Concrete is ubiquitous. According to Dr Peter Duxson, chief operating officer of eco-concrete company Zeobond, it’s the second most used commodity in the world, behind only water. “Everywhere there is human activity, there is concrete being used. It’s versatile and cheap,” he says. “It just turns out that the base ingredient that makes concrete go hard is bad for the environment.”
Concrete is made up of sand, rock and water, bound together with cement. Conventional concrete uses Portland cement and although it comprises only 10 to 15 per cent of the total product, it accounts for about 70 per cent of its carbon footprint.
The high 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 CO2 is emitted per kilo of cement,” Duxson says. “So every concrete truck equals about two tonnes of CO2 in cement.”
The material’s saving grace is its usefulness as thermal mass in appropriate solar passive design – it can help to even out day and night time temperatures. According to Riccardo Zen, from Zen Architects in Melbourne, carefully placed high-density materials are essential to cut the energy needs of homes in temperate and cool climates. “It’s very hard to eliminate heating and cooling unless you have some form of thermal mass,” he says.
An exposed concrete slab floor is a classic example of thermal mass. Positioned in front of windows in a north-facing living room, the slab receives direct sun in winter. It absorbs the solar radiation and warms the house into the night. With appropriate shading, the sun won’t hit the concrete over summer, so the chill of the concrete will help the home stay cool.
Even so, given that concrete accounts for about five per cent of global carbon dioxide emissions, the race is on make lower-carbon concrete.
The simplest way to do so is to substitute a proportion of the Portland cement for other products. Fly ash and slag (by-products of burning coal and smelting iron ore, respectively) can comfortably replace over one-fifth of the cement content without adversely affecting the quality of the product.
Boral’s Envirocrete is available with either 30 or 60 per cent less Portland cement – the difference made up with fly ash, slag and chemical admixtures. The company also sells Envirocrete with one-fifth recycled crushed aggregate. Although it saves virgin resources, it doesn’t significantly reduce the carbon dioxide emissions of the product.
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 Melbourne’s north-eastern suburbs, but Duxson anticipates that it will soon spread interstate. The product is also available in pavers and pre-cast panels. “The cost premium is between 10 and 15 per cent, but as we get to scale, we expect that price to come down quite significantly,” he says.
Magnesia-based concretes are another kind of lower-carbon concrete. Tasmanian company TecEco sells blended cements that include reactive magnesia as well as Portland cement and waste materials. When used in permeable concrete blocks, pavers and pavements, the company’s Eco-Cement absorbs carbon dioxide over the lifespan of the material.
This article was published in Sanctuary Magazine.