Combined cement and steel process halves carbon footprint

A concrete plan...
24 May 2024

Interview with 

Cyrille Dunant, University of Cambridge


A concrete raised highway.


Making cement is one of the world’s largest single contributors of greenhouse gas emissions. The iron and steel industry is also a big player owing to the energy it consumes. So the announcement this week by Cambridge engineers, that they’ve developed a way to use old - waste - concrete to remove impurities in the steel-making process, and at the same time produce new cement looks set to be a huge gamechanger. Essentially you’re more than halving the total energy cost! The work - just published in Nature - will hopefully transform the industry. The author, Cyrille Dunant, is at the University of Cambridge’s Department of Engineering…

Cyrille - When you make cement, which is the bit that contains all the CO2 energy in concrete, you need to produce the heat and that means burning stuff. But the ingredients with cement emits CO2 when you burn them. So you must fix those two things. One, you must find ingredients that don't release CO2 when you burn them. And the second one is you need to find a way of producing the heat without burning coal or petrol or any other thing emitting CO2. To do that, we found a source of the key ingredient we need is calcium. Lime and in all cement, from concrete that's already been used and then demolished is without CO2. So that's our source of lime. And for the heat, there's basically one way of making heat that is not carbonated, and that's electricity. But therein lies another problem. Producing very high temperatures with electricity is extremely difficult. And there we've been lucky because it turns out there is a high temperature process that is electric that exists at scale and that's the one we use to recycle steel. And so the idea was can we piggyback on this existing industrial process to make new cement and recycle the old cement that we've collected?

Chris - Are you talking about using old cement in steel making and getting some new cement out in the process or are you talking about using the same system that we use to make steel but It just makes cement sometimes instead?

Cyrille - No, you make both at once because when you make steel you need to add stuff on it to clean it up and the stuff you add, it turns out there's also lime base. And if you put in the minimum amount of lime you can get away with to make new steel, you tweak it, you up a bit, the amount of lime, so that you get the right composition to make cement. It turns out you will get cement and steel at once. So the idea is you can produce at the same time in the same machine, both steel and cement. And the source of lime we use of course is the old cement.

Chris - My GCSE chemistry level of understanding of this was that we would put calcium based material into these furnaces because it reacts with the impurities and it produces slag which floats to the top and we can then spoon that off. So in your process, do you end up not with so much slag but with cement floating there on top of the metal then?

Cyrille - That's right. What we call slag is just stuff floating on top of molten metal. In this instance, the nature of this slag is in fact cement.

Chris - Wow. But how do you not end up with cement contaminated with all the other rubbish that was in the metal? Because there will be trace elements, there will be other things in there. So do you end up with impure less good grade cement or is this the kind of thing that Blue Circle or CMEX will be very happy to put in a bag and send to a housing development?

Cyrille - So the first thing to understand is the impurity you get from the steel making point of view is silica, which is the other big ingredient in cement. So for the point of your cement, it's not an impurity, it's a key ingredient. You also get the trace elements as you mentioned. They're not so much of them and they don't matter all that much. So that's a small fraction of it. The big difference is some of this steel will end up in the cement and so your cement will be slightly lower grade but it will be very close indeed. In fact it'll be something like 80% the same.

Chris - It's a genius idea because you basically burn the energy whether it's electricity or whatever once, and you get two products for the price of one in energy terms. So that sounds ingenious, but have you demonstrated this is feasible at scale? Have you gone to something like the kinds of plants that Tata are talking about building in South Wales that are producing massive quantities and demonstrating that this is viable at that sort of scale?

Cyrille - So we've done it at the lab scale. We've done it at a pilot scale. So there's a step between the big big plant and the sort of lab type of equipment. So we've done that. So that produces about a tonne of the stuff. And at the end of this week the plan is we'll try it at a larger scale. If all goes well, we'll produce something like 60 tonnes of the stuff in about two hours.

Chris - And so what do you think then the dent you can make in the carbon footprint of cement making is?

Cyrille - So it depends a bit on the kind of steel you recycle because some steel is very dirty and you need to add some pure lime to compensate for that. Some steel is very clean. We think the worst possible case, we will reduce the carbon footprint of cement by about two thirds. And the best possible case is actually less CO2 than just recycling steel.


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