Climate protection: How concrete becomes a CO2 store

Around eight percent of global carbon dioxide emissions come from cement and concrete production. And although a lot of energy is required to burn the raw material, cement clinker, a large part of the released CO comes from₂ the chemical process that occurs – it escapes from the limestone. The cement industry is therefore considered a difficult sector to decarbonise. Nevertheless, research is being carried out on various methods of CO_2-Reduce the footprint of cement and concrete, for example through increased recycling of concrete from demolished buildings or through CO₂capture and storage (CCS). For the latter, appropriate underground storage facilities would also be needed.

At the Federal Materials Testing and Research Institute (Empa) in Switzerland, however, research is being carried out into how concrete itself emits CO₂-can become neutral or even CO₂-negative. This is made possible by adding biochar, which is considered a long-term carbon store. Instead of CO₂ By storing it in the ground, the greenhouse gas should be fixed in buildings. In a publication in »Journal of Cleaner Production« The Empa team of Pietro Lura and Mateusz Wyrzykowski presents a process in which biochar is added in pellet form during concrete production. You have to imagine the pellets as small balls similar to expanded clay. “With a proportion of 20 percent by volume of carbon pellets in the concrete, we achieve net zero emissions,” says Wyrzykowski. In lightweight concrete, this proportion could even be increased so that the CO₂-The balance of this concrete would even be negative in the end.

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However, carbon dioxide is still emitted during the production of concrete, it is just – at least mathematically – captured elsewhere. Biochar is produced using pyrolysis, which means that heat is added to an organic starting material, such as wood residues, in the absence of oxygen. The product of this charring process is mostly pure carbon, which remains stable over the long term. So that in this way there is actually CO₂ is neutralized, of course no trees may be felled for this purpose, and it must be ensured that the carbon remains safely stored and does not burn or rot again. The Empa research team therefore took into account waste wood that occurs in horticulture and nature conservation. According to calculations, charring it for concrete production could offset around half of the emissions from this sector in Switzerland.

The scientists also thought about how the biochar is preserved over centuries. »Our idea is that the carbon becomes part of the concrete granulate. Concrete granules are then used as aggregate for new concrete. “You could store the carbon in buildings and infrastructure for hundreds of years through multiple recycling cycles,” explains team leader Pietro Lura when asked by “nd”. However, recycling has not yet been tested.

Now concrete containing biochar is not an invention of the researchers at Empa. A Swiss company already offers such a product under the name “Klark”. However, the biochar is added there untreated, says Lura. “We are convinced that our technology is much more practical and robust.” And not only is “climate-neutral” concrete already on the market, the city of Basel would like to use asphalt with biochar in the future. In Germany, the company Ecolocked trades in additives made from biochar for the production of concrete.

The material biochar is not only enjoying growing popularity in the construction industry. Carbon obtained through pyrolysis is also used in agriculture. It contributes to the build-up of humus and can help to store water in the soil for a longer period of time – a big plus point, especially on sandy soils and as summers become drier. The use of charcoal to improve soil may have been known for thousands of years – as part of the “terra preta” developed by indigenous people in the Amazon region. According to the soil condition survey carried out by the Thünen Institute in 2018, arable soils in Germany are threatened with an average loss of humus of 0.19 tonnes per hectare per year. Building humus with organic substances such as biochar could be a remedy.

Ultimately, for the overall climate balance, it should not matter whether carbon is stored in buildings or in the fields. Since the compensation of CO₂-Emissions are usually also compensated monetarily, but it would have to be ensured that no new competition for organic material arises – and that this is then removed from ecosystems in excessive quantities.

But the concrete and asphalt researchers at Empa are already thinking one step further. »For us, biochar is more of a model material. We think that carbon from pyrolysis of other organic waste and plastic waste is much more interesting for use in building materials. Another source would be carbon from the pyrolysis of methane,” says Lura. However, the methane would have to be produced synthetically using solar energy. Subsequent pyrolysis of the gas produces hydrogen and solid carbon.

Methane pyrolysis was originally used as a chemical process to produce solid carbon, with hydrogen being the byproduct. The methane for this process usually comes from fossil natural gas. According to the categories of the German Federal Environment Agency, “turquoise” hydrogen is produced during methane pyrolysis.

Companies like BASF like to sell this as “clean hydrogen” because there is no CO during production₂ is released. What is left out of the balance sheet is that part of the potent greenhouse gas methane usually escapes from the extraction and transport of natural gas. When it comes to carbon from methane pyrolysis, it is worth taking a closer look at the manufacturing process.

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