Concrete could turn into an effective carbon sink using new additives

New carbonation pathways for creating more environmentally friendly concrete.

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Recent studies by the MIT team have shown that introducing new materials into existing concrete manufacturing techniques could dramatically lower this carbon footprint without affecting concrete’s bulk mechanical properties.

A new study suggests an optimistic future for developing carbon-neutral construction materials.

The findings were published today in the journal PNAS Nexus in an article co-authored by MIT professors of civil and environmental engineering Admir Masic and Franz-Josef Ulm, MIT postdoc Damian Stefaniuk and Ph.D. student Marcin Hajduczek, and James Weaver from Harvard University’s Wyss Institute.

Concrete is the second most utilized material in the world and is the foundation of modern infrastructure. However, it’s manufacturing currently accounts for 8% of worldwide CO2 emissions because of the use of fossil fuels such as oil and natural gas to heat a mixture of limestone and clay, which ultimately creates the familiar grey powder known as ordinary Portland cement. (OPC).

The other half of the emissions are caused by the material itself, as OPC is heated above 1,400 degrees Celsius (2,552 degrees Fahrenheit) and undergoes a chemical transformation from calcium carbonate and clay to a mixture of clinker (primarily calcium silicates) and carbon dioxide.

A very alkaline environment is produced when OPC is combined with water, sand, and gravel to make concrete. This climate is perfect for the sequestration and long-term storage of carbon dioxide in the form of carbonate minerals. (a process known as carbonation).

However, when these reactions occur naturally, primarily within cured concrete, they can weaken the material and reduce internal alkalinity, accelerating the corrosion of reinforcing rebar and negatively impacting its long-term mechanical performance. These processes ultimately destroy the building’s load-bearing capacity and have a negative impact on its long-term mechanical performance.

As a result, these slow late-stage carbonation processes, which can take decades to complete, have long been recognized as undesirable pathways that accelerate concrete deterioration.

Masic says, “The problem with these posturing 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.”

The research team found that carbon dioxide sequestration pathways rely on creating carbonates relatively early during concrete mixing and pouring, before the material sets, which may reduce the negative impacts of carbon dioxide uptake after the material cures.

Adding one simple, affordable substance, sodium bicarbonate, also known as baking soda, is the key to this procedure.
In lab testing, up to 15% of the total quantity of carbon dioxide connected with cement manufacturing was mineralized, potentially reducing the material’s global carbon footprint significantly.

He said, “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.”

Also, the resulting concrete sets substantially faster due to the creation of a previously undescribed composite phase without compromising mechanical performance. This method makes the construction industry more productive by shortening the time it takes to complete a bridge or building.

The researcher said, “The composite, a mix of calcium carbonate and calcium silicon hydrate, is an entirely new material. 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 discoveries suggest an optimistic future for the development of carbon-neutral construction materials.”

He also says. “Our 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.”

It also underscores the fact that the capacity of concrete to trap carbon dioxide has been vastly overestimated and underutilized. The study was funded by the MIT Concrete Sustainability Hub, financed by the Portland Cement Association, and the Concrete Research and Education Foundation.

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