Cementitious Slag Makers Association

Ground granulated blastfurnace slag 'GGBS' is one of the ‘greenest’ of construction materials. Its only raw material is a very specific slag that is a by-product from the blast-furnaces manufacturing iron. Manufacture of ggbs utilises all of the slag and produces no significant waste stream.

As well as the environmental benefit of utilising a by-product, ggbs replaces something that is produced by a highly energy-intensive process. By comparison with Portland cement, manufacture of GGBS requires less than a fifth the energy and produces less than a fifteenth of the carbon dioxide emissions. Further 'green' benefits are that manufacture of GGBS does not require the quarrying of virgin materials, and if the slag was not used as cement it might have to be disposed of to tip.

Each year, the UK uses up to two million tonnes of GGBS as cement, which:

• reduces carbon dioxide emissions by some two million tonnes
• reduces primary energy use by two thousand million kWhs
• saves three million tonnes of quarrying
• saves a potential landfill of two million tonnes

The following table shows the effectiveness of ggbs in reducing the embodied carbon dioxide of concrete.

Concrete		CO2 per cubic metre (kgCO2/m3)			CO2 relative to PC-only
Designation	Typical application	PC concrete	30% fly ash concrete	50% ggbs concrete	30% fly ash concrete	50% ggbs concrete
GEN1	Blinding	208	161	104	78%	50%
GEN1 S4	Trench foundations	217	174	109	80%	50%
RC25/30	Reinforced foundations	291	233	155	80%	53%
RC28/35	Suspended floors	310	252	170	81%	55%
RC32/40	Structural	342	281	196	82%	57%
Note: Calculations are based on: - the typical mix designs used by BRE for calculation of 'Ecopoints' for concrete, - the data on embodied CO2 of UK cement, additions and cementitious material, referenced below.

Further information:

• In 2007, the average emission by the membership of the CSMA to produce 1 tonne of GGBS was 52kg CO2.

[This was calculated using the DEFRA Green House Gas Conversions (June 2007) and includes the energy required for granulation of the slag, its transport to the Slag Grinding Plant and the energy used in drying and grinding but does not include any of the impacts of Iron-making because the slag evolves irrespective of whether or not it can be used]

• Higgins D D, "Sustainable Concrete: How can Additions contribute?" The Institute of Concrete Technology Annual Technical Forum, March 2006. Click here

• Data on embodied CO2 of UK cement, additions and cementitious material. Click here

• An Environmental Profile prepared by BRE for GGBS for 2004. Click here

Note that in this profile:
(i) 'Climate change' is expressed as CO2 equivalent, and therefore includes the effects of global warming gases other than CO2
(ii) there is no allocation from the impacts of Iron-making.