In Nantes, the largest hospital under construction in Europe uses low-carbon concrete

For this vast hospital site which covers 11 hectares - the largest of this type in Europe currently, according to its manufacturer Vinci - only so-called "low carbon" concrete is used: 60.000 tonnes were poured out of the 120.000 planned for here for delivery of the building at the end of 2026, indicates the construction group, responsible for nine of the fifteen future buildings.

With his feet in the gray molasses, a companion ensures that the concrete mixes well with a sort of vibrating beater. No air bubbles should form in the slab. Then it smooths like pastry dough. Above all, hurry before it freezes.

At first glance, nothing distinguishes it from concrete made with conventional cement, the manufacture of which generates, on a global level, 7% of CO2 emissions alone, i.e. more than aviation, according to a recent report from the UN Environment.

But once dry, the hospital walls take on subtly different shades depending on the binder used to replace the polluting cement: metallic gray or slightly pink.

Gray concrete is based on a waste product from steel manufacturing called slag. It replaces “clinker”, the basic element of classic cement. The manufacture of this classic clinker requires firing at 1.400°C, therefore a lot of energy, and the chemical reaction releases a lot of CO2.

“Massifying low-carbon”

Supplied by the Irish group Ecocem, the slag produces “ultra-low-carbon” cement and concrete, according to the Vinci classification, with emissions estimated to be 70% lower than those of basic concrete.

But as the steel industry itself is looking for ways to decarbonize, slag risks becoming rarer with new methods of steel manufacturing within a few years.

Vinci anticipated by looking for another binder, mineral: kaolin crushed and calcined at 700°C, supplied by the company Imerys. It produces a slightly pinkish concrete.

Two years of laboratory research to develop this “metakaolin”, the manufacture of which emits 240 kg of CO2 per tonne produced, compared to 650 kg for conventional cement, a reduction of 40% in CO2. A less efficient material than slag, but easy to find.

“Being able to access significant quantities of metakaolin means that we can massify low-carbon and keep competitive prices almost at the same level as conventional cement” explains Bruno Paul-Dauphin, director of low-carbon solutions at Vinci .

“Concrete will always have carbon emissions,” adds Rémi Lefeuvre, director of technical and operational resources at Vinci Construction: 100% reduction in CO2 emissions, “we can’t get there today.”

An objective that is nevertheless crucial for the climate. The use of concrete has increased tenfold on the planet in 65 years, according to the UN Environment. At the same time, that of steel has tripled and that of wood has remained stagnant in construction.

Cautiously, Vinci does not give any specific objective for reducing CO2 emissions for the hospital. At least 30% compared to normal concrete, says Mr. Lefeuvre. Precise accounts will be made at the end of the project.

Because reducing emissions does not only depend on materials. The weather also plays a role. If it is too cold, the "forms" - metal molds into which the walls are cast - must be heated. This increases the emission rate for the energy used. So far, no heating has taken place since construction began in March 2022.

During the coffee break, we discuss the UN report according to which global concrete consumption will have to be halved in 2060 compared to today. “We are not surprised, we are preparing,” says Bruno Paul-Dauphin.

The builder relies on other materials such as wood for certain sites. For concrete, he is also exploring other binder recipes, reusing industrial waste, in particular based on "silica fumes", residues from the silicon industry.