Concrete with lower emission of dioxide

Despite the many advantages of concrete as a modern building material, such as high strength, low cost and ease of work, its production today accounts for approximately 8% of global carbon dioxide emissions.

Scientists have discovered that the introduction of certain additives in current concrete manufacturing processes can significantly reduce this carbon footprint, without changing the mechanical properties of concrete.

The discovery was made by a team that includes Admir Masic, Franz-Josef Ulm, Damian Stefaniuk and Marcin Hajduczek, from the Massachusetts Institute of Technology (MIT) in the United States of America, as well as James Weaver from the Wyssen Institute, affiliated to Harvard. University, in the United States.

About half of the emissions that come with concrete production come from burning fossil fuels such as oil and natural gas, which are used to heat a mixture of lime and clay that eventually turns into a lightweight ash called cement. Although the energy required for this heating process could be replaced in the future by electricity generated from renewable sources (basically solar or wind energy), the other half of the emissions is inherent to the material itself: the mineral mixture with higher temperatures heated to 1,400 degrees Celsius undergoes a chemical transformation that leads to the emission of dioxide of carbon that enters the atmosphere.

When ordinary Portland cement is mixed with water, sand and gravel or similar in the production of concrete, it becomes very alkaline, as an ideal environment for the capture and long-term storage of carbon dioxide in the form of carbonate material (a process known as carbonation). Despite this good potential of concrete to absorb carbon dioxide from the atmosphere, when these reactions occur spontaneously, especially in cured concrete, they can weaken the material and reduce the internal alkalinity, accelerating the corrosion of the internal steel rods in concrete structures. Ultimately, these processes destroy the load-bearing capacity of the building and negatively affect its long-term mechanical performance. Therefore, these slow carbonation reactions, which can take place over decades, have long been recognized as an undesirable phenomenon that accelerates the deterioration of concrete.

But the new carbon dioxide capture pathway discovered by Masic and his colleagues is based on the initial formation of carbonates in the concrete mix and infusion, before killing the materials, which largely eliminate the carbonates, putting the detrimental effect of carbon dioxide absorption behind the material.

The introduction of the additive in the concrete manufacturing process could significantly reduce the carbon footprint of the material without changing the mechanical properties, as determined in the research. (Image: research team. CC BY-NC-ND 3.0)

The key to the new process is a simple and cheap ingredient: sodium bicarbonate. In laboratory tests carried out with sodium bicarbonate, the team showed that up to 15% of the total amount of carbon dioxide, with the production of cement, could be mineralized in these early stages, significantly enough to reduce the global carbon footprint of the material.

Masicus and colleagues report the technical details of their discovery in the academic journal PNAS Nexus, under the title “Co2 CO2 in CSH: A step toward concrete carbon neutrality.” (Source: NCYT from Amazing)