Concrete carbonation

Over time, CO₂ in the atmosphere reacts with the calcium oxide in concrete to form calcium carbonate; a process called carbonation. 

This is essentially a natural reversal of the chemical process that occurs when making the cement used in concrete i.e. the calcination of lime that takes place in cement kilns, which in the UK, currently accounts for around 70% of the CO₂ emissions from cement manufacture. Carbonation is a slow and continuous process that progresses from the outer surface moving inwards permanently storing the CO₂. 

Over the lifecycle of concrete, carbonation will result in the reabsorption of around a third of the CO₂ emitted when making cement, significantly reducing the whole-life CO₂ footprint of both the cement and the concrete for which it is used. For this reason, it is important to ensure the environmental benefit of carbonation is accounted for when carrying out a life cycle assessment of concrete and buildings constructed from it. 

If the carbonation front reaches steel reinforcement it can cause corrosion, so the mix design of structural concrete purposefully limits the rate of carbonation, preventing this problem from occurring during the life of buildings and infrastructure. There is, however, a greater degree of carbonation during the end-of-life stage, when concrete is crushed for reuse as an aggregate. 

The crushing process substantially increases the material’s surface area, allowing CO₂ to be more readily absorbed. Although the deconstruction and demolition process at end-of-life can be comparatively brief, the resulting carbonation during this phase is significant. 

In addition to direct absorption of atmospheric CO₂, the newly crushed concrete aggregate also undergoes carbonation as a consequence of leaching from exposure to rain; a process that has been shown to significantly increase the rate of carbonation. Further CO₂ uptake occurs during the material’s secondary-life stage, when the recycled aggregate is used in a range of applications. 

In lower strength concrete where no steel reinforcement is used, such as blocks, carbonation is more rapid during its service life, as CO₂ can permeate the material more easily. In addition to the absorption of CO₂, the carbonation process is also likely to increase the strength of these materials, and with no steel reinforcement present, their serviceable lifespan has the potential to be measured in hundreds rather than tens of years. The Pantheon in Rome, constructed around 1900 years ago provides demonstrable evidence of this. 

Concrete carbonation is scientifically well established and has been recognised by the Intergovernmental Panel on Climate Change (IPCC) as an important carbon emissions sink (AR6, WG11, Chapter 5). The IPCC has now commissioned a new Methodology Report on carbon dioxide removal technologies, carbon capture utilization and storage to be completed by 2027. 

Calculation of CO₂ uptake by carbonation  

For accurate carbon accounting, the carbon emissions sink from concrete carbonation needs to be included in concrete Environmental Product Declarations (EPDs) and project-level life cycle assessments of buildings, as well as in national greenhouse gas inventories. 

EN 16757 

Annex G in British Standard EN 16757:2022, provides a standard method to calculate the CO₂ uptake from concrete carbonation. It includes detailed worked examples applying the method to the primary use phase (EPD module B) of concrete structures. However, the method can be used to calculate the carbon emissions sink due to carbonation in any stage of the concrete life cycle. 

EN 16757 gives values for the rate at which carbonation progresses through the concrete and the proportion of CO₂ absorbed, for a range of concrete strengths and exposures. The rate of progression of carbonation is highest for concrete exposed to indoor environments. However, the proportion of CO₂ absorbed, or degree of carbonation, is only 40% of the corresponding cement calcination emissions, whereas it is 75-85% in outdoor environments or in the ground. 

In lower carbon concretes containing SCMs or additions, (such as Fly Ash and GGBS), carbonation progresses more quickly than in a CEM I mix of the same strength, but the potential total CO₂ uptake is lower – due to the lower upfront embodied carbon. 

National Provisions 

EN 16757 allows for the use of ‘national provisions’ for carbonation of end-of-life and secondary use concrete in the form of scenarios that are representative of typical practice and thus provide a unified calculation method. The end-of-life (module C) scenario should account for the post-crushing concrete aggregate sizes/distribution and the average time subsequently spent stockpiled awaiting demand. The secondary use scenario (module D/beyond the system boundary) should consider carbonation that results from reuse, recovery, and recycling of the concrete.  

Applying the calculation methodology 

Carbonation of Ready-mixed concrete 

Ready-mixed concrete used in structural applications is designed so that only the surface layer will carbonate during the primary use stage. Even so, the CO₂ uptake depends on the concrete exposure. Where air can circulate freely over the concrete surface, for example, behind plasterboard or suspended ceilings/floors, carbonation will occur. However, where concrete surfaces are tightly covered by, e.g., tiles or the metal underside of a composite frame floor, no CO₂ will be absorbed. Concrete buried in the ground, e.g., in foundations, also carbonates albeit more slowly. 

Carbonation will accelerate during the end-of-life demolition and waste processing stage, when concrete is crushed and stockpiled on site, and will continue once the crushed concrete is reused in groundworks. The CO₂ uptake in the end-of-life stage can be maximised by optimising recovery and recycling practices.  

The CO₂ uptake in different stages of the concrete life cycle was calculated and included in the recently published MPA sector EPDs for ready-mixed concrete. The table below gives indicative values, for normal strength C28/35 concrete, of the percentage of the EPD upfront embodied carbon reabsorbed during each life cycle stage. 

Carbonation of masonry concrete blocks 

Lower-strength concrete products, such as aggregate concrete blocks used in masonry construction, are relatively porous and therefore carbonate rapidly. Some CO₂ is absorbed whilst the blocks are stored outdoors in the manufacturers’ stockyard (EPD module A3). After being transported to the construction site, the blocks will be exposed to outdoor conditions until the building is made watertight: carbonation will continue during the installation phase (module A5). 

Blocks used in masonry walls will usually have carbonated through their full depth by the end of their 100-year service life, but with indoor exposure the degree of carbonation is only 40%. At the end-of-life, the concrete exposure changes allowing a higher degree of carbonation, up to 85% for outdoor or buried exposure. Around a third of the upfront embodied carbon will be reabsorbed over the whole life cycle of an aggregate concrete block. Aircrete blocks carbonate a different rate to aggregate blocks due to the lime-based content of its composition.  

UK Greenhouse Gas Inventory Improvement: Carbonation of Concrete Emissions Sink Modelling 

Although concrete carbonation has been recognised by the IPCC as an important carbon sink, there is currently no IPCC methodology for calculating national estimates of the carbonation sink for inclusion in annual national greenhouse gas inventory reports (NIR) submitted to the United Nations Framework Convention on Climate Change (UNFCCC). 

MPA were commissioned by DESNZ to develop a UK-specific model to calculate the annual national emissions sink from carbonation of concrete for inclusion in the UK NIR. For 2020, the estimated total emissions sink, and overall uncertainty is 1.548 Mt CO2 ±34%. This equates to 0.4% of the UK’s total GHG emissions. Concrete in primary use absorbed 0.862Mt CO₂ (22% of the UK’s reported calcination emissions from cement production, 17% of calcination emissions for all cement consumed). End-of-life and secondary use concrete absorbed 0.686Mt CO₂ (18% of UK cement production calcination emissions, 14% of calcinations emissions for all cement consumed). 

The model disaggregates annual national concrete consumption into five different primary use applications. For each application a CO₂ emissions sink factor - the CO₂ uptake over the service life - was calculated for a prototype structure. The CO₂ uptake by concrete being demolished and entering the waste stream was also calculated. The results are shown in the table below.  

Full details of the methodology are contained in the final report available on the National Atmospheric Emissions Inventory website. 

* For framed buildings, two calculations were carried out. The lower number represents typical concrete mixes in current use, whereas the higher value assumes a mix with a higher CEM I cementitious content typical of late 20th century construction.   

Industrial carbonation  

Innovation ways of artificially accelerating the carbonation process are in development and include the use of recycled concrete aggregate, and recycled concrete paste or fines, and also introduction of CO₂ during product manufacture. E.g CO₂ curing chambers for precast concrete.