MIT Engineers Discover New Additives for Reducing Concrete’s Carbon Footprint

A team at the Massachusetts Institute of Technology (MIT) (Cambridge, Massachusetts, USA) recently discovered that introducing new materials into existing concrete manufacturing processes could significantly reduce its carbon footprint without altering concrete’s bulk mechanical properties. Currently, concrete production accounts for approximately 8% of global carbon dioxide (CO₂) emissions. 

The MIT team revealed its findings in a paper published in the journal PNAS Nexus. The paper was written by Admir Masic and Franz-Josef Ulm, MIT professors of civil and environmental engineering; Damian Stefaniuk and Marcin Hajduczek, MIT postdoctoral and doctoral students, respectively; and James Weaver from Harvard University’s Wyss Institute. 

During the manufacturing of concrete, large quantities of CO₂ are released, both as a chemical byproduct of cement production and in the energy required to fuel these reactions. Approximately half of the emissions associated with concrete 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 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 °C (2,552 °F), it undergoes a chemical transformation from calcium carbonate and clay to a mixture of clinker (consisting primarily of calcium silicates) and CO₂, the latter of which escapes into the air. 

When OPC is mixed with water, sand, and gravel 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 in a process known as carbonation. While concrete has the potential to naturally absorb CO₂ from the atmosphere, the reactions that normally occur—mainly within cured concrete—can both weaken the material and lower the internal alkalinity, which in turn 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. As such, 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 postcuring 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.” 

In contrast, the new CO₂ sequestration pathways discovered by the authors 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 CO₂ uptake after the material cures. 

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 a sodium bicarbonate substitution, the team demonstrated that up to 15% of the total amount of CO₂ 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 multifunctional concrete by incorporating the added benefits of carbon dioxide mineralization during production and casting.” 

Furthermore, 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. Form works can be removed earlier, reducing 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,” says Masic.

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 carbon dioxideuptake after concrete is cast into its desired shape, the current discoveries by the MIT team highlight the fact that the precuring capacity of concrete to sequester CO₂ 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 the solution,” Masic says.

Source: MIT News, https://news.mit.edu.