In the wake of COP26 and a boosted focus on low carbon construction, conventional concrete with its high embodied carbon payload remains a crucial material globally. Yet at almost 1 tonne of embodied carbon per 1mᶟ of Portland cement, suppliers are under pressure to develop mixes with a reduced impact on the environment. By and large, this means replacing the cement, which is produced at temperatures of up to about 1,400⁰C, with other binders.
These typically include ground granulated blast furnace slag (GGBS) – a by-product from iron production – and pulverised fuel ash that is similarly a leftover material produced by coal-fired power stations.
As with GGBS, PFA production is reliant on a high carbon emission sector, but it’s one that’s becoming steadily less prevalent in the UK. “We’re shutting down more coal fired power stations, so we have to import this material,” says Atkins senior materials engineer Homayoon Pouya, nodding nonetheless to the carbon footprint incurred when transporting it.
Advancements With Clay
However, manufacturers also produce clay binders known as metakaolin. Although this doesn’t rely on energy-intensive steel production or coal-fired power stations, its conventional production process still requires high temperatures that also make it relatively expensive.
“It’s been around for a long time as it’s basically clay heated up to 1,000⁰C. But now [researchers] are looking at heating the clay to lower temperatures and adding chemicals to make it react,” says Pouya, explaining that with the addition of such admixtures, clay binders can be produced at just 400-500⁰C.
So far so good, so one question might be ‘why aren’t contractors already switching?’ One reason is there’s an issue with lag in being able to use newly developed concrete mixes within critical infrastructure. This is down to the lengthy period required to codify new materials that must perform throughout the operational life of an asset. While on the one hand engineers are generally conservative and don’t tend to use materials until they are fully certified, on the other, accruing information on the long-term performance and durability of a material like concrete will normally take some years.
Too Late for HS2?
“There was a great push to use these materials for HS2,” says Pouya. “They have loads of structures, so if they can [reduce embodied carbon in each] that becomes huge. But it’s hard to say whether these will become regulated materials in the next ten to 15 years and the design process for these structures has already started. They get used [elsewhere] in construction in low risk elements, but infrastructure [assets] might be expected to last for 120 years.” He says that while these low carbon concretes may have missed the boat in terms of HS2’s initial design phases, it’s conceivable that a forward thinking contractor might propose these low carbon concretes, if codified in time, for when construction begins.
Fortunately, some clients are helping to advance the certification process in the kind of low risk environments Pouya referenced. “We work with National Grid and they are trialling zero Portland cement concrete at one of their sites… in the foundations,” he says.
While infrastructure remains high on the political agenda, not least with the government backed imperative of decarbonising UK energy generation, it can only be hoped that substantially lowering the embodied carbon in such a core material as concrete can be achieved as quickly as possible.