Tales of low-carbon concrete abound, but what exactly does that mean? Kristina Smith looks at what’s in the mix.
In September 2022, HS2’s Euston station site saw the UK’s largest ever low-carbon concrete pour: a 232 cu m temporary slab that will support polymer silos for piling works.
Depending on how you look at it, the mix used – Earth Friendly Concrete (EFC) from Capital Concrete – has saved either 75% or 50% of embodied carbon, compared to a ‘standard’ mix. The press releases have, of course, gone for the former, but the question is whether a standard mix is one that contains 100% Ordinary Portland Cement (OPC) or one that contains a mix of half OPC and half ground granulated blast furnace slag (GGBS).
“Some of the big concrete suppliers are referring to concrete with 50% slag as ‘lower-carbon concrete’, but that’s an everyday concrete in London,” says specialist concrete consultant Charles Allen, director of OTB Concrete, who is an advisor to Capital Concrete. “I have been using 70% slag in foundations for 30 years.”
Materials such as GGBS and pulverised fuel ash (PFA) are often used to replace some of the OPC in large-volume concrete pours to reduce the heat of hydration. This in turn reduces the chance of cracking and can allow the amount of reinforcement to be reduced.
We need to get far more specific in our definition of low-carbon concrete, says the Low Carbon Concrete Group. In its Low Carbon Concrete Routemap, the group says that different strengths of concrete should be given an embodied carbon rating graded from A++ to G, similar to energy efficiency ratings for appliances. And the baseline should be calculated from contemporary industry values.
The other challenge is that concretes that use GGBS and PFA are not a long-term fix. There are already limited supplies – which will diminish as steel-making processes change and coal-fired power stations become fewer.
“Slag and GGBS are transition technologies,” says Noushin Khosravi, sustainable construction manager at the Concrete Centre, which is part of Mineral Products Association (MPA). “We need to invest in longer-term solutions at the same time.”
One of MPA’s current projects is to look at what type of UK clay would produce the best calcined clay – another alternative cementitious material which could be in greater supply than PFA and slag.