31 Mar 2021
Published in: Concrete Quarterly Spring 2021
Concrete mix design is an art, a science – and one of the keys to a zero-carbon built environment. When construction materials are ranked in order of their embodied carbon, people are often surprised to see how low down the list concrete is. The reason its global carbon cost is so significant is that it is used in such enormous quantities. There are good reasons for this.
Apart from the fact that its raw materials are available close to every building site in the world, concrete performs in ways that no other material yet discovered can. As we work to reduce its embodied carbon still further, what we don’t want to do is lose the properties that make it – in the words of Bill Gates – an “amazing material”.
Concrete’s embodied carbon lies predominantly in the Portland cement that makes up 10-15% of the mix. Fortunately, there are already proven ways to specify low-carbon concrete, by replacing a proportion of that cement with fly ash or ground granulated blast-furnace slag (GGBS), waste products of the coal and steel industries respectively.
They have been used for 50 years or so, and we know exactly how they perform and the very useful roles they can play. We also know that they can be stronger than conventional mixes but that they achieve strength more slowly – so one very simple step specifiers can take to lower the carbon bill of a project is to specify strength at 56 days for elements such as foundations, rather than 28.
The proportions of the typical mix will continue to evolve over the coming decades. In the UK, we are only just catching on to the potential of powdered limestone, a lower carbon alternative that can replace up to 20% of the Portland cement. It is used as standard widely across Europe – an Irish producer told me that it’s in every mix that leaves his plant.
This can be specified in the UK as part of what’s called a ternary blend, as a second replacement alongside GGBS or fly ash. There is also a growing range of other cements. Some of these are covered by a publicly available specification, but the industry is working to get them fully incorporated into British Standards.
In the UK, the cement industry is trialling fuel switching using hydrogen, and globally it is piloting carbon capture, use and storage technologies, which are at the heart of its roadmap to reach zero carbon emissions by 2050.
As this comes onstream, the embodied carbon of Portland cement will fall and it will be able to account for a higher proportion of the mix again. GGBS and fly ash will continue to play a role for the special performance properties they offer, and there will no doubt be many more innovative products coming on to the market as concrete science continues to push the boundaries.
The take-home message for the specifier of today is that concrete can already be a low-carbon material and it’s heading for zero – and that wherever you can specify 56-day strength, you should.
See our latest issue of Concrete Quarterly Spring out now.
Written by - Jenny Burridge Head of structural engineering, The Concrete Centre