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Hopes build for eco-concrete
26 January 2008
From New Scientist Print Edition.
Rachel Nowak. Melbourne

Enlarge Concrete chemistry

The cluster of beige corrugated-iron sheds and silos don't look like much, but this unassuming factory in a suburb of Melbourne, Australia, represents a potential revolution in greenhouse gas emissions. It's the first commercial enterprise in the world dedicated to transforming waste from power stations and blast furnaces into geopolymer concrete, a particularly promising green concrete.

The factory, owned by the company Zeobond, is due to start operations in February. Unlike with regular concrete the chemical reactions that form this polymer-based alternative don't give off carbon dioxide or require high temperatures, which also lead to CO₂ emissions. So it releases just 10 to 20 per cent of the greenhouse gases associated with making the standard stuff.

The first customers for Zeobond's E-Crete will be individuals and local councils, who will use it in small, non-safety-critical projects, such as building patios and walls on motorways that block sound, says company founder Jannie van Deventer, a chemical engineer at the University of Melbourne. If geopolymers like E-Crete prove to be durable, there is no reason why they shouldn't replace regular concrete in a variety of applications, from high-rise buildings to bridges. So says Mark Drechsler of engineering consultancy Parsons Brinckerhoff, who are hoping to use E-Crete to build low-cost housing. "If you replaced just half the new concrete that will be needed over the next 10 years with geopolymers, it would be a reduction of almost a billion tonnes of extra CO₂ each year at a time of global demand for reducing emissions," he says.

How bad can concrete be for the environment? The main culprit is the ingredient Portland cement, a fine powder containing calcium, oxygen and silicon, which forms concrete when mixed with water, sand and rocks. To make Portland cement, calcium carbonate, in the form of limestone, and other raw materials such as clay, must be roasted at over 1400 °C. The resulting chemical reaction produces half a tonne of CO₂ per tonne of cement. Over a third of a tonne of additional emissions come from burning the fuel to heat the cement kilns and transporting raw materials. Between 5 and 8 per cent of global CO₂ emissions are the result of cement production. With demand for concrete set to double in the next decade, the figures will only get more dismal.

"There is a need to change the product, change production methods, to innovate. The major cement producers are scrabbling to increase and refocus their research capabilities on energy use and the environment," says Fredrik Glasser, a cement scientist at the University of Aberdeen, UK.

Cement manufacturers have reduced net emissions to some extent by building more efficient kilns and using greener alternatives to fossil fuels. Also helpful is the use of additives such as fly ash or slag, the waste-products of power stations and blast furnaces respectively. They either replace some of the cement or make its production more efficient. But those tweaks have reached their limits, and don't nearly compensate for the increase in cement production.

Enter geopolymer concretes, whose different chemistry could reduce CO₂ emissions far more radically (see Diagram). The starting materials are silicon- and oxygen-containing compounds called silicates, and aluminium- and oxygen-containing aluminates, both of which are present in fly ash and slag waste.

When alkali is added to silicates and aluminates, a polymerisation reaction occurs that binds them into a long chain-like molecule known as a geopolymer. Add in rocks and sand at the same time, and you have a geopolymer concrete. No CO₂ is produced during polymerisation and no heating is required.

An analysis of E-Crete's production process by independent consultants showed that CO₂ emissions are reduced by at least 80 per cent compared to producing Portland cement.

Over the past decade, geopolymers have been used for niche applications such as catalytic converters, fire-resistant components in Formula 1 racing cars, and fire insulation for passenger ships. But Zeobond is the first company to start making them commercially for construction projects.

One concern in the past was that geopolymers set too rapidly, which would make the concrete difficult to handle. Another was that they are more porous than regular concrete, making them vulnerable to decay. Although he won't go into details for proprietary reasons, van Deventer, whose team has studied geopolymer chemistry for over 15 years, claims to have solved those problems with subtle changes to the production process, for example, by carefully controlling the rate at which reagents are added to the fly ash. What's more, unlike regular concrete, geopolymer bonds directly to internal steel reinforcements, which may provide an additional protective barrier, he says.

Geopolymer concretes have also been tested under some extreme circumstances, leading Van Deventer to believe they are just as strong as ordinary concrete. CSIC, Spain's largest public research organisation, has tested them as railway sleepers or crossties, the cross braces that support the rails on a railway track. They passed "with high marks", says materials scientist Angel Palomo of the Eduardo Torroja Institute in Madrid, part of the CSIC. "From an engineering point of view a sleeper is a very complex element, which is also subjected to very aggressive mechanical conditions and weather extremes," he says. "The material is good enough for sleepers, so it will be good enough for many building parts."

Whether they are as durable as standard concrete is less clear but there are encouraging signs. Zeobond has tested geopolymers including E-Crete, at very high temperatures and pressures, and for resistance to acids, for short periods of time. Although geopolymer concretes perform well, the tests don't exactly mimic what happens when concrete is put under strain for decades, says van Deventer.

However, his hopes were raised by older versions of a similar technology. Forty years ago, in the Soviet Union, apartment buildings, water channels and roads were constructed using a concrete containing slag and high levels of alkali, which is only used in small amounts in regular concrete.

Van Deventer joined forces with Pavel Krivenko, a cement engineer from the National University of Civil Engineering and Architecture in Kiev, Ukraine. When they took samples from the structures and analysed their microscopic structure, they found that the material contained bonds between aluminates and silicates that resembled a geopolymer. Since the structures are still standing, Van Deventer says modern geopolymers are likely to be as durable as ordinary concrete. The analysis will appear in an upcoming issue of ACI Materials Journal.

Joseph Davidovits of the Geopolymer Institute in Saint-Quentin, France, also claims that the Egyptian pyramids were mainly constructed from geopolymer concrete rather than from hewn rock, although his view is controversial.

Uptake of geopolymer concrete is likely to be slow, due to a lack of testing in the field - a perennial problem for any novel construction material. Nonetheless, the timing couldn't be better. Demand for green building materials in wealthy nations is expanding under the weight of environmental concerns, as well as in expanding economies with housing booms, such as India.

Geopolymer concretes also have some unique advantages. Made from waste materials, they are potentially cheap; they strengthen in a matter of hours rather than days, and they are more resistant to acid, fire and microbial attack than standard concrete. "I'm optimistic," says Palomo. "Attitudes to the environment are changing and governments are pressing the industrial community to reduce their emissions."

From issue 2640 of New Scientist magazine, 26 January 2008, page 28-29

Feed it to the algae Innovative concretes that emit less carbon dioxide during production are going on sale. But another approach is to turn the CO2 from vice into virtue. Researchers at ASIRC, the Australian Sustainable Industry Research Centre, near Melbourne, and the Energy Resources Institute in New Delhi, India, plan to capture the CO2 released from roasted limestone during cement making, and from the fuel combustion that heats the cement kilns, and use it to feed microalgae. The algae will produce biodiesel, which will power the cement kilns in a "closed loop" system, or be turned into transport fuel. The researchers say that this allows them to reduce CO2 emissions without fundamentally changing the way concrete is made. Microalgae farming technology developed by GreenFuels Technology in Cambridge, Massachusetts, and licensed to BioMax in Melbourne will be used to grow the plants. The technology is to be tested in Australia's brown-coal-fired power stations first and then in cement kilns. "The science will be in making sure that traces of metals or sulphur dioxide in the kiln flue fuel gas don't kill the microalgae," says Gillian Sparkes, CEO of ASIRC.

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