Cambridge University research proposes cement recycling

Cyrille Dunant, a materials scientist at Cambridge University, is one of a team of scientists working on a method to recycle cement and reduce the environmental impact of the construction industry. Dunant said, “Cement manufacturing emits CO₂ for two reasons: One is that to make it you need to burn coal to produce heat, or maybe gas or some other kind of fuel. And the other reason is the limestone that has to be burned in the process. This limestone is lime and CO₂ together. The heat separates the CO₂, is you end up emitting it.”

The reason for the high emissions from the cement is not the material itself but the sheer amount it is produced worldwide. The Cambridge University team is researching how steel recycling processes can be repurposed for cement recycling. No new machinery is needed, because it uses the same electric kiln that is used for steel. The difference between steel and cement is that the former is electrically conductive and the latter is not. However, melting the steel is only the first part of the process, after which the remains have to be cleaned, something that is normally done with lime. Dunant continues, “What lime does is to collect all the impurities. And the mixture of lime and impurities is called slag. Instead of lime, in this case we add cement, which is largely composed of lime, so it's really good for cleaning steel.”

The process as such consists of charging the electric kiln with steel, cement and some additional lime, as cement separation is not always perfect and may need to be compensated. Dunant explains didactically: "You melt the steel and, on the one hand, you have new steel to make ingots, bars and beams and, on the other hand, you have the slag that you can cool quickly and then grind to powder. You add a little gypsum and you have new cement. With that and new steel you can build a new building."

The Cambridge research team is receiving funding from the European Union to collaborate with companies across the concrete supply chain. On the steel side, the main partner is Spain's Celsa, which in addition to its headquarters in Castellbisbal (Barcelona), has a subsidiary in the city of Cardiff (Wales), with which Dunant and his team work. With Celsaestan's electric kiln they are putting cement recycling into practice on an industrial scale, and the researchers have already done enough tests to consider this process a real option for treating cement