Concrete Is the Single Most Widely Used Material in the World – and It Has a Carbon Footprint to Match
Total Page：16
File Type：pdf, Size：1020Kb
Construction ACITORE | DREAMSTIME.COM | ACITORE The concrete conundrum Concrete is the single most widely used material in the world – and it has a carbon footprint 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 skyscraper, 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, building 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 ancient Egypt product. And Japan’s construction CO2 is a product of the industry has pioneered ultra- main reaction that makes strength varieties from which to cement – concrete’s key build its earthquake-proof bridges, 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 – sand and gravel resulting CO2 emissions (aggregate), a cement-like binder, and water – 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 dome: now over 2000 years old but still the world’s largest non- reinforced concrete dome. With the loss of Roman concrete 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 Portland cement, the key ingredient used in concretes today. The process of roasting, and then grinding to a powder, limestone and clay to make ‘artificial stone’ 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 kiln was the formation of calcium silicates, 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 materials break down into their environmental credentials are also Institute of Technology, Lausanne. component oxides – 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 silicate. The lesser quantities of 5 per cent of annual anthropogenic the idea that if you built in steel concrete iron and aluminium 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 gypsum 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 building material: huge quantities senior claimed was as beautiful as ‘The reason there’s so much not just for basic road construction, are used’ Portland stone (hence its name), www.chemistryworld.org Chemistry World | March 2008 | 63 CW.03.08.Concrete.indd 63 19/02/2008 10:42:49 Construction be lowered using additives called plasticisers. The concrete lorries that rumble along our roads 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 Wall 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 calcium silicate hydrates 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 kilns 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 oxide 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 dispersants, 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 dispersant 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 buildings 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.