UK GHG Inventory Improvement project: Carbonation of Concrete Emissions Sink Modelling: BEIS Tender Reference CR21048

As the binder in concrete and products such as mortar, Portland cement is one of mankind’s most widely used materials. Portland cement clinker is made by heating limestone with clay or shale to very high temperatures. ‘Calcination’ breaks down the limestone into calcium oxide, the key ingredient of cement, and carbon dioxide, CO2. The cement clinker is ground with gypsum and other materials to make cement. In 2020 in the UK, direct emissions from cement production were 5.81Mt
CO2; 3.90Mt CO2 due to the calcination reaction and the remaining 1.91Mt CO2 from combustion of fuels within the cement kiln.

‘Recarbonation’, or carbonation, describes how concrete and other cement-containing products naturally reabsorb and permanently store CO2 during their life, in effect reversing the calcination reaction. Carbonation is scientifically well established and has been recognised by the Intergovernmental Panel on Climate Change (IPCC)1 (AR6, Chapter 5) as an important carbon emissions sink. Construction industry product standards2,3 for life cycle assessment (LCA) and
environmental product declarations (EPDs) include established methodologies for calculating concrete carbonation. However, there is currently no IPCC method for calculating national estimates of the carbonation sink in greenhouse gas (GHG) national inventory reports (NIR). In 2018, IVL Swedish Environmental Institute4,5 proposed methodologies at three tiers of increasing complexity – in line with IPCC requirements. Sweden first included a Tier 1 calculation of the carbonation concrete emissions sink – 297 ktonne CO2 in Sweden in 2018 – in its 2020 NIR
submission.