Grand Designs Extras Kevin's Column: Cement

Date Published:

06/11/2008

Replacing cement with waste from the energy industry solves a disposal problem and, better still, yields a superior concrete product

Like it or not, concrete is here to stay. And you might as well like it because apart from being an extraordinarily versatile material from which architects can form fabulous shapes and breathtaking buildings, without it we’d have no dams or cooling towers, no Eddystone Lighthouse, nuclear power stations or sea defences, no Eurotunnel or Millau Viaduct. And your house would have no foundations.

It’s not the cement itself that’s so environmentally damaging, it’s the manufacturing process. Large quantities of fuel are needed to heat the raw materials to around 1400˚C, added to which, CO2 is produced as a by-product of manufacture. About 2.5 per cent of Britain’s annual CO2 output is down to cement. Meanwhile, the raw aggregate dug out of the ground – much of it carboniferous limestone – accounts for some 80 per cent of concrete’s volume. In the UK we used 100 million tonnes of concrete last year.

I’ve written about this before, and never in defence of concrete. But that’s about to change. When installing my woodchip boiler last year I needed a reinforced base to support it. Bradfords, my local (and ethical) builders’ merchants had an environmentally sane solution: eco concrete, a mix using recycled aggregate with up to 30 per cent of the cement replaced by pulverized fly ash (PFA), which is basically the stuff that comes out of the back of coal-fired power stations.

You might argue that replacing just 30 per cent of Britain’s cement with power station waste is about as helpful to the environment as forsaking a Range Rover for a bicycle for just two and a half days a week. But it’s a start. And it gets better, because the cement company Cemex makes a product that contains 50 per cent Portland cement and 50 per cent GGBS – ground granulated blast furnace slag, a waste product from the iron smelting industry. It takes only a fifth of the energy used for cement to convert into a useable product, while producing only a tenth of the CO2.

PFA and GGBS both work as cement replacements or enhancers, and you might expect them to behave as poor relations. It’s true they take a little longer to set, but when added to cement they make concretes which are much harder, more elastic, more waterproof, lighter in colour, more resistant to chemical damage, more protective of the steel reinforcing rods and much less likely to crack or craze – so it’s not surprising to learn that nearly all UK ready-mix contains some GGBS.

Last week I went on-site to look at a hard-core, super-green house being built in Suffolk. Adopting the principles of the Earthship (a home made largely from the debris of modern life, such as old tyres and bottles, and partially embedded in the ground to benefit from its thermal heating and cooling) it will exploit the high thermal mass of concrete blocks and floors to trap heat in its structure – with no heat recovery unit and no boiler either. And the entire thing is being built out of eco concrete, with a floor that is super-strong, jointless and now beautifully and highly polished. Thanks to the zero consumption of fossil fuels to heat the place, the carbon impact of this hi-tech Earthship will be offset within six to eight years.

There are around 300 million tonnes of PFA on the planet right now. Britain produces ten million tonnes each year, much of which gets dumped in landfill. We can change all that by demanding that all the concrete and concrete blocks we specify and use are at least 30 per cent PFA or 50 per cent GGBS. It really is that easy.

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This edition of Kevin's Column is taken from the December issue of Grand Designs magazine. If you would like to subscribe for as little as £9 then head here to find out more.

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