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Posted December 2, 2021 | Published in General

The Devil in Carbon: the challenge for construction

The UK has committed to reducing its greenhouse gas emissions to net zero by 2050. For net zero by 2050 to be possible, trajectories indicate that global carbon emissions need to halve by 2030. Undoubtedly, these requirements will become clear duties, not merely exhortations – the draft COP26 Agreement confirms that – but precisely what will be required of the construction industry is yet to be made clear. What is clear is that any changes made in the sector, as a carbon intensive industry which accounts for nearly 40% of global emissions, could represent the tipping point between success or failure. 

So, what are the options? There appear to be some broad themes of what the construction industry might be required to do: (1) renovate and reuse, (2) improve material production processes (especially the production of cement), and (3) capture (carbon) to mitigate.

It seems clear, though it merits restating, that there should be a primary focus on renovation and reuse of existing sites, where the foundations and structure are preserved. The case for this is increasingly clear. Research by CPRE suggests that there is already sufficient brownfield land, including a huge amount with planning permission in place, to meet the government’s targets for new housing.

But, accepting that new buildings and infrastructure will be necessary, the industry must look to reduce the carbon footprint associated with that. In this respect, the use of concrete is the main culprit. Concrete is the most widely used building material in the world, with around 14 billion cubic metres produced each year. For good reason – it is affordable, accessible, and reliable. However, the production process emits an awful lot of carbon. By way of example, the production of Portland cement (a major component of concrete) requires large amounts of limestone to be dug out of the ground and crushed to a powder. That powder is heated to extremely high temperatures until it forms nodules of “clinker” which is then ground back to the powder that is cement. Crucially, when limestone is heated, it releases its carbon atoms and forms carbon dioxide. Almost one metric ton of carbon dioxide gets released into the atmosphere for every metric ton of cement manufactured. Not good, but what can we do about it?

" Reducing emissions by the extent necessary to achieve net zero by 2050 will require a radical shift in mindset that is way beyond just the construction industry."

When the Global Cement and Concrete Association (GCCA) announced plans to cut a further 25% of carbon emissions by 2030, this included planned reductions in the amount of carbon intensive clinker used in cement. Replacing portions of clinker in the cement mix with supplementary cementitious materials (SCMs) is one way to reduce the carbon associated with concrete. SCMs are natural materials or industrial by-products that exhibit cementitious behaviours when combined with water, or water and other compounds. They are used to enhance the results of ordinary Portland cement in concrete; for instance, by strengthening hardened concrete properties. Ground granulated blast furnace slag (GGBS) and pulverised fly ash are the main SCMs used at present. 

However, there are still problems associated with SCMs. GGBS is a by-product of steel production (which is a carbon intensive process), and pulverised fly ash is a by-product of coal combustion (which is likely to be phased out). The future of the cement industry could conceivably involve the development and greater use of other, lower carbon SCMs, such as calcined clay, but that still requires digging a natural resource out of the ground.

It seems obvious that improving current processes alone will not be sufficient, so construction must also do what it can to capture the carbon that is emitted. The cement industry seems to see carbon capture and storage (CCS) as its main hope. The GCCA’s roadmap includes actions to accelerate technologies, including carbon capture, and the government aspires for the UK to become a world leader in this area. 

One approach to carbon capture, which is relevant to cement production, is oxyfuel combustion. To briefly explain the science behind this, the primary fuel is combusted in oxygen instead of air, which produces a flue gas containing water vapour and carbon dioxide. The flue gas is then cooled to condense the water vapour, which leaves a pure stream of carbon dioxide (that can be captured). The technology is in its infancy, and initial studies show that using oxyfuel combustion would be 25% more expensive than a conventional cement plant. Nevertheless, the industry is investing in research and development to see if, how and when the technology that is there can be applied at scale, more cheaply. 

Reducing emissions by the extent necessary to achieve net zero by 2050 will require a radical shift in mindset that is way beyond just the construction industry. The sector can do its best to find concrete alternatives, and move away from traditional clinker, GGBS and fly ash in cement, but it seems inevitable that whatever the industry can do itself must be accompanied by something bigger and more fundamental – namely, a radical difference in approach to construction or even a lowering of demand for new buildings full stop.

By Catherine Simpson


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