New Formula Could Cut Pollution from Concrete

Concrete is the most-used construction material in the world and a major contributor to global greenhouse gas emissions. But by using a different chemical formula to make the material, new research suggests it's possible to significantly reduce concrete's carbon footprint.

photo by scientific american

Concrete has a complex molecular structure made of a mixture of sand, gravel, water and cement. The cement is made by heating a calcium-rich material, typically limestone, with a silica-rich material, typically clay, at temperatures of around 1,500 degrees Celsius. This process, usually fueled by coal, produces a hard mass that's then ground into a powder. Making cement accounts for a significant portion of concrete's overall greenhouse gas emissions.

More than 20 billion tons of concrete are produced each year and contribute up to 10 percent of global anthropogenic carbon dioxide production. A report released yesterday by the Massachusetts Institute of Technology suggests that reducing the ratio of calcium to silicate in cement would enhance the strength of the material, reduce material volume and cut the emissions associated with concrete by more than half.

At present rates, concrete usage is estimated to be three times that of steel, according to Roland Pellenq, MIT senior scientist and author on the report. "There's no other solution to sheltering mankind in a durable way—turning liquid into stone in 10 hours, easily, at room temperature. That's the magic of cement," he said.

Stronger and glassier
The ratio of calcium to silica in conventional cements ranges from 1.2 to 2.2, with 1.7 as the accepted standard. By creating a database of all possible chemical formulations, the MIT research team found 1.5 parts calcium to every 1 part silica to be the "magical ratio," according to Pellenq.

With this mixture the material can achieve "two times the resistance of normal cement, in mechanical resistance to fracture, with some molecular-scale design," he said.

Since concrete production is highly greenhouse gas-intensive, any reduction in calcium content in the cement will have a positive impact on emissions. According to Pellenq, reductions could be as much as 60 percent.

Using this formula would also improve the mechanical strength of concrete and give the material a glassier and less crystalline structure, which would make it more fracture-resistant. The oil and gas industry has a particular interest in using stronger concrete around well casings to prevent leakages and blowouts.

Researchers will now carry out further testing on the new formula to prove their findings, published in the journal Nature Communications, translate from the molecular scale to the engineering scale for applications such as infrastructure and buildings.