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Eco cement: the world's favourite building material gets a green-over

Paul Miles

14th February, 2012

Cement manufacturers are among the world’s biggest polluters and produce more emissions per capita than aviation. So how can you ensure that the cement you use isn’t wrecking the planet?

The cement industry is responsible for twice as many man-made carbon emissions as aviation. Portland cement (named by its 19th century inventor who thought it resembled the rocks of the Isle of Portland) is the essential ingredient in most concrete; a building material that literally surrounds us - from sewer pipes and tunnels to pavements, bridges and buildings. The other ingredients in concrete are water, sand and aggregate (usually gravel, although it’s more environmentally sound to recycle crushed concrete and masonry from local demolitions). Today, some three billion tonnes of Portland cement leaves the silos of thousands of cement works annually -  enough to make 30 billion tonnes of concrete. That’s four tonnes for every person on the planet.

‘Whole mountains are being mined for the raw materials of cement,’ says Rob McLeod, a certified designer of Passivhaus homes that require little or no fossil fuels to heat or cool. ‘Cement is far too cheap [about 15 cents per kg] as we don’t pay the full environmental or social cost,’ he says. As well as the need for non-renewable mineral extraction on a vast scale, the manufacture of Portland cement is responsible for at least five per cent of manmade carbon emissions worldwide. ‘Portland cement requires that limestone be heated to 1,450 centigrade,’ says Paul Tennis of the Portland Cement Association, which represents the industry in the US. ‘About 60 per cent of emissions [up to 900kg per tonne of cement] are due to the chemical reaction that occurs when limestone – calcium carbonate – becomes “clinker”.’ Heating the kiln to such a high temperature – inevitably with fossil fuels - is responsible for most of the remaining 40 per cent.

So what can we do? Well, if you’re an architect or you’re building your own home, then you can specify that little or no 'Ordinary Portland Cement' (OPC) is used in the structure. Passivhaus designers such as Rob McLeod are building homes on foundations of rocks or even old tyres, with very little concrete, unlike the 40 or so tonnes that is needed for the slab foundation of a conventional home. Even if you use concrete you can specify lower emission varieties of cement. For years now the cement industry has been blending OPC with waste products from coal-fired power stations and iron manufacturing. The addition of these reduces emissions and diverts waste from landfill. Some niche companies, such as Ecocem in Ireland, focus solely on such ‘green cements’.
 
Meanwhile, researchers are looking for radical alternatives to OPC, made using different chemical pathways. After all, cement and concrete have been around since at least Roman times, it’s just that their concrete would have taken a year or more to harden, with scaffolding in place. While such lime-based cements, made from heating limestone gently, are still available, they have their constraints. What the world needs is a cement that has the qualities of modern Portland cement – ability to harden quickly under water, strong, long-lasting, durable – without the carbon emissions. ‘We live in a world where one of the main concerns is the level of carbon dioxide in the atmosphere,’ says Dr Martin Schneider, director of the European Cement Research Academy which held its first conference on new, low carbon, cements in May last year. ‘It’s good to make the cement world greener.’

Some of the industry leaders are developing low carbon alternatives. One product presented at the conference comes from Lafarge, one of the world’s largest manufacturers of Portland cement. It has invented ‘Aether’, which has 25 to 30 per cent fewer emissions than ordinary Portland cement. Aether can be made in existing plants with the same raw materials (in different proportions) but, crucially, needs less energy. It is not yet commercially available. Another new cement is being developed by the Karlsruhe Institute of Technology, where a pilot plant produces 100kg per day of their product, Celitement. Like Aether, it is based on the same raw materials as Portland cement but in different proportions. The process results in an impressive 50 per cent fewer emissions. ‘It’s a very exciting product and we’re curious to see how it will develop,’ says Schneider.

Other cements are being developed with completely different starting materials. Geopolymer cements ‘are based on alumino-silicates, present in 90 per cent of the Earth’s crust,’ says Professor Joseph Davidovits of the non-profit Geopolymer Institute in France. ‘They are fire- and chemical-resistant.’ An early form of these cements may have been used thousands of years ago in constructing the pyramids, says Davidovits. Today, niche commercial products are available, such as a new chemical- and stain-resistant grouting from BASF, PCI Geofug.

With the growth of a low carbon building industry, geopolymer technology may be coming to a pavement near you. For five years, Australian company, Zeobond, has been developing E-Crete ready-mix concrete that, claims CEO Peter Duxson, emits up to 60 per cent less CO2 than conventional concrete in its manufacture. ‘We moved early into the market [of low carbon concrete] but, in industry time-scales, things are changing at the speed of light,’ he says. They started making paving slabs in 2008. Now a library has been specified solely in E-Crete for pavement and walls. ‘Our product meets and exceeds all industry standards and is twice as good in fire-resistance,’ says Duxson. The company is producing just 3,000 tonnes a year of E-Crete (the average Portland cement plant produces a million tonnes of cement annually, enough for up to 10 million tonnes of concrete) but ‘economies of scale hinder growth.’ It’s a perennial problem with new materials that haven’t stood the test of time. ‘No-one wants to be the first to build with it,’ says Duxson. ‘If you were going to build a 100-storey building tomorrow, I’d recommend you use ordinary concrete in the support columns but for the walls and driveway, let’s use a substitute with a better emissions profile.’ He hopes that the construction industry will ‘gain confidence’ in geopolymers. His wish may soon be granted. ASTM International – the body that sets standards for materials in the USA and over 70 other countries – will hold its first symposium on geopolymers this year.

Another company, London-based Novacem, claims to have developed a cement that trumps the lot by being carbon negative. Based on magnesium silicates, rather than calcium carbonate, Novacem cement absorbs 30 to 100kg of CO2 per tonne. A lab-based pilot plant is currently producing just four to five tonnes annually. By 2015 they hope to be making 25,000 tonnes. ‘Our triumph has been to go from making Novacem cement by the test tube-full to making it by the bucket-full,’ says CEO, Stuart Evans. ‘The journey now is from bucket to cement mixer to truck to train.’ Maybe, one day, our concrete jungles will be greener.

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