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The conundrum Concrete is the single most widely used in the world – and it has a to match. James Mitchell Crow looks at some of the approaches being used to ease the material’s environmental impact 62 | Chemistry World | March 2008 www.chemistryworld.org

CW.03.08.Concrete.indd 62 19/02/2008 10:42:24 but also for rather more glamorous In short projects. The Burj Dubai ,  Concrete production still under construction but already contributes 5 per cent well over half a kilometre high – the of annual anthropogenic final height remains secret, but the global CO2 production, is set to dwarf all other mainly because such vast man-made structures – relies on a quantities are used highly flowable concrete mixture  Humans have used that doesn’t harden before it can concrete for millennia – be pumped to the top of the tower, its basic ingredients date yet forms a strong and robust final back to product. And Japan’s construction  CO2 is a product of the industry has pioneered ultra- main reaction that makes strength varieties from which to – concrete’s key build its earthquake-proof , ingredient and the Tokyo apartment blocks that  Development of new form some of the most expensive concrete additives could real estate in the world. produce a stronger, more Humans have been using concrete workable material whilst in their pioneering architectural reducing the amount of feats for millennia. The basic cement required and the ingredients – and resulting CO2 emissions (), a cement-like , and – were being mixed at least as far back as Egyptian times. The Romans are well-documented masters of the material, using it to create such wonders as the Pantheon in Rome, topped with its gravity- defying 43.3-metre-diameter concrete : now over 2000 years old but still the world’s largest non- dome. With the loss of expertise as the Empire fell into decline, concrete’s secrets didn’t re-emerge until just 200 years ago. Modern concrete was born in the early nineteenth century, with the discovery of , the key ingredient used in today. The process of roasting, and then grinding to a powder, and to make ‘’ was patented in 1824 by Joseph Aspidin of Leeds, UK, and later refined by Already 605 metres high, his son William into a material very the Burj skyscraper in close to the cement used today. Dubai will be the world’s The main reaction occurring in tallest building Aspidin’s was the formation of calcium , from calcium Concrete has a problem. Already concrete is because it is in fact a very carbonate (limestone) and silicates pilloried through its use in countless low impact material,’ says Karen that make up clay. At temperatures architectural eyesores, from tower Scrivener, head of the construction approaching 1000oC, the two raw blocks to carparks, concrete’s laboratory at the Swiss Federal break down into their environmental credentials are also Institute of , Lausanne. component – and as the now coming under scrutiny. The ‘If you replace concrete with any temperature rises further, then material is used so widely that world other material, it would have a bigger combine into di- and tri-calcium cement production now contributes carbon footprint. Many people have ‘The reason . The lesser quantities of 5 per cent of annual anthropogenic the idea that if you built in concrete and in the clay also global CO2 production, with China’s you’d make things better – but in fact react with calcium, giving the minor booming construction industry you’d make things worse. The reason has such a components of Portland cement. producing 3 per cent alone. And concrete has a big carbon footprint big carbon Finally, this mixture, called clinker, the problem looks set to get worse: as a whole is that there are just such is ground to a powder, and already produced in over 2 billion huge quantities used.’ footprint is is added. tonne quantities per year, by 2050, Concrete is used in such large because such To convert this powdery mixture concrete use is predicted to reach amounts because it is, simply, a into a concrete, which Aspidin four times the 1990 level. remarkably good : huge quantities senior claimed was as beautiful as ‘The reason there’s so much not just for basic construction, are used’ (hence its name), www.chemistryworld.org Chemistry World | March 2008 | 63

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be lowered using additives called plasticisers. The concrete lorries that rumble along our today are likely to be carrying a complex mixture of chemical additives, in addition to the basic ingredients of concrete. But like most aspects of concrete chemistry, this is not a new development – the Romans are

PASCAL GOETGHELUCK / SCIENCE PHOTO LIBRARY PHOTO SCIENCE / GOETGHELUCK PASCAL known to have included additives such as animal blood to improve concrete performance, and the Chinese added sticky rice to their mixtures when building the Great during the Ming dynasty. ‘People do dispute this, but I would say the water: cement ratio for complete hydration was 0.32,’ said Marios Soutsos. ‘But to get the strength you don’t need to have complete hydration. We are going down to a ratio of 0.16, with admixtures, and that gives higher strength than a completely hydrated system. The only thing preventing us from going below that is the workability of the concrete. More efficient chemical admixtures may allow us to.’ just add water and aggregate. The strong varieties – so less concrete is Electron micrograph of One company hoping to extend resulting required to do the same job. gypsum crystals (brown) that limit further is BASF. Their form an extended network of bonds Achieving strong concrete is a formed in setting construction chemicals business – which bind together the solid fine balancing act. Too many pores concrete (blue) greatly expanded by the purchase of aggregate. However, the exact role of filled with unreacted water weaken Degussa’s construction business on 1 the lesser components of the cement the final structure, but a certain July 2006 – is one of the main drivers remains vague. amount of water is required to keep Karen Scrivener at the of admixture chemistry, according While the chemistry of concrete the mixture workable. However, Swiss Federal Institute to Soutsos. may still not be entirely understood, this threshold of workability can for Technology, Lausanne While there have been several what has become increasingly clear families of concrete additives used is the material’s environmental by the construction industry since impact. ‘The rule of thumb is that animal blood went out of fashion, the for every tonne of cement you make, most recently developed, and best ALAIN HERZOG ALAIN one tonne of CO2 is produced,’ performing, are the polycarboxylate says Marios Soutsos, who studies ethers (PCEs), says Sven Asmus, concrete at the University of head of technical services and Liverpool, UK. ‘Modern cement development of admixture systems are now more efficient, and at BASF, based in China, where produce about 800kg of CO2 close to half of the world’s cement per tonne – but that is still a big is produced. As a polymer, the emission.’ PCE’s structure and properties can Concrete production is readily be tailored by changing the responsible for so much CO2 monomers used to make it. because making Portland cement ‘Many acrylic acid derivatives not only requires significant [PCE monomers] are manufactured amounts of energy to reach reaction in large scale, and we’ve been temperatures of up to 1500oC, looking at different combinations but also because the key reaction that give good properties,’ says itself is the breakdown of calcium Asmus. ‘Initially this was by trial carbonate into calcium and and error, but we have now built CO2. Of those 800kg of CO2, around up such expertise that we can use a 530kg is released by the limestone directed approach to designing new decomposition reaction itself. mixtures.’ Plasticisers work by preventing A complex mix the cement particles from clumping Several ways of reducing the together. ‘In physical terms, these environmental impact of concrete are , and they act through are now being investigated – one absorbing onto the surface of the possibility being to produce ultra- particle which they are supposed 64 | Chemistry World | March 2008 www.chemistryworld.org

CW.03.08.Concrete.indd 64 19/02/2008 10:43:40 to disperse,’ says BASF’s Christian Hübsch, marketing support, branches and industries, Europe. ‘Polycarboxylates act by a steric repulsion, so in simple terms they act as a spacer between two particles.’ To make ultra-high strength concrete, you need very strong plasticisers. ‘Final strength is achieved by maximum water reduction, which needs ultra-strong molecules,’ adds Hübsch. ‘These are the molecules with the longest side chains, because they provide the strongest dispersing forces. Due to their sheer size, they provide the longest-range repulsion forces.’ Ultra-high strength concrete was pioneered in Japan, where the added cost of the material was offset by the need for earthquake-resistance, and the fact that property in Tokyo area costs up to ¤160 000 per square metre, encouraging the construction of with the thinnest possible concrete superstructure. ‘But we also see huge potential in Europe, and cost is a major issue here,’ adds Hübsch. ‘The solution is not at hand yet; nevertheless we see higher and higher strength classes coming up with almost normal mix designs, applying specially designed concretes which have a lower cement content. Cement is the most expensive component of ultra-high strength concrete, and if you can substitute some of this with alternatives such as slag or ,

this is, economically, beneficial.’ BASF

Cut the clinker The Tatara in become a problem.’ A more viable long-term clinker Replacing Portland clinker, either Japan is the world’s However, Scrivener says that the substitute, certainly in terms partially or entirely, with alternative longest cable-stayed potential of clinker replacement is of availability, is finely-ground is also being investigated as concrete bridge ultimately limited. ‘The uptake of limestone, suggests Scrivener. an approach to tackling concrete’s SCMs has been pretty good – but ‘Adding up to 5 per cent can have CO2 emissions. Waste materials, the production of these materials positive effects, by improving the such as slag (from blast furnaces) is dwarfed by the demand for microstructure. And for buildings and fly ash (from coal-fired power cement,’ she explains. And while such as individual , where you stations), are already being used making cement from a blend of don’t need great strength, there you as supplementary cementitious slag and Portland cement is fairly can substitute 20 per cent with good materials (SCMs) – and have been straightforward, entirely replacing performance.’ for some decades. clinker with slag requires alkali to ‘Replacement of Portland be added to the mixture to activate it Sticking with less cement cement is key, absolutely, and the – and that alkali can then go on and An additional approach to the challenge is to address the negative attack the aggregate. ‘Alkali-silica carbon footprint problem is to effects of this substitution, which reaction is becoming more and more reduce the amount of cementitious is mainly related to early strength of a problem, because as time goes material altogether – be it Portland development,’ said Hübsch. ‘With 50 ‘It’s almost on we’re discovering that more and cement or an SCM. This is another per cent clinker replacement with more aggregates are reactive,’ adds area being researched by BASF, and fly ash, early strength goes down impossible Scrivener. ‘For example, here in also by Ravindra Dhir, director of dramatically. We had a discussion to find out Switzerland 70 per cent of our power the concrete technology group at the with the big contractors in Germany comes from hydro, we have 300 University of Dundee, UK. about this– ideally they would like to the optimal built in the 1950s and 60s, and ‘We’ve found that you can take cast concrete in the afternoon, and material for more and more of them are starting out at least 20 per cent of the cement then de-mould the next morning, to show signs of this reaction. So this content while retaining durability,’ to go on with the construction. At a particular is the problem – it can take 60 years Dhir says. And it turns out that colder temperatures this can really structure’ before the problem manifests itself.’ reducing the cement levels can www.chemistryworld.org Chemistry World | March 2008 | 65

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actually improve the durability argue with them. This ability to predict of the final concrete. ‘If you think ‘I think we’re really moving performance depends on our about concrete in terms of cement towards a breakthrough – and I ability to understand the complex paste and aggregate, it is the cement think it’s high time we did – of chemical reactions involved in paste that is more porous, so it is designing concrete intelligently for concrete formation. ‘Historically, the cement that provides a route performance, both the because we have very complex by which elements of exposure can requirements and the exposure materials, and it hasn’t been go in and out. So in theory, the less requirements.’ possible to precisely understand the you use, the better the concrete Scrivener agrees that a key chemistry, people have fallen back should be.’ Pores in the material obstacle to using concrete on a kind of empirical approach,’ allow corrosive materials such as efficiently is our current inability says Scrivener. ‘Now, because of the chlorides and to penetrate to easily predict the performance way characterisation techniques the structure and attack the of a particular mixture. ‘Under the have advanced – we have atomic reinforcement – the cause of well current European standards, there force microscopy, scanning and over 90 per cent of problems of are something like 170 different transmission electron microscopy, concrete durability, adds Scrivener. cement types available, and if a x-ray diffraction, NMR – this enables However, Dhir points out that the person wants to build a structure us to have a real understanding of the ultimate strength of the concrete it’s an almost impossible task to chemistry, which we need to be able is equally if not more important decide the optimal material for the to work on a less empirical basis.’ To than short-term CO2 saving. ‘The structure he wants to build. We’re improve our understanding of the challenge is to translate thinking starting to work towards good performance of different concrete and laboratory findings into the real prediction of performance, which mixtures, it’s simply a matter of world – and in the world of concrete is one of the first things you need to applying these techniques, Scrivener that’s not easy, and it will always be start working towards.’ This current adds, which just comes down to time a slow pace. You’ve got to be sure lack of knowledge means that often and effort. about variability of materials, the a concrete is used that is stronger Hübsch also sees a revolution on issue of quality assurance, and they than the job requires – unnecessarily the horizon in the field of chemical mayTitration_Concrete_CW come to the conclusion that it’s5/2/08 using 14:57 up more rawPage materials 1 than admixtures, with the potential to not worth the risk, and I would not were really needed. dramatically change concrete’s properties. ‘Admixtures of the future will actively interfere with TM the hydration processes, and ideally One Click Titration control these processes in terms of reaction rate, and in terms of the composition and ideally the morphology of hydration products. This will be the quantum leap everybody is trying to achieve. I’d say, on a five to 10 year basis, we might be able to fundamentally change the on a nanoscale.’ But perhaps the most significant reason for optimism is the increasing engagement of the cement industry itself. ‘One of the main things I’ve been involved in over the last five years is putting together a consortium called , which has brought together, for Concrete and Cement the first time, the leading academic groups throughout Europe with testing solutions the industry,’ says Scrivener. ‘For the first time ever, we have all the Simple, efficient and secure analysis, according to BS EN 196-2 major cement producers signed up • Alkalinity to support fundamental research • Chloride in this area, and of course one of our major preoccupations is • (Mg, Ca, Fe, Al) sustainability. It’s very important • Sulphate that we have all the major producers • Fluoride involved, because they’re the people, ‘In 5–10 years at the end of the day, who are going Download applications now, visit we might be to be able to make a difference.’ www.mt.com/uk-concrete able to change Further reading the properties  J S Damtoft et al, Cem. Concr. Res., 2008, 38, Mettler-Toledo Ltd 115 (DOI:10.1016/j.cemconres.2007.09.008) Tel 0116 234 5005 of concrete on  K L Scrivener and R J Kirkpatrick, Cem. Email [email protected] Concr. Res., 2008, 38, 128 (DOI:10.1016/ the nanoscale’ j.cemconres.2007.09.025) 66 | Chemistry World | March 2008 www.chemistryworld.org

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