Ground granulated blastfurnace slag (ggbs)

Ground granulated blastfurnace slag (ggbs) is a by-product from the blast-furnaces used to make iron. These operate at a temperature of about 1,500 degrees centigrade and are fed with a carefully controlled mixture of iron-ore, coke and limestone. The iron ore is reduced to iron and the remaining materials form a slag that floats on top of the iron. This slag is periodically tapped off as a molten liquid and if it is to be used for the manufacture of ggbs it has to be rapidly quenched in large volumes of water. The quenching optimises the cementitious properties and produces granules similar to a coarse sand. This 'granulated' slag is then dried and ground to a fine powder.

The major use of ggbs is in ready-mixed concrete, and it is utilised in a half of all UK 'ready-mix' deliveries. Specifiers are well aware of the technical benefits which ggbs imparts to concrete, including:
  • Workability - making placing and compaction easier.
  • Lower early-age temperature rise, reducing the risk of thermal cracking in large pours.
  • Elimination of the risk of damaging internal reactions such as ASR.
  • High resistance to chloride ingress, reducing the risk of reinforcement corrosion.
  • High resistance to attack by sulphate and other chemicals.

In the production of ready-mixed concrete, ggbs replaces a substantial portion of the normal Portland cement content, generally about 50%, but sometimes up to 70% or more. The higher the proportion, the better the durability. The disadvantage of the higher replacement level is that early-age strength development is somewhat slower.

Ggbs is also used in other forms of concrete, including site-batched and precast. Unfortunately, it is not available for smaller-scale concrete production because it can only be economically supplied in bulk. Ggbs is not only used in concrete: other applications include the in-situ stabilisation of soil.

Environmental sustainability and GGBS

Because ggbs is derived as a by-product from another industry its use is an example of industrial ecology. It can be used un-ground as a coarse aggregate or as a supplementary cementitious material, where it can replace up to 70% of cement in a concrete mix and reduce carbon dioxide emissions per tonne of concrete by up to 60 or 70%.

 

Map of UK plants and depots (Glasgow and Belfast are depots)

 

Diagram showing the production of ggbs

 

 

Further information

For more information on ggbs, visit the CSMA website.

Download table: Embodied carbon dioxide of concrete mixes including ggbs.