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Decarbonising the construction sector, episode 1: low-carbon concretes

15 December 2021 - Sustainability - France

Construction emits particularly high amounts of greenhouse gases. In France, estimates suggest, the sector is responsible for 8% of total emissions.* The fuel used to power machinery accounts for some of those emissions, but materials account for the bulk of them. Most building and civil engineering companies have aligned their targets with the goals in the 2015 Paris Agreement and are thus aiming for carbon neutrality by 2050. In this series, we will be looking at the various levers the sector can use to tackle the challenge of low-carbon construction. The first episode is about concrete.

Concrete production amounts to 14 billion cu. metres, making it the most widely used construction material worldwide. It is versatile – it can be prestressed, reinforced, cast and sprayed, for example – so it works well in all kinds of solutions and situations. And its cost is relatively modest compared to other materials.

Concrete accounts for 8% of greenhouse gas emissions

On the flip side, concrete is responsible for 8% of the world’s greenhouse gas emissions. Here’s why: it is made of inert components (sand or aggregates) but they need to be mixed with an ingredient that has the right physical and chemical properties to bind them together. In most modern concrete, the binder is cement. And cement production is energy-intensive: making its main component, clinker, involves heating limestone and clay to 1,450°C, which releases substantial amounts of CO2.

The big challenge for the construction sector, on the road to fulfilling its commitments to lowering greenhouse gas emissions, is to use less clinker in its cement. “When you’re talking about cement, low-carbon basically means ‘low-cement’. Or, more specifically, ‘low-clinker’,” sums up François Cussigh, concrete engineering director at VINCI Construction. Concrete can be replaced with biosourced materials such as timber or baked clay in buildings. But concrete is often the only option in public works – for example on underground construction projects. This is why the public works sector needs to look into alternative cements.

Substituting clinker, the source of concrete’s CO2 emissions

Fortunately, there are alternatives to clinker. One of them, granulated blast furnace slag, is both tougher and environmentally friendlier. It is a by-product of iron melting processes, has the same characteristics as clinker and has a much smaller carbon footprint. Production of alternative binders was fine-tuned and scaled up to industrial levels in the mid-2010s.

Using this by-product from the steelmaking industry, VINCI Construction has designed, secured certification for and started selling its range of low-carbon concretes, Exegy, in partnership with Ecocem, the leading independent producer of granulated blast furnace slag. The Exegy range includes low-carbon, very-low-carbon and ultra-low-carbon concretes (the lower the carbon, the more slag and less clinker it contains). Exegy ultra-low-carbon concrete releases 70% less CO2 than traditional concretes and is just as strong.

It is the partnership between granulated blast furnace slag suppliers and construction companies such as VINCI that made it possible to fine-tune greener concrete formulas. “Many of VINCI Construction France’s project contain granulated slag from Ecocem. We started working with theit teams in 2009, as soon as we opened our first production plant in France, in Fos-sur-Mer. We have since worked on several building, civil engineering and underground projects, so we have got to know each other well,” says Laurent Frouin, research and innovation director at Ecocem.

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Low-carbon concrete is gaining ground at worksites

VINCI is already using this low-carbon concrete on many of its projects, including:
- Its new head office in Nanterre, which has six structural posts, holding up eight storeys, poured with low-carbon concrete;
- Works package 1 of Line 18 of the Grand Paris Express, which has 40 voussoirs (prefabricated wedge-shaped components lining a tunnel) made of low-carbon concrete – a first worldwide in a metro system! The goal here is to test low-carbon concrete’s strength underground, with Société du Grand Paris, with a view to mainstreaming it;
- Phase III of the extension of the port of Le Havre (“Port 2000”); the trial here involves casting an ultra-low-carbon concrete base 17 metres underwater.

Low-carbon concrete was used to pour the foundations for the To-Lyon tower.

* Source: Ademe (France’s Agency for Ecological Transition)https://ile-de-france.ademe.fr/sites/default/files/neutralite-carbone-batiment.pdf.

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