Team Finds Cement’s ‘Magical’ Mix
Tweaking the amount of just one ingredient in cement could go a long way toward reducing concrete’s environmental wallop, scientists at MIT have discovered.
Better yet: The recommended recipe actually calls for less of the ingredient.
Best of all: The resulting material could be even stronger than its current counterpart.
The secret: Less calcium.
'The Magic of Cement'
Concrete (made of portland cement and water) is not only the world's most-used construction material, but it is also a leading contributor to global warming, accounting for up to one-tenth of industrial greenhouse-gas emissions, the Massachusetts Institute of Technology reports.
A new study from an MIT team suggests that cutting the ratio of calcium to silicate in cement could halve those emissions and produce a stronger, more durable concrete in the process.
"Cement is the most-used material on the planet," used about three times as often as steel, says Roland Pellenq, the senior research scientist who led the project.
"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," says Pellenq.
Building a Database
The team's new cement recipe is the result of "the most detailed molecular analysis yet of the complex structure of concrete," according to MIT.
The research shows that most of cement's greenhouse gas output come from the process by which the material is made, particularly the decarbonation of limestone and the heating process.
"[A]ny reduction in calcium content in the cement mix will have an impact on the CO2," Pellenq says.
Conventional cement has a calcium-to-silica ratio that ranges from about 1.2 to 2.2 parts calcium for every one part silica, Pellenq said.
However, the molecular structures throughout the range had not been compared in detail.
So the team did just that by building a database of all the chemical formulations. What they discovered was an optimal mixture ratio of about 1.5, not the accepted standard 1.7.
A Magical Mix
According to Pellenq, the ratio of 1.5 parts calcium for every one part silica is "magical," because at that point the material can achieve "two times the resistance of normal cement, in mechanical resistance to fracture, with some molecular-scale design."
As the ratios change, the hardened material's molecular structure progresses from a "tightly ordered crystalline structure to a disordered glassy structure."
With the 1.5 ratio, the material would be more glassy and less crystalline, and there would be "no residual stresses in the materials, so it would be more fracture-resistant," Pellenq says.
"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," says Roland Pellenq of MIT.
The revised formulation's improved resistance to mechanical stress could be of interest to the oil and gas industry, where cement around well casings is crucial to preventing leakage and blowouts, Pellenq notes.
So far, the work has focused on the molecular level of analysis[ the next step is to make sure the properties of the new ratio work in engineering applications.
Aiding Pellenq on the research were professors Krystyn Van Vilet, Franz-Josef Ulm, Sidney Yip, and Markus Buehler and eight co-authors at MIT and the National Center for Scientific Research (CNRS) in Marseille, France.
A paper on their findings, "Combinatorial molecular optimization of cement hydrates," was published Sept. 24 in the journal Nature Communications.
MIT and CNRS, where Pellenq is research director, have collaborated on the research for five years. The two institutions have a joint laboratory at MIT called the Multi-Scale Materials Science for Energy and Environment. The lab is hosted by the MIT Energy Initiative and run by Pellenq and Ulm, who is the director of MIT's Concrete Sustainability Hub.
The Concrete Sustainability Hub is supported by the Portland Cement Association and the Ready Mixed Concrete Research and Education Foundation.