aggregate (BFS) or (2) granulated blast furnace slag (GBFS). To produce GBFS, liquid slag must be rapidly quenched using large volumes of high-pressure fresh water sprays, instantly cooling the molten material to produce a sandy material with a maximum size of 6 mm.

Typically BFS is then collected from the manufacturing source or blast furnace granulation bays and transported to the nominated processing facilities. The GBFS is de-watered and dried to reduce the moisture content before further processing (grinding) in a traditional cement clinker grinding plant, then ground to a fine powder. At this point the ground granulated blast furnace slag or GGBFS is stored in silos awaiting delivery to customers.

The reactive properties of GBFS depend upon a number of factors, including:  

1. Temperature of slag before granulation.

2. Chemical composition.

3. Condition during granulation (flow rates and temperature).

4. Fines content.

GGBFS is mostly used in the production of quality-improved slag cement, namely blast furnace cement (BFC).