After water, concrete is the world’s most consumed material and is the mainstay of construction everywhere, and concrete production’s impact on the environment is significant. Researchers have discovered that introducing an inexpensive, readily available ingredient into its production may make concrete a climate solution rather than a climate villain.
As a construction material, concrete has many advantages: it’s very strong, easy to manufacture, and inexpensive. But its downside is that it’s not very environmentally friendly. Concrete production is the single largest industrial cause of carbon pollution, accounting for 8% of global carbon dioxide emissions.
Concrete is made by mixing cement with aggregate, a grainy blend of materials like stone and sand. Firing limestone, clay, and other materials in a kiln creates that familiar gray powder, ordinary Portland cement (OPC). Carbon dioxide is emitted when the material is fired and from the chemical reaction that takes place when the mixture is exposed to heat. According to the National Ready Mixed Concrete Association, each pound (0.45 kg) of concrete produced releases 0.93 pounds (0.42 kg) of carbon dioxide.
While the firing process can be modified from using fossil fuels to using electricity from renewable energy sources, the second contributor to carbon dioxide emissions, the chemical reaction, presents more challenges. As the mineral mixture is heated to above 2,552 °F (1,400 °C), it’s transformed from calcium carbonate and clay to a mixture of clinker – calcium silicates, primarily – and carbon dioxide. This carbon dioxide then escapes into the air.
Over time, carbon dioxide in the atmosphere reacts with calcium oxide in the concrete, mineralizing the carbon dioxide into calcium carbonate in a process known as carbonation. Carbonation is the reverse of what occurs when concrete is made.
While carbonation allows concrete to sequester (capture and store) carbon dioxide, it can also weaken concrete, especially cured concrete, lowering its internal alkalinity and leading to corrosion of steel that’s been used as reinforcement. Curing is the process of maintaining moisture levels inside cast concrete, which results in stronger, more durable, less porous concrete.
A team of researchers at MIT have devised a way of addressing the issue of carbon dioxide in the early stages, during concrete mixing and pouring before the material sets, by introducing a very inexpensive ingredient: sodium bicarbonate, otherwise known as baking soda.
The researchers found that by adding a sodium bicarbonate substitute, up to 15% of the total amount of carbon dioxide associated with cement production could be mineralized during the early stages. The composite used by the researchers is a mix of calcium carbonate and calcium silicon hydrate, an entirely new material.
“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,” said Admir Masic, corresponding author of the study.
Moreover, the "new" concrete sets more quickly without losing any of its mechanical performance. This, the researchers say, would allow the construction industry to complete work faster.
Early-stage concrete carbonation is not new, but MIT’s discovery highlights the ability to sequester carbon dioxide in the pre-cure stage.
“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 said.
Research into the long-term performance of the concrete is ongoing.
The study was published in the journal PNAS Nexus.
Source: MIT News
