Embodied carbon: The Complicated Case of Concrete

The case against concrete is fairly simple: it is responsible for around 8% of manmade CO₂.

This is because it comprises 10-15% cement, the manufacture of which is energy-intensive and also generates CO₂ as a result of the chemical processes involved. It follows that a building with lots of concrete will probably have high embodied carbon — a fact that has led some architects to declare their intention to avoid specifying concrete wherever possible. Is this sensible?

Research scientist Jeremy Gregory is director of MIT’s Concrete Sustainability Hub and, perhaps unsurprisingly, he thinks not. “It’s not usually appropriate to generalize about building design that way,” he says. “Whether you use concrete, steel or timber, each will have its good and bad points and your decision will depend on what kind of building it is, where it is and how it will be used. So if you say you are just not going to use concrete, then you’re not properly framing the trade-offs between performance, cost and environmental impact. You can design a heavily insulated timber-framed building that will be just as energy-efficient to run as a well-designed concrete building. But will it work in a hot climate? Is it a large building? If so, is a timber solution scalable?”

Much of the case for concrete hangs on claims regarding its thermal mass. Exposed concrete columns and slabs absorb heat during the day, which can either be stored overnight to reduce future heating requirements, or vented to allow more heat to be absorbed the next day. Concrete frames are used in this way to even out temperature differentials, reduce both heating and air conditioning costs and thus reduce operational carbon. As Gregory points out, the US Department of Energy’s own National Renewable Energy laboratory deploys a labyrinth of concrete slabs to do just this. “For a building in Colorado, hot in summer, cold in winter, concrete’s thermal mass is ideal. The relatively high embodied carbon is working hard and will continue to do so for the lifespan of the building.”

Thermal mass also facilitates the use of renewable energy, he adds. “Most solar, for example, is generated in the middle of the day, but peak energy demand is often in early evening. So you can use solar to store heat or coolness in your concrete for when the sun isn’t shining.”

Concrete is getting greener too. Cement replacements such as pulverized fuel ash (PFA) or ground granulated blast-furnace slag (GGBS) are now routinely specified in many countries to replace 30-50% of the cement. Considerable research is underway to discover scalable alternatives to cement, but while cement-free or ultra low carbon concrete is possible, it is not yet commercially viable at scale.

Finally, adds Gregory, there is some intriguing research indicating that concrete exposed to the air actually absorbs CO₂ over time through carbonation. “It’s a slow process, but researchers in China have estimated that over the past 80 years, 43% of cement’s process emissions — those resulting from the chemical transition of limestone into clinker — have been reabsorbed by cementitious materials worldwide. In fact, in Scandinavia, demolished concrete is deliberately crushed to increase its surface area and then left a while to encourage this process.”

This article appeared in The Possible issue 06, as part of a longer feature on embodied carbon

Main image: New carbon capture technology called “direct separation” is being tested at the HeidelbergCement plant in Lixhe, Belgium, under the LEILAC (Low Emissions Intensity Lime and Cement) project. As limestone is heated, the carbon dioxide that is released is collected separately from furnace exhaust gases. Photo: Carbon Trust