Despite the many advantages of concrete as a modern building material, including its high strength, low cost and ease of manufacture, its production currently accounts for about 8% of global carbon dioxide emissions. Recent findings by
a team at MIT show that introducing new materials into existing concrete manufacturing processes can significantly reduce this carbon footprint without changing the overall mechanical properties of concrete.
Concrete is the second largest consumable material in the world after water and the cornerstone of modern infrastructure. But in the process of its manufacture, a large amount of carbon dioxide is released.
About half of the emissions associated with concrete production come from the burning of fossil fuels such as oil and gas, which are used to heat a mixture of limestone and clay that ends up as the familiar gray powder known as ordinary Portland cement, or OPC. While the energy required for this heating process can eventually be replaced by renewable solar or wind power generation, the other half of the emissions are inherent to the material itself: when the mineral mixture is heated to temperatures above 1400 degrees Celsius, it undergoes chemical transformation from calcium carbonate and clay to clinker and carbon dioxide. Carbon dioxide is released into the air. When OPC is mixed with water, sand and gravel materials during
concrete production, it becomes highly alkaline, creating a seemingly ideal environment for sequestration and long-term storage of carbon dioxide in the form of carbonate materials. Although it is possible for concrete to absorb carbon dioxide naturally from the atmosphere, when these reactions normally occur, mainly within the cured concrete, they both weaken the material and reduce the internal alkalinity, thus accelerating the corrosion of the steel. These processes ultimately destroy the load-carrying capacity of the building and negatively affect its long-term mechanical performance. As a result, these slow, late-stage carbonation reactions, which can occur for decades, have long been recognized as a poor pathway for accelerating concrete deterioration.
The authors found that the new CO2 sequestration pathway relies on the early formation of carbonate during concrete mixing and pouring before the material sets, which may largely eliminate the adverse effects of CO2 absorption after the material sets. The key to the
new process is the addition of a simple, inexpensive ingredient: sodium bicarbonate, also known as baking soda. In laboratory tests using sodium bicarbonate substitution, the team demonstrated that as much as 15% of the total CO2 associated with cement production could be mineralized at these early stages, which could significantly reduce the global carbon footprint of the material.
Furthermore, by forming a composite phase not previously described, the resulting concrete sets faster without affecting its mechanical properties. As a result, the process makes the construction industry more productive: formwork can be removed earlier, reducing the time it takes to complete a bridge or building.
The composite, a mixture of calcium carbonate and calcium silicate hydrate, "is a completely new material," Masic said. "In addition, through its formation, we can double the mechanical properties of early concrete." However, he added, the study is still ongoing. "Although it is not clear how the formation of these new phases will affect the long-term performance of concrete, these new findings show that the development of carbon-neutral building materials is promising." The composite, a mixture of calcium carbonate and calcium silicate hydrate, is an entirely new material, the
researchers said. In addition, through its formation, we can double the mechanical properties of early concrete.
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