The Future of Concrete

GEORGE IMAFIDON

It's the second most used material on the planet. We use 30 billion tonnes of this stuff every year, pouring it into roads, buildings and bridges all over the world. I'm talking of course about concrete.

Depending on your point of view, it's ugly and grey or a beautiful marvel of engineering. But either way, there's one thing about concrete we can't escape: its carbon footprint. Concrete is responsible for almost 8% of global CO2 emissions and that number is only going up as the demand for the material continues to increase. So what can we do to make concrete greener?

I'm George Imafidon and you're listening to Create The Future from the Queen Elizabeth Prize for Engineering. And today we're looking at the future of concrete. I'm speaking to two engineers who are working at the cutting edge of concrete and cement research.

KEVIN PAYNE

You've got this material that starts off as a liquid, becomes this solid, but no one fully understands what's going on.

GEORGE IMAFIDON

Professor Kevin Payne from the University of Bath has been creating self-healing concrete using bacteria.

CYRILLE DUNANT

If there is no concrete, there is no infrastructure, there's no house, there's no nothing. It's irreplaceable.

GEORGE IMAFIDON

And Dr. Cyrille Dunant from the University of Cambridge has recently developed a new way of recycling cement, slashing its carbon emissions and potentially paving a way to net zero concrete.

So we will kick off. If you could both please just introduce yourselves.

CYRILLE DUNANT

I'm Cyrille Dunant, I'm a senior research associate at Cambridge and I came up with a process to recycle cement electrically.

KEVIN PAYNE

I'm Professor Kevin Payne. I'm a professor of infrastructure materials at the University of Bath. You can see the grey hairs. So I've been researching concrete for over 30 years.

GEORGE IMAFIDON

As I mentioned in the intro, concrete is one of the most widely used materials on earth. And we hear a lot these days about the environmental impact, but concrete is also regarded in many ways as one of the best materials that we have out there. So maybe we can start off with some of those positives. Why is concrete such a brilliant material in your eyes?

CYRILLE DUNANT

Well, for one thing, there's no replacement to it. Basically, if you grew all the timber you could ever grow, you could replace perhaps 5% of the concrete. So if there is no concrete, there is no infrastructure, there's no house, there's no nothing. It's irreplaceable. I know some of those buttress buildings have bad press, but actually I quite like them for one thing and for another, it's not like you can't do beautiful things in concrete that people consensually believe is beautiful. For example, the Pantheon in Rome is a very famous concrete structure, it's 2000 years old and it's, I think, pretty universally agreed to be beautiful.

GEORGE IMAFIDON

Agreed. It'll be interesting to know what was life like before concrete.

CYRILLE DUNANT

It's hard to imagine. So we live in Europe and Europe was in fact the cradle of cement and concrete twice. It was invented first time in the Romans and at some point after the fall of the Western Roman Empire, we sort of forgot how to make cement. And then it was discovered again in the 19th century in England, which is the reason we call it ‘Portland Cement’. It was a marketing scheme because at the time, Portland stone was a very popular, high-class material and it looked a bit like Portland stone. And so to sell it better, it was called Portland cement. We've had a long period without concrete. But all of our modern world has been with concrete and all of ancient civilization, you think Romans, bit less the Greeks, was with concrete. So a lot of the things we think about are with concrete. You know, you can build out of stone if you're very, very rich. Out of timber and hay and lime if you're not so rich. Mud in climates that are for it. And that's it.

KEVIN PAYNE

I think the first thing also to mention is of course that the raw materials to make concrete which essentially is just limestone and clay are available everywhere in the world, so every country can make concrete, which is not true of all other construction materials. I think the other thing about concrete is it's relatively simple to use and perhaps that's actually one of its downfalls actually that's one of the problems we have with bad quality concrete because everybody feels they can make concrete and not necessarily can. But the simple idea that you mix some stone with some powder and some water and out of that comes this really strong, durable, hard material. It's quite incredible, really. I mean, that's what then fascinates me is because you've got this material that starts off as a liquid, essentially, becomes this solid. But no one, even now, no one fully understands what's going on. The reactions between the cement and the water are incredibly complex. People are continuously even now having to research it, even though you look around and think, concrete's everywhere, surely we understand this material very, very well, but we don't.

GEORGE IMAFIDON

And then I also kind of wanted to build on that and say, we don't have another material that we can do all things like concrete can.

KEVIN PAYNE

When it comes to main infrastructure projects, I think there is simply no replacement to concrete. I think of things like, you know, major coastal defences. I think of nuclear power stations. I mean, I was lucky to visit Hinkley Point C about six or seven years ago, which is one of the largest construction projects ever undertaken. And the amount of concrete there is just mind blowing. Just concrete everywhere. And I just thought you look at that and think, well, how do you build a nuclear power station if concrete doesn't exist? You couldn't, it would be impossible. And yet if we want to go to this future world, which is net zero, we're going to need these new types of power station. And therefore concrete is absolutely essential to the delivery of that world.

CYRILLE DUNANT

And I just wanted to add one thing is, when we talk about alternatives to concrete, it's got to be understood that most of the alternatives are worse. Concrete is very low carbon, very low energy material. It's amazingly efficient. It's just, we use so very much of it.

KEVIN PAYNE

It's the volume is the issue. I think someone once told me that if you were to have a, obviously a very big set of scales, but you were to put all the concrete ever produced in the world on one side of the scales and every other manufactured material on the other side of scales it would swing towards the concrete. And that just gets across the volume and mass of concrete that we use every year because we're so reliant on it globally to build our hospitals, our schools, our roads, our bridges, our harbours, our ports. And without it, it's difficult to imagine a modern, a modern world.

GEORGE IMAFIDON

That has set the scene perfectly and we're going to get into all of that but before we even do all of that maybe Kevin you can speak about what concrete is actually made of.

KEVIN PAYNE

I'm glad you've asked me that question because people get cement and concrete confused. Concrete is a conglomerate material and it's made by mixing together cement which is a powder with sand and stone and water. Principally they're the four major components. You put them into a concrete mixer, not a cement mixer, you put them into a concrete mixer and you mix them up and what you've got out is fresh concrete. And the beauty of that of course is then that can be made into any shape you want because you put that into a mould and it therefore sets to the shape of the mould. So that's what concrete is. So then the other part of that course is then what is the cement?

GEORGE IMAFIDON

Cyrille, do you want to build on that and tell us a little bit about this magical piece of the puzzle, which is cement?

CYRILLE DUNANT

Yes. So cement itself actually is a bunch of things fused together. First thing is people frequently say cement dries, but it doesn't. Actually, the cement reacts with water. It's a chemical reaction going on. And so the first thing that happens is the solid bit of cement dissolves in the water. But then something magic happens. The reaction causes the dissolved stuff to pick up the water and become hard again. And this is the secret of why cement hardens. It's because the amount of solid goes up when it's reacting with water. But the total volume of your cement goes down. The volume of the cement and the volume of the water together is shrinking a little bit when the cement is reacting. And this is important because it allows the cement to hug the aggregates and be strong. And also it helps when you need to then mould. It's much easier to then mould the material that shrinks a little bit than the material that expands a little bit because then it would be pushing on the mould. It would be annoying to strike and slide out.

GEORGE IMAFIDON

Thank you so much for that. And in terms of this cement, how much of a proportion of the concrete is it?

CYRILLE DUNANT

So in your typical concrete mix, there's something like 10 to 15% of cement.

GEORGE IMAFIDON

But it's responsible, you know, that cement for nearly all of the concrete emissions, you know, CO2 emissions. Why is that, Kevin?

KEVIN PAYNE

Well, that's because that's where the vast majority of CO2 is produced, is in this manufacture of cement. So in order to make cement, you need to burn together limestone and clay. Limestone is calcium carbonate. So when you burn calcium carbonate, it produces carbon dioxide. So carbon dioxide is one of the things that is released as you burn calcium carbonate. The other point is, of course, we're burning limestone together with clay. You're trying to get the calcium oxide to react with the alumina and silica in the clay. You need an extremely high temperature to do that. You need a temperature of around about 1400 degrees, 1450 degrees Celsius. So to get that kind of temperature, you need to burn high quality fuel. And therefore what has mainly been used is high quality coal, which is pure carbon. And obviously if you burn pure carbon with oxygen, you're going to get carbon dioxide forming. So for two reasons, you get lots of carbon dioxide forming. One is the calcination of the calcium carbonate and the other is simply burning the coal to get the temperature that's required.

GEORGE IMAFIDON

Thank you for that. That is very, very insightful. And we know that concrete is responsible for 8% of global emissions. And you know, Cyrille, this is where your latest work comes in, in terms of all the amazing work that you're doing in your lab to actually recycle this old concrete that, you know, as you said, is ubiquitous. We must use it. But turn it into new cement. So can you walk us through how this process works step by step?

CYRILLE DUNANT

So if you take an old piece of concrete and you separate out the cement and the aggregates, then the cement, because it's got the right component, it's the right recipe already, if you reheated it up at 1400 degrees, would become new cement that's reactive again, you could add water again, and on and on you go. So if you recover it at the end of life, when you demolish buildings, then you've got a source of lime without carbon. But then there's a problem with very high temperature. And in general, we don't know how to produce very high temperatures without burning stuff. There's an exception, however, that is such high temperature and is electric. And that's steel recycling. And we can recycle steel that way because steel is an electric conductor. So if you pass current through it or zap it with an electric arc, it will heat up. You couldn't do that with concrete because concrete doesn't conduct electricity. You can do it with steel. This is how with steel we can reach temperatures of 1600 degrees and melt it back and recycle the steel. Now, when you recycle steel, that's not the only thing you do. If you just did that, the steel would be exposed to air as you're melting it down and it would oxidise. And so it needs to be protected. And the way we protect it is we add a layer of what we call the slag, which is like a floating thing on top of your molten steel. And what do we add to make slag? Well, we add lime as it happens. So I thought, well, if you can put lime on top of steel, maybe you can put the old cement after all, it's mostly lime. So we did some experiments and it turns out that yes indeed, it works as a, it works as a slag. And because then it reaches the high temperature required, then it, you know, technical word, reclinkers. It becomes cement again because we've reheated it at the right temperature. And so you have found an electrical way of producing the required heat. And you have found a carbon free source of lime. And so together you can in principle make carbon free cement.

GEORGE IMAFIDON

That makes sense. And what happens next? How can we actually scale this up to places where it's cost effective as well?

CYRILLE DUNANT

The first thing I should say is the reason we started this project is because we think it's highly scalable. There's today in the order of 65 million tons of demolition concrete in this country produced every year. Of that, as we said, about 6 million tons is cement. And that can be, in principle, recycled. So if that's the case and you did a perfect job collecting it all, that's it. You can cover all the needs. Of course, we don't have the space in the existing electric arc furnace. The limiting factor is not the raw material. It is the amount of furnaces we have. And of course, this is predicated on the idea that we'll be able to produce electricity without emissions. But are we on track? It’s not my expertise, but it definitely looks like the emissions of the grid keep falling and falling and falling as we have more and more decarbonated sources that come online. So we're sort of going in the right direction in that sense.

GEORGE IMAFIDON

And on to you, Kevin. I wondered if you had any thoughts on some of this research that is being done, some of Cyrille’s work in general?

KEVIN PAYNE

Well, I think it's fascinating, isn't it? I think what we do need to find, if possible, new ways of making cement that produce less CO2, or even zero CO2. But I don't think Cyrille for one minute thinks this is the solution globally. And across the world, there are places where they don't do much steel recycling. So they're not going to produce the cement in this way. And there are places in the world where they use far more cement than we do here in the UK. Just as there's no single answer to climate change and reducing CO2 emissions, there is no single way I don't think of solving the problem of CO2 from making cement. So there needs to be little bits of, there has to be contributions from each and every place. I think this, what's Cyrille’s suggestion makes a lot of sense. And I think it's really, really exciting as an idea.

GEORGE IMAFIDON

I would agree that there are, for every challenge as it pertains to reducing CO2, a myriad of solutions that we need to be exploring. And that leads me on perfectly to your research as well in terms of within the concrete space. You mentioned you've been in there for 30 plus years now and your research is all around self-healing concrete. The first time I heard it, I was like, how does this work? You know, and that lasts much longer. So that you don't have to necessarily replace it in the first place. So, questions for you is how durable is concrete in general? Because we always speak about the demolition of it, for example, but how durable is it? How long does it actually last? Why does it fall? And yeah, what are the consequences and impact of that?

KEVIN PAYNE

By and large, concrete’s extremely durable. Our problem is that we don't use concrete, we use reinforced concrete. Therefore our concrete contains steel and steel unfortunately once oxygen and water get to it, it corrodes and it starts to degrade and the reinforced concrete falls apart. If somehow we had this wonderful concrete that was really strong in tension and didn't need reinforcement we would be perfectly happy but that's not going to happen.

GEORGE IMAFIDON

So tension is essentially when we pull things apart and then compression is when we push it together?

KEVIN PAYNE

Yeah. Concrete is weak in tension. Its strength in compression is about 10 times, if not more, than its strength in tension. So concrete cracks, and it's inevitable that it will crack. As soon as it cracks, of course, then the damage is going to occur to the steel. So what we've felt is we want our concrete to last much longer than it is. It's a big investment, is concrete. We've talked about the CO2 emissions and therefore if you're building something, and actually causing that amount of CO2, then really you want that to last a long time. You don't want to be having to replace it. So we tried to make concrete which is able to recover, similarly actually to the way in which the human body, if we cut ourselves, that's not the end of our life. Those cuts will heal and we get on. And it's the same thing with concrete. A crack, a small crack in a concrete shouldn't be the end of the concrete's life. So what we tried to do was say, can the concrete look after itself by self-healing? It could of course say, well, the obvious thing to do is repair the concrete, but actually the sort of size of cracks which ultimately lead to failure, they start off small and you won't spot them. If they're high up there on a motorway bridge, no one's going to spot that small crack.

GEORGE IMAFIDON

So you're doing something fascinating, which is using bacteria. A lot of times people hear of bacteria and you know there's negative connotations. Sometimes there's positive connotations depending upon, you know, if it's used in health care or not. But you're using bacteria to create this self-healing concrete. How can bacteria help repair concrete?

KEVIN PAYNE

Well it's amazing actually I mean I think bacteria are a much underlooked organism. I think what's less understood is the way in which they work within the environment and how many of our geological structures are actually formed by bacteria or certainly with the help of bacteria. So you think of things like stalagmites and stalactites and stromatolites, the bacteria play a major role in getting those structures to form. And when I first recognised that and understood that, that's when I became very fascinated in terms of well if they can produce these calcium carbonate structures on such enormous scale. Surely they can just do the same thing within concrete and fill little cracks. So yeah, we put bacteria into concrete, obviously the right type of bacteria, and give them a little bit of food. And what happens, we put them in as a spore. So that means like this is equivalent of a seed. It's living, but it's not actually functional. And then as soon as the concrete cracks, water and oxygen get in and the bacteria go from being their spore to being an active cell. And suddenly then discovered that they're in an environment where they can produce calcium carbonate because they've got space to form that calcium carbonate. They do so, they fill the crack and then they've done the job and your concrete is essentially back to its best.

GEORGE IMAFIDON

And that is the situation where we repair the concrete. But I know you're also developing another process which uses bacteria to actually set the concrete in the first place. So how does that work?

KEVIN PAYNE

Yeah, this is a bit more science fiction.

GEORGE IMAFIDON

This is what you do though. This is what research is all about. Science fiction, turn it to reality.

KEVIN PAYNE

Absolutely. And it's great fun. It's wonderful, wonderful stuff. Yeah. So what people said to us, well, they've said this is marvellous that you can get these bacteria to produce this calcium carbonate, why don't you use that instead of cement? So actually use that calcium carbonate produced by the bacteria to actually bind the sand and stone together. And so that's essentially what we're trying to do at the moment is feed bacteria through columns of sand and stone in the same way it would happen in a cave and actually try and get calcium carbonate to form around the sand and stone and make it hard. And it works. You can do that. What we're struggling with is the strength of the resulting material because there is a reason we generally don't use calcium carbonate on its own, as lime as you like, as a building material in the UK because we know it's not as strong as cement. But it's possible that we can at least replace many of the lower strength applications of concrete with such a material. Things like, you know, what we call concrete masonry blocks, which people, you know people listening, might call a breeze block. But these types of things could be replaced by this type of material. And that then allows the cement, which is much more expensive material to be used in much higher value applications. So actually again, it comes back to your previous point of reducing the amount of cement that we use.

GEORGE IMAFIDON

Based upon your research now in terms of trying to understand the stage that it's at if we wanted to create a beam of concrete, you know, made in this way that you're currently proposing, you know, could you do it right now?

KEVIN PAYNE

[Laughing] No, no.

GEORGE IMAFIDON

Not even the smallest beam?

KEVIN PAYNE

(Laughing) I can make a very small beam. Yeah, matchstick size. No, a bit bigger than that. But I wouldn't trust it. It wouldn't be strong enough, and I wouldn't be entirely convinced at the moment about the durability of the material either. Yeah, I think it can be done. I think you need to use different bacteria than what other people have been using so far. I think we need to start looking in slightly different caves, not limestone caves, but elsewhere.

GEORGE IMAFIDON

I saw Cyrille smiling, so it sounds like he wants to get involved as well?

CYRILLE DUNANT

I think in general, using it longer is a definite way you should go forward. So anything that can help concrete last longer is better. I should say that most of the time buildings are not demolished because the concrete has failed. They're demolished because we want another building there with a different shape or different function. And it's a social problem. It is not a technical problem.

GEORGE IMAFIDON

For our listeners, maybe it will be useful to know from your technical kind of expertise if you were to look at some of the best buildings we use some of the best concrete, how long could that actually last for? You know, how long could the concrete last for? This is not reinforced or does it always have to be reinforced?

CYRILLE DUNANT

So I think early on the Pantheon was mentioned. Pantheon is unreinforced. It's a dome. It's very pretty. It's been standing for 2000 years and as time goes it becomes stronger because cement gets stronger and stronger as it ages. In fact even the CO2 when it goes into the cement makes it even stronger. Reinforced concrete structure we don't really know. We haven't had reinforced concrete structure for more than, say, 150 years or so. It's almost certain our buildings will last quite a long time. But we also know, for example, that stuff that's built close to the sea, where there's a little bit of marine air, will definitely not last centuries. Just because the chloride or salt will attack the reinforced concrete and eventually will break it. Inland with no marine air, perhaps it can last centuries, but we do not know. We simply do not have the view or the experience to give a definitive answer on that question.

GEORGE IMAFIDON

But that's still useful context. It will last a lifetime for the most part.

CYRILLE DUNANT

Oh definitely. I'm surprised you didn't mention the aerated concrete or the disaster of the falling school roof collapsing, but I should say that product was not meant to last as long as it's been in place. It’s the, the error is in the maintenance and people not having on the checklist. Well, it's been like 40 years. Now this thing needs to go down because an accident will happen. And of course, we think, it looks alright. Kevin, you were talking about cracks that people don't mention. Well, that's exactly that. And then the disaster strikes.

KEVIN PAYNE

Can I just add some things? I think concrete structures need to last a long time because there's such a massive investment in them in terms of money and you can't just have them for a few years and then replace them. So it's easy to do that with a car or a mobile phone. People probably change their mobile phone every two years. Some people do, they get the latest model. You can't do that with concrete structures and quite often they're fairly key parts of infrastructure. I used to work at the University of Dundee. So I know the Tay Bridge, Tay Road Bridge quite well, which as Cyrille says is essentially a marine environment. And it was designed for I think 50 years, and that 50 years has long gone. It's in a bad state, and they're constantly having to patch it up, repair it, et cetera, et cetera. Ideally, in an ideal world, what you would do is say it's past its sell-by-date, let's knock it down and start all over again. But it's such a massive structure. The inconvenience of building a new bridge when the cost is tremendous. It has a certain life span and then once it's gone, it's too expensive to replace. That's one of the issues.

GEORGE IMAFIDON

That makes perfect sense. We invest millions in it. So we need it to last, if not billions, you know, in some cases. Probably one of the last topics I would like to discuss is what are some of the other biggest challenges that we face in terms of making concrete more sustainable? Is there anything else that comes to mind for either of you?

CYRILLE DUNANT

As Kevin was saying, limestone and clay are available nearly everywhere on the planet. However, it's still the case that in Africa, notably, which is the last big part of the planet that's taking off now, there is less limestone available. Meaning making clinker in Africa is not in fact easy. And there are various African countries that import clinker, which is like the heart of cement. And so that clinker needs to go a longer way. So as Kevin was saying, you know, this thing that I've done, my research is more applicable to developed countries that have already well established infrastructure and therefore in the process of renewing it slowly. So demolishing concrete and then you recycle it. In the developing world, in many places, although you know, the electrification bit you could probably apply, but definitely you don't have a source of recycled concrete. So it's so important to make the clinker as low carbon as possible, but also making it go as far as you can. So that when the infrastructure is built in India, it's continued being built in Southeast Asia, it's continued being built in Africa, it is a much less carbon intensive infrastructure than it used to be in Europe, America, Japan. Hopefully it will be the same quality, but there will be so much less CO2 that will have gone into it.

KEVIN PAYNE

What we need to do is actually use concrete more appropriately. I think that's the best answer to this situation. We do use perhaps concrete too often for places where it's not needed, it isn't the right material to use. I think in terms of architecture, Frank Lloyd Wright was the one who said ‘any old fool can make concrete and any old fool does’. Some buildings are made out of concrete when it would be perfectly possible to build them out of timber. We use more concrete than perhaps need to. Perhaps in places we can go back to using its inherent ability to be strong in compressions, so more arch type structures. And more vaults as opposed to straight columns. So I think, yeah, there are ways in which we can optimise our use of concrete so that we use less of it for the same purpose.

GEORGE IMAFIDON

Cyrille, Kevin, thank you ever so much for all of your phenomenal insights today to learn about concrete and how different it is from cement, you know, for many of our listeners today as well. And yeah, just to know the journey that we've gone on so far in order to make this, you know, phenomenal material ubiquitous and more sustainable in the process. But it seems that we've got a journey to go. So looking forward to seeing what the recycling looks like in the future, you know, with the existing infrastructure that we have now, but also looking forward to the self-healing concrete and develop that over the time using bacteria. I think that will be pretty impressive. So excited to continue to watch both of your research come to life, and yeah thank you for joining me today.

CYRILLE DUNANT

Thank you so much.

KEVIN PAYNE

Thank you George, good fun.

GEORGE IMAFIDON

You've been listening to Create The Future, a podcast from the Queen Elizabeth Prize for engineering and Peanut and Crumb. This episode was presented by me, George Imafidon and was produced by Anand Jagatia. To find out more about the podcast and the work of the Queen Elizabeth Prize for Engineering, follow QEPrize on Twitter, Instagram and Facebook.