Get Sust! Issue 30

How big is a concrete footprint?

The British Cement Association (BCA) says around 20% of the direct carbon dioxide (CO2) emissions from the manufacture of cement is reabsorbed by concrete over its lifecycle, and concludes that, in whole-life terms, this reduces the impact of one tonne of cement from an initial 0.8 tonnes of CO2 to approximately 0.65 tonnes.

Re-carbonation - the process by which carbon dioxide is re-absorption from the atmosphere by cementitious materials during their service life or following recycling - is sometimes overlooked in the environmental debate, so the BCA commissioned research to find out what contribution re-carbonation should play in calculations of the materials ‘carbon footprint’.

Their study found that of the 0.8 tonnes CO2 emitted during the manufacture of one tonne of UK cement around 40% is from the fuel used, and 60% from the thermal decomposition of calcium carbonate (CaCO3), otherwise known as calcination. This latter reaction is partly reversible, absorbing CO2 from the atmosphere.

Re-carbonation can only occur where air can circulate around the material, so this process is less significant for structural concrete; but where concrete is recycled at the end of its structural life (e.g. when it is crushed for reuse as an aggregate) CO2 is more readily absorbed, even when the aggregate is used in ground works.

And in low-strength un-reinforced concrete such as blocks, and cementitious materials such as mortar, re-carbonation is much more rapid during the service life, because CO2 can permeate the material more easily.

The BCA concludes that, although re-carbonation has no impact on the CO2 emissions incurred during cement manufacture, it is certainly worth taking account of the effects when reviewing the whole-life performance the materials.

Learn more:

	•	For further information please contact Tom De Saulles at the BCA: 0126 608714.
	•	Source: Innovation and Research Focus, No. 70 (August 2007) www.innovationandresearchfocus.org.uk.