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Construction is a cause of global warming, but is concrete really the problem?

Paddington maintenance depot
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With cement blamed for 8 per cent of world CO2 emissions, Aidan Reilly and Oliver Kinnane examine the options facing the construction industry if it is to help prevent irreversible climate change

‘Architects are fetishising a material that is killing the planet,’ wrote the AJ’s Will Hurst following the report by Chatham House calling for decarbonisation strategies in the face of rising global demand. Less than a week later, Paul Finch claimed it wasn’t that bad, saying: ‘I don’t think any architect designing a house in concrete for its aesthetic value should feel at all guilty about proceeding’. Who is right? Can the use of concrete be sustainable? These are urgent questions, but to address them we need facts as well as opinions.

Comparing concrete with other materials

One measurable aspect of environmental impact is the emission of greenhouse gases, assessed using the embodied carbon of a product. Embodied carbon refers to the CO2 emitted during production; the ‘embodied carbon equivalent’ refers to the total greenhouse gas effect (as other gasses besides CO2 contribute to global warming, primarily methane, nitrous oxide and hydrofluorocarbons).

Compared with other construction materials, the total impact of concrete production and use is worse; but in large part, that is because we use so much of it. On an embodied carbon per-kg basis, concrete can appear rather benign (see table).

MaterialCO2e/kg

Concrete

0.107

Steel

1.46

Aluminium

9.16

Asphalt

0.066

Plastics

3.31

However, displaying these values on a per-kg basis hides the fact that the structural properties of concrete are not as good as steel, for example, and less steel would be used by mass in a comparable framed building.

To measure the impact of concrete, we must look at the impact of the industry as a whole. Any reductions in carbon intensity have to be placed in the context of total production – which is rising. For assessing environmental impact, it is the total CO2 output that matters, not the output per kg of concrete. If emissions per kg halve, but production doubles, the environmental impact continues to accumulate at the same rate.

Cement is the problem

Annual cement production

Annual cement production

Source: USGS

Annual cement production

In terms of CO2 emissions, the dominant component of concrete is cement. Figure 1 shows global cement production for the last two decades, showing that it has risen dramatically in the last decade. Is this rise sustainable?

In order to meet targets set out in the IPCC 2018 report, CO2 emissions from concrete must essentially fall to zero by 2055. Unless we stop using concrete, the concrete industry must either find a way to produce cement without CO2 as a byproduct, or it must find an alternative to cement.

How much progress has been made to date?

Alternative binders, such as fly ash (FA) and ground granulated blast-furnace slag (GGBS), can partially replace cement, and production of these binders emits less CO2 than production of cement. On average, the possible reduction in emissions is around 27 per cent from current levels. However, the supply of these alternatives is limited.

The manufacture of cement releases CO2 due to (a) the thermal decomposition of limestone (CaCO3) and (b) the energy required to heat the kilns. The heating energy could, in principle, be replaced by renewable energy but the decomposition emissions are unavoidable. Some improvements are possible (for example, depending whether the cement kiln dust is added back to the kiln or sent to landfill). Losses attributable to the loss of cement kiln dust typically amount to 2 to 6 per cent of the CO2 emissions from clinker production.

Concrete absorbs CO2 from the atmosphere during curing. Under optimum conditions, concrete can absorb up to 17 per cent of the CO2 emitted during production; but a more realistic figure for in-service concrete is between 5 and 7 per cent.

If all these strategies were applied together, this would allow for a maximum reduction of embodied carbon per kg of traditional concrete of about 37 per cent. This is roughly half of what would be needed merely to keep emissions at 1990 levels. And global production is still rising.

Global extraction of nonmetallic minerals

Global extraction of nonmetallic minerals

Source: Miatto 2017

Global extraction of nonmetallic minerals by type, 1970–2010

Is recycling the solution?

Recycled, crushed concrete from demolition projects can have a very low impact by comparison with virgin aggregates. However, there are limits to recycling concrete while the building stock is growing – even if we recycle 100 per cent of demolished concrete. Furthermore, crushed concrete is often regarded as suitable only for low-strength applications. Even in Japan, where 95 per cent of concrete is recycled, most ends up as general fill.

Where structural conditions allow, timber might be suggested as an alternative to concrete. Could timber replace a significant proportion of concrete? What sort of volumes are required?

Global concrete production is approximately 11,000 million m3/year. Current timber production is around 3,800 million m3/year – and our use of wood is causing deforestation even at present levels. Significant replacement of concrete with timber might reduce CO2 emissions, but it would dramatically increase deforestation.

If we wish to reduce our impact we need to reduce our consumption, our production, and live within the planet’s means

So far, this may seem bleak – and rightly so. Concrete has an enormous impact on the environment not because it’s intrinsically bad, but because we use so much of it. Possible replacements also have huge impacts when used at scale. There are no technological fixes to make its impact sustainable; no magic bullets. If we wish to reduce our impact we need to reduce our consumption, our production, and live within the planet’s means.

Efficiency improvements alone will not be enough. Our overall consumption of concrete must fall, and in the context of a rising population, this means a dramatic fall in per-capita consumption.

Any use of a non-renewable resource is fundamentally unsustainable. Concrete uses fossil fuels to make cement, and sand and gravel for the aggregate. CO2 is emitted and limited resources are consumed, so any use of concrete cannot be sustained on long timescales; it is therefore incumbent on people to use as little as possible, and to make the very best use we can of the quantities that we use.

It is important that architects understand, and advise on, the wider impacts of their projects – which means sometimes placing such issues above the commercial interests of their clients. There is now a clear opportunity for the profession to accept a leading role, and its inherent responsibility, in the future development of our planet.

Aidan Reilly is a research fellow and Oliver Kinnane is assistant professor at the School of Architecture, University College Dublin

  • 4 Comments

Readers' comments (4)

  • Why on earth has been EMC cement been actively ignored by the European cement industry for the last 26 years. EMC cement - OPC (Ordinary Portland Cement) finely inter-ground with sand or puzzoulanes with up to 50% OPC reduction (and hence CO2 reduction). Developed by scientists in Sweden in 1992. Bridge built in Sweden 20 years ago, some roads and bridges Texas, USA. see wiki en.wikipedia.org/wiki/Energetically_modified_cement

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  • I would like to know more about EMC? So we may have that low embodied cement in the US!
    This sounds like a plug, and it is... however I’ve been looking for a healthy low embodied sustainable cement for years.. and I believe it’s here... see below;

    It’s called Ecorock and is a multi-purpose, non-toxic, fast-setting, water-activated, geopolymer blend specifically designed for the eco-safe repair, replacement and resurfacing of Portland concrete and asphalt, reaching 6000 psi in 3-hrs, 7500 psi in 24-hrs, 8500 psi in 7-days, and 10 to 13,000 psi in 28-days.

    EkoRok is conveniently applied using standard concrete technology, is impervious to salt erosion and rebar corrosion, and strongly bonds to mineral, metal, cellulose and itself.

    We now use it in Seattle Washington, the department of Transportation updates and repairs all roads and cement and can be driven on within 2 hours, stays mailable in all temps... bridge, road, nuclear storage repair, and has been used for a thousand years!

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  • Unfortunately architects can't do much about population growth, which is the most significant factor in respect of climate change, but doesn't get much discussion because possible strategies are certain to cause offence. Easier to hammer the architects instead.

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  • You stopped short of the obvious conclusion. Architects must argue for more re-use of old buildings.

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