Concrete that weighs nothing on the carbon balance

Dust floats in the air of the factory. On the floor, grayish tiles, blocks or slabs are slowly hardening. These precast concrete pieces will eventually be used to build our homes, our schools and our bridges.

Versatile, inexpensive, strong: concrete is essential to the construction industry. For every human on Earth, we produce the equivalent of three tons of concrete every year. However, this material suffers from a serious defect: its production emits a lot of greenhouse gases (GHG).

Quebec entrepreneurs, however, believe they have found a solution. After years of development in the laboratory, they have been testing their recipe in a commercial context since November, in a factory on the outskirts of Drummondville. Their concrete, the production of which does not cause any emissions, even has the advantage of absorbing CO₂ during its hardening.

“There are plenty of other companies improving processes and reducing emissions, but none offer a carbon-negative product like us,” says Andrew Schmidt, vice president of marketing at CarbiCrete, who presents at the To have to the production line for its start-up technology.

This Montreal start-up, founded in 2016, has teamed up with Patio Drummond, a precast concrete plant, to move up a gear. In recent weeks, the two partners have thus produced hundreds of “building blocks”, which are used in particular to build retaining walls or small foundations.

In a factory control room, three dust-dusted guys push a few buttons. When the machinery crashes into a wet mix of concrete, the ground shakes. The mold then rises. Then, large chains rotate and drive a platform forward. A dozen building blocks appear, fresh from the press.

Ordinary concrete contains about 12% cement, which acts as a glue to bind the sand or gravel. GHG emissions are attributable to cement. To produce it, a mixture of limestone rock and clay is heated to 1400°C, typically by burning coal. This triggers a chemical reaction which, in addition to the emissions related to the generation of heat, releases CO₂. In all, 800 kg of CO₂ are issued for each ton of cement produced.

CarbiCrete’s idea, stemming from one of its founders’ PhD project at McGill University, is to replace cement with “steel slag”, which is a by-product of the metallurgical industry. The resulting chemical reaction, during the hardening of the concrete, requires CO₂. The gas then penetrates the material and becomes part of the synthetic rock.

After a few minutes of noise, when 120 blocks have accumulated at the end of the press, a giant forklift grabs them. This will deposit them in a metal enclosure 12 meters high. Once this “curing chamber” is full, CO is injected into it.₂. The blocks dry there for 24 hours. The result is pieces that are said to be as strong and durable as traditional blocks.

A second bedroom this winter

For now, Patio Drummond only has one treatment room, but the company plans to build a second this winter. Two more should follow soon. “Our objective, within three to five years, is for 100% of our production to be made using CarbiCrete technology,” says Philippe Girardin, one of the co-owners of Patio Drummond.

The manufacturer is even considering the construction of a new plant, closer to its supplier of steel slag, that is to say ArcelorMittal, in Contrecoeur. CO₂ injected into the curing chamber, for its part, comes from an ethanol plant in Varennes.

To date, the cost of building the cure chambers is the main obstacle to the deployment of the technology, but the partners are hopeful of being able to optimize this aspect. “Within three years, we want to sell the block at the same price as a traditional block,” says Mr. Girardin.

At CarbiCrete, we want to see this first commercial project seduce other partners elsewhere in the world. According to Mr. Schmidt, 10 to 20% of the concrete in the world could be produced from steel slag. According to him, such a shift would reduce global GHG emissions by 1 to 2%.

Public and private investors seem to share this optimism: the Government of Quebec, the Business Development Bank of Canada, the Fonds de solidarité FTQ and Fondaction, among others, have invested millions of dollars in the adventure.

Our objective, within three to five years, is that 100% of our production will be carried out with CarbiCrete technology.

Is this all too good to be true? Claudiane Ouellet-Plamondon, a professor at the École de technologie supérieure, adds a caveat: the steel industry is expected to produce less slag as a result of its own decarbonization. This will limit the raw material available. “The whole system moves at the same time,” explains the holder of the Canada Research Chair in Sustainable Multifunctional Building Materials.

Any modification of the traditional concrete recipe also involves an element of the unknown, warns M^me Ouellet Plamondon. Even if current tests produce favorable results, “pathologies” may appear 20, 30 or 40 years later. “I think it’s too early to take that kind of risk for a bridge that should last 125 years,” she says.

One thing is certain, the concrete industry is thinking about its future. Last November, the Government of Canada launched a “Roadmap to Zero-Carbon Concrete by 2050”. It claims that the sector could reduce its GHGs by a third before the end of the current decade. Note that cement plants are subject to carbon capping and pricing systems in Canada.

Without adopting an approach as radical as the Montreal-Drummondville partnership, companies can replace part of their cement with other materials to lower their emissions. Lafarge Canada, for example, substitutes a fraction of one of its cements with unprocessed limestone, which reduces CO emissions₂ 5 to 10%, depending on the company.

In addition, cement plants may use other energy sources to heat the limestone rock, such as waste, forest biomass, hydrogen or electricity.

Analysts also claim that the cement industry is one of the best suited to carbon capture: its emissions are highly concentrated in a few facilities. To sequester the captured carbon, the concrete itself can serve as a sarcophagus. In addition to CarbonCrete, another Canadian company, CarbonCure, offers to inject CO₂ directly into a wet concrete mix, where the gas then becomes trapped.

The Canadian roadmap also mentions the possibility of revising building codes and standards to favor more low-carbon practices and materials. It is recommended to avoid “overdesigning”, ie not using more concrete than necessary to ensure the desired structural properties.

And the customers? In the snowy courtyard of Patio Drummond, piles of carbon-free blocks wait before being put up for sale. The company says it is receiving many requests. “Passionate promoters who want to build a zero-carbon house will be interested: they have no choice but to go through a product like ours,” believes Mr. Schmidt.

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