The invisible carbon sink
For many materials, demolition and waste processing at the end of a building’s life results in the release of CO2. But with concrete, the opposite is the case. Tom De Saulles considers what carbonation means for designers
The absorption of carbon dioxide (CO2) by plants and trees is biology 101, but we could be forgiven for not knowing that concrete and other cement-based materials also soak up a significant amount of CO2 from the air. This naturally occurring chemical process is called carbonation and happens all around us in our buildings and infrastructure.
The carbonation process is the reaction between calcium oxide (CaO), an alkaline product of hardened concrete, and CO2, which results in the formation of calcium carbonate (CaCO3), or limestone. At a worldwide level, the CO2 uptake from carbonation is equivalent to roughly 23% of the net CO2 sink from global forests between 1990 and 2007. This surprising figure comes from a new study led by Chinese Academy of Sciences researcher Fengming Xi, which also estimates that between 1930 and 2013, CO2 uptake from cement-based materials was in the order of 4.5 billion tonnes.
To those unfamiliar with carbonation, these figures may sound fantastic, as if perhaps the cement and concrete industry has marked its own homework. So it is important to point out that overall emissions from the manufacture of cement continue to be more significant than that from carbonation. More specifically, the study estimates that over the lifecycle of these materials, carbonation accounts for around a 43% takeback from the fuel used. If included, you get a more representative figure of around 26%, which broadly aligns with studies from other countries. In mentioning this, it is worth pointing out that regional differences in the use of concrete can affect results. For example, the Chinese study includes cement-based renders, which absorb CO2 rapidly due to their high exposed area.
Accounting for carbonation
Carbonation has historically been viewed by engineers and designers in the context of concrete durability, and more specifically, the necessity of protecting steel reinforcement from its effects. This is addressed in structural design standards, which set minimum requirements for reinforcing cover and concrete mix design. Greater awareness of carbonation does not influence design decisions – all design standards already account for it, and resilience of the structure remains paramount. Moreover, cover is not an issue for mass concrete, nor for concrete block products that contain no reinforcement.
Discussion of carbonation from a sustainability and whole-life CO2 perspective is a more recent development. This does not necessarily seek to encourage carbonation, but does aim to ensure the implications of this naturally occurring process are acknowledged, and the resulting CO2 uptake is included in whole-life performance assessments. This is already the case with the BRE Green Guide ratings, which use an environmental profiling methodology that accounts for carbonation. More recently, the development of environmental product declarations (EPDs) allows the process to be fully accounted for in the lifecycle of concrete products, which includes the end-of-life phase.
As a general point, it is worth noting that unlike many of the environmental performance considerations associated with the use of construction materials, the carbonation of concrete is always going to occur, and is not particularly dependent on any specific whole-life scenario. The average reduction in embodied CO2 for structural concrete is about 7.5% over the lifecycle of a building. Beyond the lifecycle stage, a much greater reduction ultimately occurs that can reduce the initial embodied CO2 by around a third.
Alternative construction materials rely on unknown assumptions and scenarios for end-of-life impacts and emissions associated with landfill or incineration. In contrast, whatever the whole-life scenario or time period considered for concrete, the fact that the carbonation process will always occur means this end-of-life uncertainty avoided. This provides a useful degree of confidence when undertaking whole-life CO2 calculations for concrete.
Some 91B tonnes of crushed concrete were in the creation of the Olympic Park in London. Researchers at Newcastle University are investigating the absorption benefits of the material on hard landscaping. Photo: Queen Elizabeth Olympic Park
Looking specifically at concrete-frame buildings in the UK, lifecycle carbonation is currently estimated to be closer to 30%, with only a modest amount occurring during their operational life. This type of building doesn’t generally feature a render finish, and in any case, the mix design of structural concrete purposefully limits the carbonation process to prevent corrosion of any embedded steel reinforcement, which might otherwise be affected (see box below) uptake during the in-use phase of reinforced-concrete buildings is generally limited.
The other side of carbonation
While concrete provides a carbon sink by the process of carbonation, talk to any structural engineer and their response to carbonation is that it is bad and to be avoided at all costs, writes Jenny Burridge. Stronger, more dense concrete will resist the passage of carbon dioxide into the body of the concrete so, where resisting carbonation is important, the concrete can be specified to do just that.
Carbonation does not weaken the concrete but it does lower the pH, making any steel reinforcement liable to corrosion. When steel reinforcement is placed in concrete a passive protective layer of an alkaline film is formed, but this requires a pH of 10.5 or above. Normal concrete has a pH of around 13, but when carbonated this falls to below 9. If this happens, and there is oxygen and moisture at the surface of the steel, this will lead to rust formation and the resulting expansion can cause concrete to fail.
The normal method of specifying concrete to resist carbonation is by increasing the strength of the mix and increasing the cover to the reinforcement. The cover is the distance from the face of the concrete to the reinforcement. BS 8500-1:2015 contains guidance on the different ways of specifying against damage by carbonation. There are four exposure classes for carbonation: XC1 to XC4 (see table 1). Tables A4 and A5 in BS 8500 give the minimum strengths and covers for the exposure classes for a working life of 50 years and 100 years respectively.
For an XC1 exposure class, the rate of carbonation is relatively slow, and either the oxygen or the moisture required for the steel to rust is not present. So, the concrete that needs to be specified tends to be a lower strength and with a small cover – for example, at least C20/25 strength with a minimum cover of at least 15mm. By contrast, a concrete in an XC4 exposure would need (for a working life of 100 years) to be C40/50 with a minimum cover of 30mm. Lower strength concretes can also be used, but the cover required is increased.
Tests have shown that for concretes with high levels of fly ash (greater than 35%) the resistance to the rate of carbonation is slightly lowered in comparison to other types of cement. So for exposure classes XC3 and XC4, the tables specify that larger covers are needed.
For the structural engineer, carbonation is not all bad, though. At the surface of the concrete, it significantly reduces the possibility of the very rare thaumasite form of sulfate attack for foundations. And once the reinforced-concrete structure has been demolished, the concern about carbonation is no longer relevant. Then, as is explained elsewhere in this article, the carbonation of the crushed concrete, with its larger surface area, can capture some of the CO2 in the atmosphere.
However, more significant carbonation occurs during the end-of-life phase, when the concrete crushing process greatly increases its surface area and exposure to the air. While deconstruction and demolition may only last a matter of weeks or months, this is long enough for the resulting carbonation to offset around 5% of the material’s initial embodied CO2. Further CO2 uptake occurs beyond the building’s lifecycle, when the crushed concrete goes on to be used in other applications, particularly groundworks. It is during this secondary-life period when most of the carbonation will ultimately occur.
In addition to atmospheric CO2 uptake, two further absorption mechanisms can come into play, particularly at the end-of-life and secondary-life stages. The first of these involves leaching from exposure to the rain, and the other is a newly discovered process involving microbial soil activity where crushed concrete is present in hard landscaping or brownfield sites. This process is the subject of research at Newcastle University, which is investigating methods of increasing inorganic soil carbon storage through soil engineering. It has estimated that this can result in around 150 tonnes of CO2 being absorbed per hectare annually.
Finally, returning to the Chinese study, the research paper finishes with an interesting thought: if carbon capture and storage were applied to the cement-manufacturing process, concrete and other cement-based materials might actually represent a source of negative CO2 emissions when carbonation is factored in. That’s something to think about.
The Hayward Gallery under a heavy London sky. BS 8500 states that reinforced concrete exposed to direct rain requires 30mm of cover to resist damage by carbonation over a 100-year lifespan. Photo:Mark Hamilton / Alamy Stock Photo
Whole-Life Carbon and Buildings, The Concrete Centre, available to download from www.concretecentre.com
Fengming Xi et al, Substantial global carbon uptake by cement carbonation, Nature Geoscience, advance online publication, 21 November 2016