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One of the most difficult challenges in climate mitigation is cement: making cement releases emissions regardless of how its powered, there are currently no alternative options available at scale and we don’t know how to install new renewables or make new energy efficient buildings without it. All existing forms of cement production are incompatible with zero emissions and therefore will, by law, no longer exist by 2050

Dr Cyrille Dunant, based in the UK FIRES project at the University of Cambridge, pondered this problem: “People have always known you could recycle cement. In principle: it’s got all the right ingredients to make cement ! But there is no cement to recycle, only concrete, which is 80% rock and only 20% cement.

A decade before, during my PhD, I had separated paste and aggregates, and it had been pretty easy – involving manual labour and a microwave. And a number of initiatives showed promising avenues. Conversations with CDW processors speed the expertise existed at the industrial scale, but there was no commercial value in the cement powder.

But this is still no solution: when reclinkering cement, the result is a lower grade, high belite cement, because there is not quite enough calcium for the process. And anyway, a flame and associated emissions are still required.

But reading articles on steel recycling (an electric processes involving no combustion), I noticed the reported slag compositions were very close to cement. In fact, some papers even reported Alite, the main component of Portland cement in EAF slags. Perhaps the higher temperature and the presence of large amount of molten steel would solve the clinkering problem?

The first experiments showed paste could be reclinkered, but this yielded only Belite, Alite’s less reactive cousin. However, applying the empirical recipes of the cement industry, the experiments was run again, with added lime. In principle, this should not have worked: the system would be too lime rich.

But lo and behold, Portland cement was produced, with no more energy than required for the steel recycling. A second trial with another cement produced the same results. The slag, ground and blended with calcium sulphate behaved like a Portland cement setting and gaining strength, as hoped.

There is still a road ahead, understanding the new clinkering process, and making paste separation into an industry. But cement is now a recyclable material.”