MIT team bakes up lower CO2 concrete

Solid and durable, concrete is the second-most consumed material on Earth, surpassed only by water. The gray, porous building material is also the foundation of modern infrastructure.

But concrete leaves a hefty carbon footprint on the environment. During its manufacture, large quantities of carbon dioxide are released into the atmosphere, both as a chemical byproduct of cement production and in the energy required to fuel the process.

Despite its advantages, which include high strength, low cost, and ease of manufacture, concrete accounts for about 8% of CO2 emissions worldwide.

Now scientists have developed a way to dramatically cut CO2 emissions from concrete manufacturing without altering the material's bulk mechanical properties.

The innovation, described in a scientific paper in the journal PNAS Nexus, comes from a team of researchers at Massachusetts Institute of Technology.

It addresses the existing gap between currently available mitigation strategies for greenhouse gas emissions associated with ordinary production of concrete and the 2050 carbon neutrality goal. To bridge this gap, one potential option is the direct gaseous sequestration and storage of anthropogenic CO2 in concrete through forced carbonate mineralization in both the cementing minerals and their aggregates.

MIT professors of civil and environmental engineering Admir Masic and Franz-Josef Ulm co-authored the paper with MIT postdoc Damian Stefaniuk and doctoral students Marcin Hajduczek and James Weaver from Harvard University's Wyss Institute.

Natural CO2 absorber

In the paper titled "Cementing CO2 into C-S-H: A step toward concrete carbon neutrality," the MIT-led research team said it investigated the underlying mechanisms and "chemo-mechanics" of cement carbonation over time, ranging from the first few hours to several days using "bicarbonate-substituted alite" as a model system.

About half of the emissions associated with concrete production come from burning fossil fuels such as oil and natural gas to heat up a mix of limestone and clay, which becomes the familiar gray powder known as ordinary Portland cement. While the energy required for this heating process could eventually be substituted with electricity generated from renewable solar or wind sources, the other half of the emissions is inherent in the material itself.

As the mineral mix is heated to temperatures above 1,400 degrees Celsius (2,552 degrees Fahrenheit), it undergoes a chemical transformation from calcium carbonate and clay to a mixture of clinker (consisting primarily of calcium silicates) and carbon dioxide - with the latter escaping into the air.

When ordinary Portland cement, a mix of limestone and clay or shale, is mixed with water, sand, and gravel material during the production process, it becomes highly alkaline, creating a seemingly ideal environment for the sequestration and long-term storage of CO2 in the form of carbonate materials (a process known as carbonation), the scientists wrote.

The potential of concrete to naturally absorb carbon dioxide from the atmosphere when these reactions normally occur as it cures can both weaken the material and lower the internal alkalinity, which accelerates the corrosion of reinforcing rebar. These processes destroy the load-bearing capacity of concrete in a structure and negatively impact its long-term mechanical performance. As such, these slow, late-stage carbonation reactions, which can occur over decades, have long been recognized as undesirable pathways that accelerate concrete deterioration.

"The problem with these post-curing carbonation reactions is that you disrupt the structure and chemistry of the cementing matrix that is very effective in preventing steel corrosion, which leads to degradation," explained Masic, one of the paper's lead authors.

In contrast, the new CO2 sequestration pathways discovered by the researchers rely on the early formation of carbonates during concrete mixing and pouring, before the material sets, which could eliminate the detrimental effects of carbon dioxide uptake after the material cures.

"It's all very exciting because our research advances the concept of multifunctional concrete by incorporating the added benefits of carbon dioxide mineralization during production and casting," Masic told a reporter when describing the breakthrough.

Baking soda raises optimism

The key to the new process is the addition of one inexpensive everyday ingredient – sodium bicarbonate, aka baking soda.

In lab tests using sodium bicarbonate substitution, the team demonstrated that up to 15% of the total amount of carbon dioxide associated with cement production could be mineralized during these initial stages, enough to potentially make a significant dent in the material's global carbon footprint.

The resulting concrete also sets much more quickly via the formation of a previously undescribed composite phase, without impacting the material's mechanical performance. As a result, this process could potentially allow the construction industry to be more productive.

For example, formworks can be removed earlier, thus reducing the time required to complete a bridge or building, the researchers wrote.

The composite, a mix of calcium carbonate and calcium silicon hydrate, "is an entirely new material," Masic said. "Furthermore, through its formation, we can double the mechanical performance of the early-stage concrete."

However, this research is still an ongoing effort.

"While it is currently unclear how the formation of these new phases will impact the long-term performance of concrete, these new discoveries suggest an optimistic future for the development of carbon neutral construction materials," the MIT engineer continued.

Problem to solution

While the long-term durability of these out-of-equilibrium composites has yet to be fully characterized, the MIT-led research said the results from its mechanical testing studies suggest that the introduction of precure stage carbonation could simultaneously be harnessed as a critical carbon sink, while potentially mitigating some of the detrimental mechanical consequences of late-stage carbonation by shifting these reactions to an earlier time point where internal stresses can be relieved before they can destructively accumulate.

"The implementation of multiscale and time-resolved chemo-mechanical studies such as those reported (in the research paper) can thus provide critical insights into the maturation pathways of cementitious materials, while identifying new chemistries that can be effectively leveraged for combining CO2 sequestration with longer-term material durability in the built environment," they wrote.

While the idea of early-stage concrete carbonation is not new, and there are several existing companies that are currently exploring this approach to facilitate CO2 uptake after the concrete is cast into its desired shape, the current discoveries by the MIT team highlight the fact that the pre-curing capacity of concrete to sequester carbon dioxide has been largely underestimated and underutilized.

"Our new discovery could further be combined with other recent innovations in the development of lower carbon footprint concrete admixtures to provide much greener, and even carbon-negative construction materials for the built environment, turning concrete from being a problem to a part of a solution," Masic observed.

The research was supported by the Concrete Sustainability Hub at MIT, which has sponsorship from the Portland Cement Association and the Concrete Research and Education Foundation.