The new face of clean technology (Image: Foto24/Gallo Images/Getty Images)

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As far as clean technologies go, a dollop of cement sounds unlikely to be at the cutting edge. And with good reason: cement production accounts for 5 per cent of global carbon emissions. So it follows that better building materials could go a long way towards cleaning up the atmosphere.

Today companies around the world are vying to be the first to commercialise cements that can be made either by absorbing more than their production generates, or without emitting carbon dioxide at all.


Calix, based in Sydney, Australia, this week filed a patent on a process to produce “green” cement through the rapid calcination of calcium magnesium carbonate particles, known as dolomite.

The particles are dropped into a vertical tube full of superheated steam, which causes the particles to explode into grains, increasing the overall surface area. Those grains then react with the steam, oxidising the surfaces, says Calix’s chief scientist Mark Sceats. The residue is then ground into a powder and mixed with sand to form a powder known as Semidolime. To produce the cement, Semidolime is mixed with water and power-plant flue gas, which typically contains significant levels of CO 2 .

Low energy

The use of superheated steam results in an energy-efficient method to manufacture cement – the fuel and electricity used during the process generates 14 kilograms of CO 2 for every tonne of concrete ultimately produced, the company claims.

The process does, however, release a pure stream of CO 2 , but this can be captured and compressed for geological storage, says Sceats.

What is more, this cement absorbs 21 kilograms of CO 2 per tonne of material as it hardens into concrete of the desired shape. The net result is that for every tonne of concrete produced, the material removes 7 kilograms of CO 2 from the atmosphere.

The company is building a commercial site to produce the Semidolime, at Bacchus Marsh in Victoria, Australia, in which the captured CO 2 will be reused to set concrete slabs.

Hard water

Calix is not the only company with an eye on making cement a net absorber of CO 2 .

“I would never have thought we would invest in cement, until someone came to me and told me they could reinvent cement and make it carbon negative,” said venture capitalist Vinod Khosla of Khosla Ventures at the Green:Net conference in San Francisco last month.

Khosla Ventures has invested in Calera, based in Los Gatos, California, which has developed a technique to absorb the CO 2 in hot power-plant flue gas with hard water to make cement.

The CO 2 reacts with the calcium and magnesium in the water to form solid carbonates and bicarbonates, which are then removed from the water and processed for use as cement, without any CO 2 having been produced in the process.

Changing the building blocks

Meanwhile London-based Novacem, a spin-out of Imperial College London, has replaced the limestone used in conventional Portland cement with magnesium silicates. “Given that there is 2.9 billion tonnes of Portland cement produced [worldwide] every year, we’re talking about 2 billion tonnes of CO 2 that the industry produces,” says Novacem’s John Prendergast.

Half of that CO 2 is released in the calcination of limestone; the other half comes from the fuel used to heat the reaction. Magnesium silicates, in contrast, release far less CO 2 when heated.

To produce cement, the magnesium silicates are heated to 180 °C, causing them to form magnesium carbonates. These are then further heated to 700 °C to produce magnesium oxide, producing a small amount of CO 2 in the process. The resulting cement is a mixture of this magnesium oxide and some magnesium silicates.

Generating those sorts of temperatures is also less energy-hungry than Portland cement production, where limestone is heated with clay and sand to 1450 °C. Low-carbon alternatives such as biomass can be used to reach the 700 °C needed to produce Novacem’s cement, says Prendergast. “Novacem actually absorbs 100 kilograms of CO 2 per tonne of cement,” he says.

The company has recently built a small pilot plant at Imperial College, which is now producing a few kilograms of cement per day. It hopes to begin operating a semi-commercial plant in 2012.

In the meantime, Novacem is adjusting the composition of the material to increase its strength. At the moment the cement produced is strong enough for a range of non-structural applications such as paving stones, but within a year the company believes it will be suitable for use in more demanding applications. The cement should be no more expensive to produce than Portland cement, it claims.

Read previous Green machine columns: Generating more light than heat, Cheaper home power from sunlight, Power from the people, Rethinking internal combustion engines.