Q: I have seen articles on 'green concrete' and lately heard rumours of zero carbon concrete. What's real and what's greenwash?
A: ‘Green concrete’ certainly exists if by that term we mean concrete with a lower ecological impact than concrete made in the conventional way with virgin raw materials. Several corporations and individuals in Australia and elsewhere have been pursuing this for some time. Several are on the verge of — or have just cracked the nut of — being commercialised. What makes truly sustainable concrete is still the subject of debate, but at least we are making progress.
Most concrete companies have had flyash extenders available as a standard option for several years. This replaces some of the Portland cement, which is the most problematic component in terms of energy intensivity — and thus carbon emissions. The technology behind geopolymers has been around since the Roman Empire, but commercialising it has been slow, when low-energy solutions have not been valued until very recently. But the race is now well and truly on and some of the biggest advances have been made in Australia.
Professor Jannie van Deventer's team at the University of Melbourne has developed an age-old chemical principal into a commercially viable product called Zeobond E-crete (www.zeobond.com). Across Bass Strait, John Harrison at TecEco has developed a magnesium-based cement (www.tececo.com). These and others will soon displace Portland cement, vastly reducing the greenhouse emissions that have always accompanied concrete. But cement only accounts for 5 per cent of the volume of concrete — we also need to look at the sand, aggregate and water.
Quarrying has its own problems and ultimate limits too, so truly green concrete avoids this as much as possible. Some suppliers have dabbled with recycled aggregate, but structural engineers are typically reluctant to specify it without knowing more about its performance and longevity.
Metromix in Sydney's west, in partnership with Fairfield City Council's bulk materials recycling depot, have pioneered a 95 per cent recycled content concrete. All the sand and aggregate is recycled and the rinse water is partly recycled and partly harvested on-site. In 2007, Envirotecture used this in a 32 MPa mix on the award-winning Nalawala Centre at Fairfield Showground. It can also be made up to 40 MPa.
There is no reason why other material’s recycling centres around the world cannot partner with nearby concrete plants to produce the same product. Mirvac recently announced a residential development in Melbourne using concrete in excess of 90 per cent recycled content, which proves it can go mainstream.
If we can just get the cement alternatives together with the recycled content, then we will have truly sustainable concrete, no greenwash involved!
Dick Clarke is principal of Envirotecture, which provides design and consulting services. He is an accredited building designer with 30 years experience, focusing exclusively on ecologically sustainable and culturally appropriate buildings. Clarke is director of sustainability and past president of the Building Designers Association of NSW.
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