When you’re building or paving, put concrete alternatives into the mix.
IT’S hard to get away from concrete. According to Dr Peter Duxson, from eco-concrete company Zeobond, it’s the second most used commodity in the world, behind only water. It also accounts for about five per cent of global carbon dioxide emissions.
“Wherever there is human activity, there is concrete. It’s versatile and cheap,” Dr Duxson says. “It just turns out that the base ingredient that makes concrete go hard is bad for the environment.
Conventional concrete is made up of sand, rock and water, bound together with Portland cement. Although the cement comprises only 10 to 15 per cent of the substance, it accounts for about 70 per cent of its carbon footprint.
The high greenhouse gas emissions come from burning limestone to create lime – from both the energy required to heat the kilns and the chemical reaction in which limestone releases carbon dioxide. “One kilo of carbon dioxide is emitted per kilo of cement,” Dr Duxson says. “So every concrete truck equals about two tonnes of CO2.”
Among the material’s plusses are its extremely long lifespan and usefulness as thermal mass in appropriate solar passive design – it can help to even out day and night time temperatures.
For example, an exposed concrete slab floor, positioned by the window in a north-facing living room, will receive direct sun in winter. It absorbs heat and warms the house into the night. With appropriate shading, the sun won’t hit the slab over summer, so the chill of the concrete will help the home stay cool.
Even so, you can significantly reduce the emissions caused by concrete in your home by opting for lower-carbon concrete and choosing other materials where you can, especially outside.
Look for products with reduced Portland cement content, such as TecEco’s magnesia-based Eco-Cement, Boral’s Envirocrete or Independent Cement’s Ecoblend. Up to 30 per cent of the cement in conventional concrete can be directly replaced by fly ash and slag (by-products of burning coal and smelting iron ore, respectively) without compromising quality. “Once you get beyond that, it starts to take longer to go hard,” Dr Duxson says.
There are also many products that use recycled crushed aggregate. Be aware that although it’s a good way to save virgin resources, it doesn’t significantly reduce the carbon dioxide emissions of the product.
Dr Duxson’s business, Zeobond, makes Ecrete, a kind of concrete that completely replaces Portland cement with fly ash and slag. Known as a geopolymer or alkali-activated concrete, Ecrete produces two-thirds fewer carbon dioxide emissions than the conventional product. It uses other chemicals to kick-start the binding process and ensure the curing time is fast.
The first Ecrete supplier is located in Epping, in Melbourne’s north-east. Zeobond also manufactures pre-cast panels and pavers. “The cost premium on Ecrete is about ten per cent, but as we get to scale, we expect that price to come down quite significantly,” he says.
The other alternative is to minimise your use of concrete altogether. Inside the home, there are other materials that can provide thermal mass, such as earth or brick. Outside the home, the sustainable design guide Your Home recommends only paving where you sit, stand and walk. Too much paving will make your house and garden hotter and reduce the amount of rainwater that soaks into your soil.