How recycled cement could cut carbon

Laura Puttkamer

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major sources of CO2 emissions. A new solution from the University of Cambridge suggests recycling these materials, which could result in carbon-zero cement.

Concrete is the most used building material in the world. It contributes to around 8 per cent of total global CO2 emissions, making the construction sector much more polluted than it already is. What is more, it is also very difficult to recycle cement so that it can be reused for new concrete structures. Similarly, steel contributes to CO2 emissions, and given that concrete and steel are major construction materials, it is easy to condemn these materials.

A new solution from the University of Cambridge suggests recycling these materials, which could result in carbon-zero. When this is done with renewable energy, the researchers suggest that completely carbon-zero cement could be the result.

Using existing furnaces to recycle steel and cement

Scientists have been looking for ways to make concrete more environmentally friendly. Already, there are ideas to swap the most polluting ingredients, such as limestone, or to design concrete that absorbs CO2 from the air. A new study, published in the Nature journal, shows how old concrete can be turned back into clinker, the dry component that is ready for reuse.

According to Dr. Cyrille Dunant, first author of the study, crushing old concrete allows experts to take out the sand and stones. By then heating the cement and removing water, the result is clinker. A bath of liquid metal helps this chemical reaction – something that is used in recycling steel.

This kind of electric arc furnace typically uses a molten rocky substance like lime to purify the steel. The flux material can capture impurities before bubbling to the surface, where it forms a protective layer to prevent the new pure steel from air exposure. In the end, the used flux material becomes waste. But according to the Cambridge method, lime flux can be replaced with recycled cement paste. t showed to purify the steel correctly and ended up as new Portland rather than waste material.

Typically, electric arc furnaces use lime to purify steel. But they can also recycle cement to produce both steel and cement in a sustainable way. Photo: pixabay

One billion tons of recycled cement by 2050

Experiments have shown that this method of purifying steel can also serve to recycle cement of high quality. Importantly, the technique does not appear to add a lot of costs to concrete and steel production. Instead, it significantly lowers CO2 emissions compared to the conventional methods of producing steel and cement. If the electric arc furnace can be powered by renewable sources, this recycling process can even result in zero-emission cement.

So far, the Cambridge method is working in smaller furnaces that produce double-digit amounts of kilograms of cement. Next, industrial-scale trials are necessary to see whether it might be possible to produce as much as 66 tons of recycled cement in two hours. Scaling up the process could allow the world to produce up to one billion tons of recycled cement by 2050, according to the researchers.

Industrial trials will show whether the Cambridge method can be scaled up. Photo: Haneen Krimly via unsplash

Reducing concrete consumption

While the recycling method proposed by the Cambridge research team might not require a lot of investment, it is still not free. Convincing manufacturers to make the change could be a challenge.

Similarly, while it is almost miraculous to produce zero-emission cement, it is important to reduce the overall amount of cement and concrete. As the key ingredient in concrete, cement is responsible for around 90 per cent of the material’s substantial carbon emissions. If the industry were a country, it would rank as the world’s fourth largest greenhouse gas producer after China, the USA, and India.

And with the demand for concrete soaring, it is likely that the carbon footprint of the construction industry will keep growing, emitting more CO2 in the production of concrete. While the Cambridge research can address both major sources of the emissions, limestone’s chemical reaction and fuel firing to power high-heat kilns, it will take some time to catch up to demand. In order to meet carbon neutrality by 2050, we need to reduce cement and concrete production while making it more sustainable at the same time.

A carbon-zero alternative

The researchers are hoping that their experiment will serve as a breakthrough for the construction industry.Dubbed “Cambridge Electric Cement”, the material will showcase that the zero-emission innovation can go far beyond the energy sector. Already, the research team has filed a patent for the process, which is the first step towards commercialisation.

Ultimately, the goal is to cheaply produce low-carbon concrete at scale by recycling old cement from demolished buildings. The innovative element, piggybacking off the existing electric-powered steel recycling furnaces, means that the process can be easily and affordably implemented. The research could present a breakthrough for the construction industry. Within ten years, about a third of the UK’s cement needs could come from Cambridge Electric Cement, if all further tests go according to plan. Since the product has the same chemical composition as any other, it is expected to be as durable.

So far, most other solutions for making cement and concrete more sustainable have involved replacing part of the cement content with alternative materials. However, the cement component cannot be replaced completely due to the necessary chemical processes, resulting in emissions. This new product works with existing infrastructure and readily available materials to provide a potentially carbon-zero alternative.