Assessing the value of concrete and cement as a carbon sink

This week, the Mineral Products Association (MPA) prepared a 112-page report for the UK Department of Energy Security and Net Zero (DESNZ) to ascertain a two-tier recarbonation model that measures the amount of CO2 absorbed from the atmosphere by concrete in the UK.

'Recarbonation' or carbonation describes how concrete and other cement-containing products naturally absorb and permanently store CO₂ during their lifetime, reversing the calcination reaction. While the Intergovernmental Panel on Climate Change (IPCC) recognises carbonation as a carbon emission sink, there is no current IPCC method for calculating natural estimates of the carbon sink. The MPA and a consortium of experts including Ricardo, the UK’s greenhouse gas (GHG) inventory agency, and others, have developed a UK-specific model  to calculate the UK emissions sink from the carbonation of concrete for consideration by the DESNZ. 

Developing the model
To develop a Tier-2 model, annual concrete consumption needed to be disaggregated into at least five primary concrete applications covering 65 per cent of national cement consumption. The primary applications included:
• building of steel composite frame construction type, for example, offices and other commercial buildings
• buildings of concrete-frame construction type, for example, high-rise residential
• buildings of masonry construction type, for example, low-rise residential
• infrastructure projects
• mortar and merchant sales of bagged cement.

For each primary use application, a typical UK-specific building/infrastructure prototype was identified and a CO₂ emission sink factor (EF) calculated. Crucially, the model re-adjusts the annual distribution of concrete between applications to ensure consistency between the top-down cement production and bottom-up concrete consumption. The adjusted activity data for each application is then multiplied by the EF to calculate the carbon sink. The MPA also developed a UK-specific model for end-of-life and secondary use concrete.

UK carbon sink results
The report concluded that in 2020 the UK’s estimated emission sink for concrete carbonation was calculated as 1,548kt CO₂, comprising 862kt CO₂ from concrete in primary use and 686kt CO₂ from concrete at endof-life and in secondary use. This equates to 40 per cent of the CO₂ calcination emissions from UK cement production, or 31 per cent of the CO₂ calcination emissions from UK cement consumption, and 0.4 per cent of the total UK GHG emissions.

Summary
While the MPA suggests that its model would benefit from improvement in bottom-up activity data, with a view to further disaggregation, especially for the infratsructure sector and more experimental data on the rate and degree of carbonation within UK concretes for end-of-life and secondary use concrete, some wider implications can be deduced from the modelling.

The carbon sink can be incorporated within the next UK National Inventory Report, and into other UK GHG evidence and reporting systems, such as carbon budgets. International collaboration will help other countries to develop similar models, with the support of the IPCC TFI Technical Support Unit and emission factors database (IPCC EFDB) Panel. Furthermore, it will be possible to maximise the permanent carbon sink potential of concrete by encouraging and incentivising optimal practices in concrete demolition, end-of-life, and reuse, and by R&D and scaling up of industrial enhanced carbonation technologies, for example, CO₂ curing of concrete products.