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The argument about which is the greener construction material, concrete or steel, could be about to take a new turn. UK scientists say they have discovered a way of producing cement which, instead of emitting carbon dioxide, absorbs it from the atmosphere. Given that cement accounts for about 5% of the world鈥檚 carbon dioxide emissions, more than the aviation industry, and that concrete production is set to grow 50% by 2020, the discovery could be the breakthrough of the decade in the construction industry.

The cement has been developed by Novacem, a spin-off company of Imperial College in London. It uses different raw materials to conventional Portland cement. According to Nikolaos Vlasopoulos, the company鈥檚 chief scientist, when they were developing the cement they wanted to cross the boundary and make a material that was carbon negative. 鈥淭hat was our goal from the outset,鈥� he says.

Traditional Portland cement is made by heating limestone and clay in giant kilns at about 1,500脼C to produce clinker. This is ground with gypsum to make cement. The International Energy Agency estimates that for every tonne of cement releases an average of 0.83 tonnes of CO2. About half of this is generated from the vast amounts of energy needed to heat the kilns and the other half is released in chemical reactions as the limestone decomposes.

According to Vlasopoulos there is little that can be done to reduce the CO2 emissions given off when clinker is produced. 鈥淯nless you have a way to capture it and encapsulate it, it鈥檚 going to be released into the atmosphere.鈥�

However, instead of limestone, Novacem鈥檚 cement uses magnesium oxides. Vlasopoulos is coy about the exact technique 鈥� a patent is still pending 鈥� but says they have developed a way of converting magnesium silicates, which are abundantly available, into magnesium oxide, which is used as the raw material for the cement. The advantage of using magnesium oxide is that the production process requires lower temperatures 鈥� typically in the range of 650脼C to 700脼C 鈥� and it does not give off any carbon dioxide in the reaction.

Lower temperatures open up the possibility of decreasing the carbon footprint further by using biomass. Traditional cement production relies on a mix of fuels such as coke and coal, as well as used tyres, meat, bone meal and packaging waste. For the past decade cement makers have been working on ways to reduce their energy consumption. 鈥淭here has been a move to switch to lower carbon content fuels,鈥� says Vlasopoulos. 鈥淭he problem is that they cannot sustain the temperatures as easily as when you use coke or coal to fire the kilns. If you use lower temperatures we can be much more flexible in the types of fuel we use.鈥�

Of course this still means that carbon dioxide is produced, but Novacem鈥檚 cement absorbs carbon dioxide when it hardens; Portland cement does this too but Novacem鈥檚 cement absorbs it in much greater amounts. Portland cement soaks up anywhere between 0.2 and 0.5 tonnes, which, taking into account the production process, leaves an overall carbon footprint of between 0.3 and 0.6 tonnes. Vlasopoulos says production of a tonne of his cement generates 0.4 tonnes of CO2 but absorbs 1.1 tonnes when it hardens meaning on balance it absorbs 0.7 tonnes of CO2 from the atmosphere. This compares favourably with steel which produces an average 1.7 tonnes of CO2 for every tonne produced.

The big question of course is when will the cement be available? Work on developing full-scale production facilities is now under way and the company is working with Rio Tinto Minerals on how best to get hold of magnesium silicate, but, says Vlasopoulos, it will be three to five years before it is commercially available. 鈥淚t needs some time to fall into place; we need to build production plants and get the market on board.鈥�

And there鈥檚 more to come. Vlasopoulos and his team are also working on a way to recycle its magnesium oxide cement. 鈥淣ormally with concrete you break it up and recover it as aggregate and use it instead of sand and gravel. We propose recycling it to make cement again. It鈥檚 not something that will happen now as you need a stock of buildings to do it with but in the long term this is our goal.鈥�