Proposed Zero-Carbon Cement Solution Called 'Absolute Miracle'

"Concrete and steel production are major sources of CO2 emissions," writes New Atlas, "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... 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... 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 first industrial-scale trials are underway this month," the article adds. "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. And the professor has also recorded a thoughtful video visualizing the process - and explaining the significance of their breakthrough.

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