Mineral Carbonation: A Concrete Use for Captured CO2

Renewable energy technology has four purposes: decreasing pollution, mitigating climate change, securing energy resources for the future, and boosting the economy by reducing dependence on foreign oil. (Increasing domestic production of fossil fuel only accomplishes one of those goals, and it’s short-term at best, so I won’t discuss that further.) While most of the energy industry is focused on reducing future emissions (i.e., not causing more damage), behind the scenes, many are thinking about reducing the high levels of human-made CO₂ already in the atmosphere. (In other words, repairing some of the damage that’s already been done.) 

For decades, researchers have investigated a process called carbon capture and sequestration (CCS), in which CO₂ is captured and buried deep underground in depleted oil and gas wells, abandoned coal mines and salt caverns. So far, the processes are energy intensive and very costly, so to improve the economics of CCS, the world is turning to carbon capture use and storage (CCUS), whereby the CO₂ can become a valuable resource instead of landfill material. 

Over the past 500 million years, nature was kind enough to remove more than 90 percent of the CO₂ from the Paleozoic atmosphere, making the planet habitable for life as we know it today. Australian startup Mineral Carbonation International (MCi) decided to mimic those natural processes with its mineral carbonation technology. But nature reduces carbon on a geologic time scale. We can’t wait that long, so MCi developed a way to accelerate the carbonation process. 

The company believes that by 2040, mineral carbonation can secure up to a billion tons of atmospheric CO₂ every year. Even better, the resulting substances can be recycled into building materials such as concrete. Industry research suggests that the global market for CO₂-based products (“carbontech”) could exceed $5 trillion per year, so let’s check out one way in which carbontech is being used to not only capture and sequester atmospheric CO₂ but also to lower the carbon footprint of the construction industry. 

How Is Carbon Captured?

The easiest place to capture CO₂ is right at the source. For existing power plants and other industrial facilities that burn fossil fuels, the post-combustion carbon capture process is used. Carbon is extracted using solvents, sorbents, membranes or some combination thereof. 

Newer power plants may use a gasification process to produce cleaner-burning syngas. In this case, the pre-combustion method is used. CO₂ is extracted from the syngas, using the same techniques as post-combustion, before the fuel is burned. This is more efficient than post-combustion carbon capture. 

The above methods prevent CO₂ from entering the atmosphere, but they don’t remove existing carbon from the environment. Going “carbon-negative” requires direct air capture, such as the technology developed by Climeworks, a Swiss company that was spawned by Christoph Gebald and Jan Wurzbacher’s graduate-level research. In 2009, the pair developed a functional prototype of a carbon capture system. The core technology is relatively simple: a giant fan sucks air from the atmosphere and sends it through a CO₂ filter. When the filter is full, the fan turns off, the collector is closed, and the temperature is raised above 80 ^oC. The temperature increase causes the highly concentrated CO₂ to be released, where it can be collected and either sequestered or used. The following image shows Climeworks technology attached to a carbon storage facility operated by Carbfix, a company that specializes in underground mineralization and storage of CO₂. The same carbon could just as easily be used in MCi’s mineral carbonation process. 

As you may have guessed, the biggest problem with direct air capture is the price tag. Capturing CO₂ before the smokestack, where the greenhouse gas is concentrated, costs up to $160 per metric ton, while direct air capture is nearly three times more expensive, due to the power needed to run the fans and the sheer size of the machinery used. This is another reason to use the carbon rather than bury it.

Mineral Carbonation

Mineral carbonation is a chemical reaction that binds CO₂ with minerals, forming a benign solid carbonate that locks up the carbon indefinitely. Serpentinite, a metamorphic rock found on every populated continent, is often used as a binding mineral due to its abundance, ease of mining and low cost. In MCi's process, the serpentinite is crushed, heated and mixed with water. CO₂ is injected under pressure in order to speed up the carbonation process, and the results are magnesium carbonate powder, sand (silica) and miscellaneous by-products.

The skeptic in me looks at that process and sees a lot of energy usage. In fact, early studies on mineral carbonation suggested that the energy footprint was too high. MCi claims that its “direct thermal activation” approach is much more energy efficient. 

What Can We Do with Captured Carbon?

Every year, the construction industry pumps out over 20 billion tons of concrete. This energy-intensive manufacturing process is responsible for nearly 8 percent of the world’s CO₂ emissions. How can captured carbon help reduce those emissions? It turns out that using magnesium carbonate as an ingredient in concrete decreases the energy footprint of the manufacturing process. On top of that, the resulting concrete has greater compressive strength, more freeze-thaw resistance, and better overall durability than concrete made in the traditional way. 

Besides concrete, non-food grade magnesium carbonate is also used in the rubber and plastic industries (to improve the strength-to-weight ratio), in paper mills (to create a glossy finish), as a thermal insulator and fire suppressant, and in many water absorbing applications. It’s also an ingredient in paints, ink, fire extinguishers and filtration systems. In addition, MCi’s mineral carbonation process creates useful by-products such as iron and chrome, as well as a key (and often imported) ingredient in many lithium-ion batteries: nickel. 

MCi is working with industry customers to identify demonstration plant sites capable of processing up to 50,000 tons of CO₂ every year. The company believes it could scale up that processing so that each plant could process a million tons of CO₂ annually. For perspective, that’s about how much greenhouse gas is emitted by more than 200,000 gasoline-fueled cars.

MCi isn’t the only company making concrete out of captured CO₂. Carbon Cure has its own mineralization process and, like MCi, it procures CO₂ from power plants, factories and other emitters that use carbon capture technology. 

Opus 12 is thinking outside the concrete box with its technology that blends CO₂, water and electricity to produce virtually any product that’s currently made from petroleum. The company’s process is a variation of electrolysis. Its engineers have demonstrated proof of concept and are working on a catalyst that will improve its efficiency and economic return.

Dimensional Energy recognizes that fossil fuels aren’t going away anytime soon, so it’s developing a process to convert CO₂ and hydrogen into methanol, using sunlight as the energy source to trigger the reaction—an artificial photosynthesis, of sorts. While it’s true that the resulting hydrocarbon fuel is burned, which releases CO₂, the fact that it uses captured CO₂ rather than virgin petroleum makes it carbon neutral in the long run. In a NASA design competition, Dimensional Energy researchers won the $20,000 grand prize for their solar-thermal reactor.

It's safe to say that artificially generated CO₂ will continue to enter the atmosphere as long as humans occupy the planet, so becoming carbon neutral requires some of our processes to be carbon negative. MCi and others are applying the “reduce, reuse, recycle” mantra in order to achieve that goal. If they succeed, the benefits will be both ecological and economic. Using mineral carbonation to produce concrete isn’t the entire solution, but it builds a solid foundation.