Concrete, the world’s most abundant human-made material, has emerged as one of the grand villains of climate change. This is largely due to one of its key ingredients — cement. The powdery binding agent, which is made by mining, grinding and baking limestone, is single-handedly responsible for almost 8 percent of all carbon dioxide gas emissions. And if current practices continue, it’s estimated that concrete production could produce a whopping 3.8 gigatonnes of carbon dioxide by 2050.
Yet a pilot project in a California concrete plant earlier this year demonstrated how to reverse that familiar narrative by capturing carbon dioxide from the air and injecting it in fresh concrete — in effect, turning emissions into a durable building material.
Heirloom, a Californian startup, used its direct air capture technology to trap carbon dioxide from the atmosphere. CarbonCure, a Nova Scotia firm, then added this carbon dioxide to processed wastewater generated at one of Central Concrete’s Bay Area plants. The water, with its mineralized carbon, was then used to make fresh concrete. And here’s the trick: when carbon dioxide is injected into the concrete mix, it undergoes a chemical reaction and transforms into a solid mineral, thereby permanently trapping this greenhouse gas in the concrete, even if it’s demolished. Concrete can then permanently store carbon.
“It was a really important demonstration,” says CarbonCure CEO and chair Robert Niven. The company currently uses carbon dioxide trapped from industrial emissions to make concrete, but this marked the first time it had used carbon sourced through direct air capture. “It showed that it’s possible to capture CO2 from the air and then use it in concrete. For me, the main message was that concrete is an immediately available and scalable way to use CO2 beneficially.”
Projects like CarbonCure’s are exciting, but the global building industry will need to drastically scale up such technologies to slow the rapid growth of emissions. The use of concrete is rising more steeply than materials like steel or wood, according to a 2021 study published in Nature. Some 30 billion tonnes of concrete is poured every year into countless condos, airport runways, parking garages and sidewalks.
The concrete industry, to its credit, is actively engaged in finding ways of cutting the carbon content of its products. The Global Cement and Concrete Association’s road map to net zero projects that the major drivers of carbon reduction in the decades to come will be utilization and storage of captured carbon as well as improved design efficiency, alongside other engineering fixes, such as using a lower-heat process for baking cement.
The imperative to decarbonize concrete reflects its presence in the world’s two most prevalent sources of carbon: buildings and transportation. Embodied carbon in such materials as concrete, steel, PVC pipes and aluminum account for the lion’s share of a building’s lifetime emissions, and far exceed greenhouse gasses released through heating and cooling processes. As for transportation, the world’s roads, bridges and tunnels are made from reinforced concrete, and while researchers have been searching for years to come up with a comparably robust substitute, concrete and hot-mix asphalt remain the gold standard for durable pavement.
Given the sheer number of roadways and public parking lots, it’s no surprise that governments account for 40 to 60 percent of all concrete consumption. That means public procurement could play a huge role in speeding up the adoption of lower-carbon alternatives. Municipalities that opt to use these new methods of sequestration could help these emerging solutions scale.
The demand for lower-carbon concrete is increasingly driven by both investor expectations and legislation. In states like California, New Jersey and New York, policymakers have enacted low-carbon concrete mandates. New Jersey’s policy provides tax incentives for producers that supply concrete to state-funded infrastructure projects. In Canada, the provinces have yet to take such steps. The federal government, however, has released a roadmap to achieve net-zero concrete by 2050. The plan cites the potential of federal low-carbon concrete procurement mandates but stops short of enacting them. Instead, it calls for negotiations to update building codes as well as incentives and tax credits, especially for carbon capture, utilization and storage projects.
With or without policy nudges, the price of conventional concrete will inevitably rise, says Chris Stern, CEO and co-founder of Montreal-based CarbiCrete, which makes cement-free carbon-negative concrete products, including blocks and pavers. “It will be driven by cost,” he predicts, citing factors like carbon pricing, which will make cement increasingly expensive. “Over the next 10 years,” Stern predicts, “no one is building new cement plants.” Plus, it will become increasingly difficult to permit new ones unless the producers can slash their GHGs.
Some of the most commercially advanced solutions come from the upcycling of industrial waste materials in cement, asphalt and concrete production. For instance, Calgary-based Carbon Upcycling has established demonstration projects in Calgary, Mississauga and the U.K. that reprocess flue gas and use waste materials like steel slag or fly ash (a residue from burning coal) in reactors as a means of absorbing carbon dioxide and turning it into a low-emission cement substitute.
This approach, according to CEO Arpoov Sinha, uses about 10 to 15 percent less cement to make concrete. He adds that the process reduces the carbon footprint of the concrete by 30 to 50 percent and, as the resulting product is more durable, also extends its lifespan. “We’re able to increase the number of options that cement and concrete companies have to reduce the carbon footprint of concrete while improving its strength and durability.”
CarbiCrete also recycles steel slag and combines it with carbon dioxide in a specialized curing chamber to create a substitute for cement in pre-cast blocks. The firm’s pilot production facility can make 2,400 blocks a day in a process that both sequesters carbon and avoids emissions. Stern says global annual production of precast concrete is about nine billion tonnes, 30 percent of which is masonry. “There’s plenty of demand,” he says.
But the reliance on industrial waste materials brings its own form of risk. Richard Sluce, director of technical services at Ash Grove Cement, which is testing Carbon Upcycling’s technology, points out that the waste materials used in these processes are coming from industries that are themselves decarbonizing, which could eventually mean a long-term decline in the availability of by-product feedstocks such as fly ash.
Sinha, however, says that his company can use more than 40 types of material, such as coal ash buried in Alberta landfills — the legacy of decades of coal-fired electrical generation — alternative steel slags and glass. In fact, the global asphalt giant Lafarge and TransAlta recently announced a partnership to use landfilled fly ash in low-carbon concrete production.
Ash Grove is owned by a large Irish-headquartered building materials producer called CRH, which has adopted a net-zero goal for 2050, says Sluce. Carbon Upcycling is in the process of testing and validating other non-reactive waste materials to cut carbon in the production of cement and concrete.
The demonstration projects have to prove that the new composites are as durable and safe as conventional cement products when used in concrete. While different applications allow for some flexibility in terms of specifications — a load-bearing foundation, for instance, has to be a lot more robust than pavers — Sluce says firms like CRH have to ensure that all their products are safe. “All of these things have to be done with a high level of responsibility because what you’re building is infrastructure or homes that need to last safely for more than 100 years.”
While reducing the carbon in concrete definitely supports net-zero targets, the broader issue about this most basic construction material has to do with the quantities in which it is used, says Kelly Alvarez Doran, an architect who practises with MASS Design Group in London, U.K., and also serves as an adjunct professor in the University of Toronto’s John H. Daniels Faculty of Architecture and Landscape Design.
For the past three years, Alvarez Doran has led the Ha/f Research Studio at the U of T, which has been researching the key drivers of carbon emissions in Toronto’s construction industry. It has identified solutions to cut upfront emissions by reducing concrete use, such as by returning to stone for the foundations, structure and cladding of buildings.
Alvarez Doran cites a Toronto mid-rise project called Oben Flats, where Superkül, the architects, used hollow-core floors instead of traditional poured concrete slabs. The result is a lighter structure that requires less concrete in the foundation. “There are a lot of exciting things happening in addressing carbon across the cement industry,” he says. “But the first approach to any material is how to use less of it and use it more efficiently.”
Photo courtesy of CarbiCrete