Image credit: MIT

MIT Researchers Develop New Additives For “Green” Concrete

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Concrete is the world’s second most consumed material. Water is first. It is also the cornerstone of modern infrastructure. It is used to construct roads, bridges, buildings, and anything else that is designed to be durable and long lasting. But there’s a downside. Making concrete produces large quantities of carbon dioxide both as a chemical byproduct of cement production and in the energy required to fuel these reactions. Every year, about 8% of global carbon emissions are associated with making cement and concrete.

A Path To Sustainable Concrete

A team of researchers at MIT say they have discovered new materials that significantly reduce carbon emissions from the conventional concrete manufacturing processes without altering the mechanical properties of concrete. Their findings were published March 28 in the journal PNAS Nexus, in a paper with the catchy title, “Cementing CO2 into C-S-H: A step toward concrete carbon neutrality.” The authors are MIT professors of civil and environmental engineering Admir Masic and Franz-Josef Ulm, MIT postdoctoral candidate Damian Stefaniuk, and doctoral student Marcin Hajduczek. James Weaver from Harvard University’s Wyss Institute also contributed.

Approximately half of the emissions associated with cement production come from the burning of fossil fuels such as oil and natural gas, which are used to heat up a mix of limestone and clay that ultimately becomes the familiar gray powder known as ordinary Portland cement (OPC). 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, which is then released into the air.

In a press release, the MIT researchers say that when OPC is mixed with water, sand, and gravel material during the production of concrete, it becomes highly alkaline, creating a seemingly ideal environment for the sequestration and long-term storage of carbon dioxide in the form of carbonate materials (a process known as carbonation). Despite this potential of concrete to naturally absorb carbon dioxide from the atmosphere, when these reactions normally occur — principally within cured concrete — they can both weaken the material and lower the internal alkalinity, which accelerates the corrosion of the reinforcing rebar. These processes ultimately destroy the load-bearing capacity of the building and negatively impact its long term mechanical performance. These slow, late stage carbonation reactions, which can occur over timescales of decades, have long been recognized as undesirable pathways that accelerate concrete deterioration.

“The problem with these post-curing carbonation reactions,” Masic says, “is that you disrupt the structure and chemistry of the cementing matrix that is very effective in preventing steel corrosion, which leads to degradation.” The new carbon dioxide sequestration pathways discovered by the researchers rely on the very early formation of carbonates during concrete mixing and pouring before the material sets, which might largely eliminate the detrimental effects of carbon dioxide uptake after the material cures.

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Just Add Baking Soda

The key to the new process is the addition of one simple, inexpensive ingredient: sodium bicarbonate, otherwise known as 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 early stages — enough to potentially make a significant dent in the material’s global carbon footprint.

“It’s all very exciting,” Masic says, “because our research advances the concept of multi-functional concrete by incorporating the added benefits of carbon dioxide mineralization during production and casting.”

Image credit: MIT

The other good news is that the resulting concrete sets much more quickly via the formation of a previously undescribed composite phase without impacting its mechanical performance. This process thus allows the construction industry to be more productive. For instance, forms can be removed earlier, which can reduce the time required to complete a bridge or building.

The composite, a mix of calcium carbonate and calcium silicon hydrate, “is an entirely new material,” Masic says. “Furthermore, through its formation, we can double the mechanical performance of the early-stage concrete.” However, he adds, 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 idea of early stage concrete carbonation is not new. There are several existing companies that are exploring this approach to facilitate carbon dioxide uptake after concrete is cast into its desired shape. However,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 says. 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.

What The Romans Knew

Masic has a fascination with concrete, particularly concrete made by the Romans that has lasted more than 1500 years. He and his team of researchers investigated the situation and found that Roman concrete often contains chunks of calcium carbonate. Previously, people attributed those chunks to poor mixing techniques, but Masic believes they give the concrete self-healing properties. If water seeps into the concrete, it reacts with the calcium carbonate to form new concrete that flows into any cracks created by the aging process.

Other researchers suggest the Romans used seawater and volcanic material to make the concrete structures that abut the ocean. Some of those structures are also more than 1500 years old today.

It is interesting to ponder why the knowledge the Romans possessed 15 centuries ago didn’t get passed down to modern builders. The Dark Ages seem to have obliterated much of the collective knowledge accumulated by humanity up until that point. Now researchers are struggling to understand what was common knowledge in 500 A.D. Maybe we modern folk aren’t half as smart as we think we are.

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Steve Hanley

Steve writes about the interface between technology and sustainability from his home in Florida or anywhere else The Force may lead him. He is proud to be "woke" and doesn't really give a damn why the glass broke. He believes passionately in what Socrates said 3000 years ago: "The secret to change is to focus all of your energy not on fighting the old but on building the new." You can follow him on Substack and LinkedIn but not on Fakebook or any social media platforms controlled by narcissistic yahoos.

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