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Concrete-brittle

Published on April 12th, 2016 | By: Stephanie Liverani

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The more brittle, the better—Defects key to ‘greener’ concrete manufacturing practices

Published on April 12th, 2016 | By: Stephanie Liverani

[Image above] Credit: Yamanak Tamaki; Flickr CC BY-NC-ND 2.0 

 

 

Concrete is one of the most widely used construction materials in the world. And it has been since the ancient Romans invented the ductile, durable compound they used to build structures like the still-standing Pantheon and Colosseum.

 

But Roman concrete was much cleaner to produce than today’s standard Portland cement, which is extremely carbon-intensive to make and accounts for nearly 7% of the world’s total carbon emissions.

 

So researchers are focusing on cleaner, greener ways to manufacture concrete and reduce this material’s carbon footprint.

 

Earlier this year, we reported on research from Northwestern University’s Center for Sustainable Engineering of Geological and Infrastructure Materials (SEGIM) in Evanston, Ill., where engineers set their sights on outer limits and developed a method for making Martian concrete using materials that are available in generous supply on Mars and without using water—a resource that will be limited and precious when humans colonize the red planet someday.

 

But this month, researchers at Rice University in Houston, Texas, are bringing the concept of ‘greener’ concrete production down to earth with the goal of reducing the impact that concrete manufacturing has on our blue planet.

 

Rouzbeh Shahsavari, theoretical physicist at Rice, and his team are taking an atomic-level look at the details of how concrete is produced. The lab recently published the results of computer modeling studies that reveal how dislocations—or screw-like defects—in raw crystals used for concrete affect manufacturing efficiency, according to a Rice news release.

 

Shahsavari and his team found that tricalcium silicates (C3S) that consist of pure rhombohedral crystals are better than others for producing “clinkers”—round lumps of C3S that, when ground into a powder, mix with water to make cement, the glue that holds gravelly concrete together. When a clinker is easy to grind, manufacturers don’t need to work as hard, the release explains.

 

And when it comes to clinkers, hotter is better. Last year, the Shahsavari lab reported that hot clinkers are easier to grind. At the time, the team also looked at the detrimental effects of screw dislocations on how well the resulting powder mixes with water. 

 

“This time, the lab built computer models of the molecular structures that make up several commonly used types of C3S to see which were prone to be more brittle, despite the inevitable dislocations that twist the crystals into unpredictable formations,” the release explains. The scientists found that the more brittle the C3S, the better it was for more efficient grinding.

 

C3S-with-defect

A computer model shows tricalcium silicate with a defect known as a screw dislocation, which influences the brittle properties of the crystal structure. The atoms on the periphery are faded because they are little affected by the dislocation core at the center of the disc. Credit: Lei Tao/Rice University

 

But the team also wanted to better understand how defects in the microscopic crystals influence the powder’s ability to react with water.

 

Through the study, the scientists found that rhombohedrals—crystals with edges that are all the same length—are more reactive to water than the two monoclinic clinkers they studied, which don’t have edges that are consistent in length.

 

“Understanding and quantifying the structure, energetics and the effect of defects on mechanics and reactivity of cement crystals is a fundamental and engineering challenge,” Shahsavari says in the release. “This work is the first study that puts an atomistic lens on the key characteristics of screw dislocations, a common line defect in C3S, which is the main ingredient of Portland cement.”

 

Energy-efficient methods for producing concrete is important, says Shahsavari, as annual worldwide production of more than 20 billion tons of concrete contributes 5%–10% of carbon dioxide to global emissions—third in line behind transportation and energy generation.

 

Shahsavari says this study could also lead to further investigation of other defects, such as edge dislocations, brittle-to-ductile transitions, and twinning deformations in cement, which could help lower the energy consumption currently needed for global concrete production.

 

The research, published in the Journal of The American Ceramic Society, is “Structure, energetics, and impact of screw dislocations in tricalcium silicates” (DOI: 10.1111/jace.14255).

 

Interested in the latest research and developments in advanced cement-based materials? The 7th Advances in Cement-Based Materials meeting will be held in Evanston, Ill., July 10–13. The call for papers is currently open—submit abstracts by April 29!


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