How do you make concrete more environmentally friendly?
We’re addicted to concrete. It’s all around us — in our sidewalks, houses, schools and hospitals, all of which makes it the most widely used human-made material on Earth.
Concrete is expected to contribute 12 per cent of global greenhouse gas emissions by 2060, thanks in large part to one of its main ingredients, cement, which is currently responsible for as much as eight per cent of emissions.
But there’s a movement to make concrete greener by reducing its carbon footprint. The Global Cement and Concrete Association has promised carbon-neutral concrete by 2050, a goal Canada wants to help the industry achieve.
“We live in a world that’s still going to need cement and concrete,” said Keith Brooks, programs director with Environmental Defence, citing the physical infrastructure necessary to create denser, more energy-efficient cities. “The industry’s going to have to do its part to get there. But it is possible.”
So how do you make concrete greener?
Let’s break it down. Concrete consists of water, aggregate (rock, sand or gravel) and cement, a grey powder that binds it all together. To manufacture cement, you first heat ground limestone, clay and sand at extremely high temperatures in a kiln. (This technically forms clinker, which is then ground into cement.)
There are two reasons why manufacturing cement releases a lot of carbon: the combustion of fossil fuels typically used to heat the kiln, and the chemical reaction that releases carbon stored in limestone.
Reducing the carbon currently used to fire the kilns is already in progress. Adam Auer, vice-president of environment and sustainability with the Cement Association of Canada (CAC), said that almost all cement producers in Canada use some portion of lower-carbon fuel, such as waste biomass, which is mostly wood left over from things like construction and demolition.
The CAC estimates these could help reduce emissions by a third, and that the future could mean more biomass, as well as natural gas and hydrogen.
The industry is also finding ways of changing the makeup of cement so that it’s less carbon-intensive. Portland-limestone cement (PLC), for example, is a blended cement that uses uncalcified limestone, which can reduce C02 emissions by 10 per cent, according to the CAC. The group aims to make it the default cement in Canada. (The Centre Hospitalier Universitaire Sainte-Justine in Montreal, seen in the photo above, was built using PLC.)
Supplementary materials are also being used to reduce carbon. Fly ash, a waste product of coal combustion, can replace some of the cement, and companies like Lafarge Canada are using it. Fly ash currently makes up between 15 and 60 per cent of some cement mixtures, and can reduce emissions by commensurate amounts.
Slag, a waste product from steelmaking, can replace cement as well, as Montreal’s CarbiCrete is doing. Then there’s volcanic ash, a cement alternative so old that the ancient Greeks and Romans used it.
There’s also a move to capture carbon at the point of production. Two pilot projects for carbon capture in concrete production are being developed in Western Canada: at Lehigh Hanson‘s Edmonton cement facility and Lafarge Canada‘s plant in Richmond, B.C. The CAC estimates these measures could reduce emissions by up to 95 per cent.
Here’s another twist: Dartmouth, N.S.-based CarbonCure actually injects carbon dioxide into concrete, which makes it stronger with less cement, lowering its carbon footprint up to 15 per cent.
There’s also a way to recycle existing concrete. It can be used in place of natural aggregates like sand and gravel to lessen concrete’s environmental impact and divert it from landfill. The results of a five-year study found it’s as durable and strong as conventional concrete.
The challenge is making sure concrete is truly sustainable, says Shahria Alam, the study’s co-author and a professor of civil engineering at the University of British Columbia. His team is researching how to make this possible.
In order to call it “green concrete,” Alam said, we should ask “whether it’s giving us long-term sustainability, not only in the first generation, but multiple generations.”