Minimising the carbon impact of concrete

We often villify “concrete” with the same high-carbon credential as “cement”, but concrete is a composite of a number of different materials, many of which are commonly used for other purposes in construction and of comparatively low embodied carbon. The way we put these materials together defines the carbon impact of the concrete we use. Useful guidance on the embodied carbon of the commoner concrete components can be found in the Mineral Products Association Factsheet 18, from which the following list is drawn:

  • Cement (OPC Cem I): 913 kgCO2e/tonne
  • Limestone fines: 75 kgCO2e/tonne
  • Ground granular blastfurnace slag (GGBS): 67 kgCO2e/tonne
  • Coarse natural aggregates: 5 kgCO2e/tonne
  • Fine natural aggregates: 5 kgCO2e/tonneConcrete carbon
  • Pulverised fuel ash (PFA) or flyash: 4 kgCO2e/tonne

Designing concrete mixes carefully to minimise the quantity of cement used by replacing part of it by other pozzalanic materials such as GGBS and PFA can have a significant effect on the overall embodied carbon of the mix, typically by 30% or more. This can be seen by reference to the MPA’s Sustainable Concrete Forum’s publication on the embodied CO2e of concretes used in building, from which the data in the table above right has been abstracted (Units: kgCO2e/m3 of concrete). Combined with careful structural design to optimise the volumes of concrete used, careful mix specification can have a dramatic effect on the overall embodied carbon whenever concrete is used.