'Absolute miracle' breakthrough provides recipe for zero-carbon cement

Concrete and steel production are major sources of CO2 emissions, but a new solution from Cambridge could recycle both at the same time. Throwing old concrete into steel-processing furnaces not only purifies iron but produces “reactivated cement” as a byproduct. If done using renewable energy, the process could make for completely carbon-zero cement.

Concrete is the world’s most used building material, and making it is a particularly dirty business – concrete production alone is responsible for about 8% of total global CO2 emissions. Unfortunately it’s not easy to recycle back into a form that can be used to make new concrete structures.

Scientists have of course been investigating ways to make concrete greener. That can include changing the recipe to switch out the most polluting ingredients – specifically limestone – or designing concrete so that it absorbs more CO2 from the air after it’s laid. For the new study, Cambridge researchers investigated how waste concrete could be converted back into clinker, the dry component of cement, ready to be used again.

“I had a vague idea from previous work that if it were possible to crush old concrete, taking out the sand and stones, heating the cement would remove the water, and then it would form clinker again,” said Dr. Cyrille Dunant, first author of the study. “A bath of liquid metal would help this chemical reaction along, and an electric arc furnace, used to recycle steel, felt like a strong possibility. We had to try.”

An electric arc furnace needs a “flux” material, usually lime, to purify the steel. This molten rocky substance captures the impurities, then bubbles to the surface and forms a protective layer that prevents the new pure steel from becoming exposed to air. At the end of the process, the used flux is discarded as a waste material.

So for the Cambridge method, the lime flux was swapped out for the recycled cement paste. And sure enough, not only was it able to purify the steel just fine, but if the leftover slag is cooled quickly in air, it becomes new Portland cement. The resulting concrete has similar performance to the original stuff.

Importantly, the team says this technique doesn’t add major costs to either concrete or steel production, and significantly reduces CO2 emissions compared to the usual methods of making both. If the electric arc furnace was powered by renewable sources, it could essentially make for zero-emission cement.

The technique has already been trialed in furnaces that produce a few dozen kilograms of cement, and the researchers say the first industrial-scale trials are underway this month, where it would be producing about 66 tons of cement in two hours. The researchers say that the process could scale up to produce one billion tonnes of "electric cement" by 2050.

"Producing zero emissions cement is an absolute miracle, but we’ve also got to reduce the amount of cement and concrete we use," said Professor Julian Allwood, who led the research. "Concrete is cheap, strong and can be made almost anywhere, but we just use far too much of it. We could dramatically reduce the amount of concrete we use without any reduction in safety, but there needs to be political will to make that happen.

"As well as being a breakthrough for the construction industry, we hope that Cambridge Electric Cement will also be a flag to help the government recognize that the opportunities for innovation on our journey to zero emissions extend far beyond the energy sector." A patent has been filed for the process, as the first step towards commercialization.

The research was published in the journal Nature. The team describes the work in the video below.

Source: Cambridge University