Engineers Discover New Concrete Additives
A team from the Massachusetts Institute of Technology has recently found that introducing new materials into the concrete manufacturing processes could significantly reduce its carbon footprint without altering its properties.
The study, recently published in the journal PNAS Nexus, was conducted by MIT professors of civil and environmental engineering Admir Masic and Franz-Josef Ulm, MIT postdoc Damian Stefaniuk and doctoral student Marcin Hajduczek, and James Weaver from Harvard University’s Wyss Institute.
About the Findings
The production of concrete reportedly accounts for about 8% of global carbon dioxide emissions. About half of these emissions associated with concrete production come from the burning of fossil fuels such as oil and natural gas, while the other half is produced from the material itself.
According to the release from MIT, when ordinary Portland cement (OPC) is mixed with water, sand and gravel material during the production of concrete, it becomes highly alkaline, creating an environment for the sequestration and long-term storage of carbon dioxide in the form of carbonate materials, or a process known as carbonation.
Despite this, the study notes, when these reactions occur they can 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.
“The problem with these postcuring carbonation reactions,” Masic said, “is that you disrupt the structure and chemistry of the cementing matrix that is very effective in preventing steel corrosion, which leads to degradation.”
However, the new carbon dioxide sequestration pathways discovered by the authors rely on the very early formation of carbonates during concrete mixing and pouring, before the material sets. This could largely eliminate the detrimental effects of carbon dioxide uptake after the material cures, MIT reports.
The new process involves sodium bicarbonate, also known as baking soda. According to the study, 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
The resulting concrete reportedly sets more quickly without impacting its mechanical performance. The researchers say that this allows the construction industry to be more productive, with form works being able to be removed earlier and reducing the time needed to complete a bridge or building.
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, he added, the research is still an ongoing effort and 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.”
MIT notes that while the idea of early-stage concrete carbonation is not new, the latest discovery by the team shows that the capacity of concrete to sequester carbon dioxide pre-cure 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 said.
The research was supported by the Concrete Sustainability Hub at MIT, which has a sponsorship from the Portland Cement Association and the Concrete Research and Education Foundation.