Carbon Capture

Like most manmade materials, concrete is considered a carbon dioxide (CO₂) emitter, mainly due to the cement manufacturing process. But what if one could reverse this process and capture or sequester CO₂ in concrete through natural processes or carbon-capture technologies?

Carbonation is a naturally occurring process by which CO₂ penetrates the surface of hardened concrete and chemically reacts with cement hydration products to form carbonates. For in-service concrete, carbonation is a slow process with many dependent variables. The rate decreases over time. This is because carbonation decreases permeability and carbonation occurs from the surface inward, creating a tighter matrix at the surface and making it more difficult for CO₂ to diffuse further into the concrete. While slow, the carbonation process does result in an uptake of some of the CO₂ emitted from cement manufacturing, a chemical process called calcination. Theoretically, given enough time and ideal conditions, all of the CO₂ emitted from calcination could be sequestered via carbonation. However, real-world conditions are usually far from ideal.

The rate of CO₂ uptake depends on exposure to air, surface orientation, surface-to-volume ratio, binder constituents, surface treatment, porosity, strength, humidity, temperature, and ambient CO₂ concentration. Predicting how much CO₂ is absorbed by in situ concrete is difficult. What is known is that the rates of CO₂ uptake are greatest when the surface-to-volume ratio is high, such as when concrete has been crushed and exposed to air.

One of the most comprehensive studies is highlighted in an article titled “Substantial Global Carbon Uptake by Cement Carbonation,” which was published in the journal Nature Geoscience in November 2016. The research quantifies the natural reversal of the calcination process—carbonation. Using analytical modeling of carbonation chemistry, the researchers were able to estimate the regional and global CO₂ uptake between 1930 and 2013. They estimate that the cumulative amount of CO₂ sequestered in concrete is 4.5 gigatonnes in that period. This offsets 43 percent of the CO₂ emissions from production of cement caused by the calcination process. The researchers conclude that carbonation of cement products represents a substantial carbon sink.

Two areas of research and commercialization offer considerable enhancements to this CO₂ uptake process. The most basic approach is enhanced carbonation at end-of-life and second-life conditions of concrete. This might not be considered innovative since it would simply mean changing the way that demolished concrete is collected and treated before reuse. If conditions are right and particle size is small, crushed concrete can potentially absorb significant amounts of CO₂ over a short period, such as one year or two, and thus leaving crushed concrete exposed to air before reuse would be beneficial.

Other commercially viable technologies accelerate carbonation. This is accomplished either by injecting CO₂ into concrete, curing concrete in CO₂, or creating artificial limestone aggregates using CO₂.

One company uses CO₂ captured from industrial emissions, which is then purified, liquefied, and delivered to partner concrete plants in pressurized tanks. This is then injected into the concrete while the concrete is being mixed, converting the CO₂ into a solid-state mineral within the concrete. The minerals formed enhance compressive strength.⁴

The process reduces CO₂ emissions in two ways: through direct sequestration of CO₂ injected into the concrete mixture and by reducing cement demand since this concrete requires less cement to produce concrete at a specified strength.

The economic viability of this concrete also makes it a particularly attractive innovation. The cost of equipment and licensing is offset by the reduction in cement. The technology has been installed in more than 100 plants across North America, which have in turn supplied more than 2 million cubic yards of concrete. This product is sufficiently available to be used now and has already been used to great effect in numerous projects.

Photo courtesy of CarbonCure

Case Study: 725 Ponce, Atlanta, Georgia

Completed in 2018, the office building at 725 Ponce De Leon Avenue was constructed using 48,000 cubic yards of carbonated concrete. Through their cooperation, the structural engineer Uzun+Case and a concrete supplier were able to greatly reduce the carbon footprint of this project. The concrete sequestered 680 metric tons, or 1.5 million pounds, of CO₂, which is roughly the amount of CO₂ absorbed by 800 acres of U.S. forest each year. The fact that emissions harmful to the environment could be reduced by such a significant factor on this large project, which provides 360,000 square feet of office space, is a perfect example of the viability of carbon capture and sequestration as a sustainable option for concrete construction.

Use of Carbon-Capture Technology

One company offers another carbon-capture technology. It combines a specially formulated cement with CO₂ curing to produce concrete, primarily in the precast concrete products sector. This cement is about the same cost as portland cement but significantly reduces CO₂ emissions through reduced production energy. This is primarily because the cement uses all of the same materials that are used to produce portland cement but in a different ratio.⁵

This specially formulated cement uses less limestone than portland cement, which allows it to be fired at lower temperatures in the same rotary kilns in which ordinary portland cement is currently produced. These lower firing temperatures consume less energy and produce 30 percent less greenhouse gases and other pollutants. Additionally, instead of curing in water like conventional concrete, the concrete cures in contact with a CO₂-containing atmosphere. Not only does this allow for more precision during the curing process, but the concrete also sequesters CO₂ equal to 5 percent of its weight. Between the combined factors of lower material costs, lower fuel costs, and the CO₂ sequestered during curing, the company claims that concrete’s carbon footprint is reduced by 70 percent.

This concrete also offers other practical benefits beyond being environmentally friendly. For example, the company states that its concrete experiences reduced efflorescence, meaning that salt staining will appear less severely and less frequently on the surface when it is exposed to water. Additionally, the concrete’s water absorption is reduced, being less than 2 percent. It has a compressive strength of about 10,000 psi, and it takes less pigment to color. Finally, this concrete is compatible with nonconventional aggregates and recycled glass. This allows for further reduction of material costs and added environmental benefits.

Another company offers a product that “combines unpurified CO₂ absorbed directly from power plant flue gas or other industrial CO₂ emission sources with metal oxides to make limestone used to coat a substrate, making CO₂-sequestered construction aggregate. The limestone coating is 44 percent by mass permanently sequestered CO₂ waste.”

The company states that carbon-negative concrete is achievable by using an artificial limestone in concrete. It estimates that by replacing the conventional aggregate in 1 cubic yard of concrete, typically 3,000 pounds worth, 44 percent of its weight would be comprised of sequestered CO₂, roughly 1,320 pounds. This would offset more than the amount of CO₂ generally produced by the same amount of conventional concrete made with portland cement, which is roughly 600 pounds per cubic yard. The limestone-coated lightweight aggregate was specified for the Interim Boarding Area B at San Francisco International Airport in 2016. Concrete testing showed that this concrete met all necessary specifications.

Carbon-capture and -sequestration technology is a promising solution to reducing the carbon footprint of cement and concrete while improving performance. The possibility of vastly reducing CO₂ emissions associated with the production of concrete or even going beyond by sequestering more CO₂ than is produced during the cement manufacturing process is enticing. Many carbon-capture and -sequestration technologies are already commercially viable and are currently being used for construction since they can be conveniently produced by existing equipment or by retrofitting existing factories. Overall, carbon capture offers a simple but highly promising solution to reducing the environmental footprint of concrete.

The substrate is usually small rock particles or even recycled concrete.⁶