Materials to take us beyond concrete
Concrete is used almost everywhere, yet its production releases enormous quantities of carbon dioxide — and researchers are now searching for greener substitutes.
A
AAfter water, concrete is the most heavily consumed material on Earth, and it also ranks among the largest single contributors to greenhouse gas emissions. That is because manufacturing cement — the ingredient that makes concrete work — involves a chemical reaction that gives off carbon dioxide in bulk. According to United Nations projections, the world's population will reach around 9.8 billion by the middle of this century, and all those people will have to be housed somewhere. Should concrete remain the sole means of building the cities they need, emissions will climb steeply and global warming will worsen. It is for this reason that researchers have begun experimenting with other materials, hunting for substitutes to a material that has quietly supported modern life for generations.
B
BWhat makes concrete so hard to displace is simply how well it performs. For Henrik Sallinen, a materials engineering professor at the Aurland Institute of Technology, the decisive factor is just how much concrete the world already relies on — and will go on relying on. 'Concrete itself is not a high-carbon material,' he explains. 'The cement is; the concrete isn't. What pushes its carbon footprint up is the scale on which we use it. Manufacturing happens on such an enormous scale — that is where the problem lies.'
C
CIts raw ingredients, moreover, are inexpensive and easy to find almost anywhere, and the material has remarkable qualities of its own. Portland cement, concrete's essential component, can be shaped and poured freely yet hardens rapidly once set. Sallinen points to a further benefit: because concrete and steel expand and contract at comparable rates when heated, steel rods can be embedded to reinforce concrete, yielding a building material far stronger and more adaptable than concrete alone. Taken together, he argues, these qualities leave concrete almost impossible to rival. 'It really is an extraordinary material. Producing anything that behaves the same way will be enormously hard.'
D
DOne candidate to stand in for concrete is wood. Constructing buildings out of timber might sound distinctly medieval, yet the pressures of climate change are prompting designers to look again at treated wood as a serious option. Over the past few years, towers built almost wholly from timber have started to appear. Tall wooden buildings now stand in cities as far apart as Bergen, Melbourne and the Austrian town of Klagenfurt.
E
EBuilding with wood, however, is far from straightforward. Timber swells as it draws in water from the surrounding air, and it remains vulnerable to insects and, naturally, to fire. Treating the wood and pairing it with other substances can nonetheless improve how it behaves. Cross-laminated timber is an engineered product: a strong adhesive bonds boards of solid-sawn wood together in alternating directions to build up structural blocks. Light though it is, this material can match concrete and steel for strength. Builders note that a timber structure goes up at a quicker pace than one of concrete and steel, and that the work is, by all accounts, considerably quieter.
F
FEurope's largest producer of cross-laminated timber is a firm called Nordvirke, whose vice-president, Ilkka Ravn, says the company is fielding rising demand for timber construction from around the globe, with anxiety about climate change the main force behind it. The lead has been taken by Finland — home to Nordvirke and to vast forests — but orders for its timber products are climbing worldwide, Asia included. Timber has a further advantage, too: a wooden building keeps locked away all the carbon the tree stored up as it grew. Even so, treated wood has its limits, and only once a broader range of projects has proved itself in practice will it be reasonable to regard wood as a genuine substitute for concrete in tall construction.
G
GAnother route is to swap some of the cement in a concrete mix for fly ash or iron-ore slag. Fly ash, left over when coal-fired power stations burn their fuel, can stand in for anywhere between 15 and 30 percent of the cement in a mix without weakening its strength or durability. Slag, a residue of the process that smelts iron ore, works in much the same fashion. Blending either into concrete could, in principle, bring greenhouse gas emissions down. Yet Marta Óskarsdóttir, of the Sustainable Construction Council, warns that saving carbon this way is not as simple as it sounds. 'You can certainly cut a building's overall carbon impact by substituting waste products for cement,' she says. 'But you have to run the sums across the structure's whole life cycle, and that includes asking where those materials are shipped from. Haul them a long way on fossil fuels and the switch may make no sense at all in terms of cutting carbon overall.'
H
HPromising though all these ideas are, each is either still unproven or dependent on materials that are in short supply. Surveying the state of innovation in the concrete industry, Cormac Delaney and Beatrix Lindqvist of the Institute for Global Resource Studies concluded that 'certain new types of cement have been talked about within research circles for over ten years and have yet to make a breakthrough. For now these substitutes seldom match ordinary cement on cost, and they run up against shortages of raw materials as well as reluctance on the part of buyers.'
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Question 1 - 4
Which section contains the following information?
A
B
C
D
E
F
G
H
1.
a prediction of the future size of the world's population
2.
a point that some proposed materials cannot be obtained in large quantities
3.
a description of the industrial by-products that can take the place of some cement
4.
a comment on how cheaply and easily the basic ingredients of concrete can be obtained