How to make stronger, 'greener' cement

September 25, 2014 by David L. Chandler, Massachusetts Institute of Technology
Credit: iStock

Concrete is the world's most-used construction material, and a leading contributor to global warming, producing as much as one-tenth of industry-generated greenhouse-gas emissions. Now a new study suggests a way in which those emissions could be reduced by more than half—and the result would be a stronger, more durable material.

The findings come from the most detailed molecular analysis yet of the complex structure of concrete, which is a mixture of sand, gravel, water, and . Cement is made by cooking calcium-rich material, usually limestone, with silica-rich material—typically clay—at temperatures of 1,500 degrees Celsius, yielding a hard mass called "clinker." This is then ground up into a powder. The decarbonation of limestone, and the heating of cement, are responsible for most of the material's greenhouse-gas output.

The new analysis suggests that reducing the ratio of calcium to silicate would not only cut those emissions, but would actually produce better, stronger concrete. These findings are described in the journal Nature Communications by MIT senior research scientist Roland Pellenq; professors Krystyn Van Vliet, Franz-Josef Ulm, Sidney Yip, and Markus Buehler; and eight co-authors at MIT and at CNRS in Marseille, France.

"Cement is the most-used material on the planet," Pellenq says, noting that its present usage is estimated to be three times that of steel. "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."

In conventional cements, Pellenq explains, the calcium-to-silica ratio ranges anywhere from about 1.2 to 2.2, with 1.7 accepted as the standard. But the resulting molecular structures have never been compared in detail. Pellenq and his colleagues built a database of all these chemical formulations, finding that the optimum mixture was not the one typically used today, but rather a ratio of about 1.5.

As the ratio varies, he says, the molecular structure of the hardened material progresses from a tightly ordered crystalline structure to a disordered glassy structure. They found the ratio of 1.5 parts calcium for every one part silica to be "a magical ratio," Pellenq says, 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."

The findings, Pellenq adds, were "validated against a large body of experimental data." Since emissions related to concrete production are estimated to represent 5 to 10 percent of industrial greenhouse-gas emissions, he says, "any reduction in calcium content in the cement mix will have an impact on the CO2." In fact, he says, the reduction in carbon emissions could be as much as 60 percent.

In addition to the overall improvement in mechanical strength, Pellenq says, because the material would be more glassy and less crystalline, there would be "no residual stresses in the material, so it would be more fracture-resistant."

The work is the culmination of five years of research by a collaborative team from MIT and CNRS, where Pellenq is research director. The two institutions have a joint laboratory at MIT called the Multi-Scale Materials Science for Energy and Environment, run by Pellenq and Ulm, who is director of MIT's Concrete Sustainability Hub, and hosted by the MIT Energy Initiative.

Because of its improved resistance to mechanical stress, Pellenq says the revised formulation could be of particular interest to the oil and gas industries, where cement around well casings is crucial to preventing leakage and blowouts. "More resistant cement certainly is something they would consider," Pellenq says.

So far, the work has remained at the molecular level of analysis, he says. "Next, we have to make sure these nanoscale properties translate to the mesoscale"—that is, to the engineering scale of applications for infrastructure, housing, and other uses.

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4 comments

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EyeNStein
not rated yet Sep 25, 2014
hope their 5 years work going from 1.7 to 1.5 isn't wasted; by degraded thermal expansion cracking or other long term factors they have yet to measure.
yep
1 / 5 (1) Sep 26, 2014
Hempcrete http://www.natura...ouse.htm
http://www.innova...building
The "25,000 specific applications for industrial hemp" along with the Oil and gas industries are some of the many reasons it remains illegal.
Eikka
not rated yet Sep 26, 2014
Hempcrete http://www.natura...ouse.htm
The "25,000 specific applications for industrial hemp" along with the Oil and gas industries are some of the many reasons it remains illegal.


Hemp isn't illegal.

Just the drug variety of it. You can apply for a permit to grow industrial hemp, and they test it for THC content so it doesn't exceed 0.3% per dry weight. People grow it, they sell it and use it in various domestic and industrial applications. There's no real conspiracy against it.

The biggest problem for those "25,000 specific industrial applications" is that there's generally something cheaper and better available for the purpose than hemp. There's plenty of other fibers that are easier and less controversial to grow, to put in concrete for example, that perform just as well or better than hemp.

yep
1 / 5 (1) Sep 27, 2014
Eikka you are incredibly misinformed. As of last year there were only nine states that had recently passed bills to grow Hemp but it is still federally illegal.
The biggest problem is that you think you have a clue do a little research before you spout off nonsense.
A few years ago General McCaffrey was trying to ban hemp imports because "kids are boiling down their hemp shirts and mixing the essence with alcohol to make marijuana"
The conspiracy will be over once it is re-legalized
1937 Du Pont's annual report "the revenue raising power of government may be converted into an instrument for forcing acceptance of sudden new ideas of industrial and social reorganization."
The tax act ended hemp production in America. Corporations then had a synthetic patent monopoly on fuel, paper, fiber, and medicine since the only natural competitor for thousands of products was forbidden.
Read some history.

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