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Cement Is Stronger When Its Molecules Are Busted

Tue January 10, 2017 - National Edition

Cement being poured.
Cement being poured.

ONE CHEMICAL REACTION rules the world: Water plus rocks plus cement. That's concrete, and it equals most of the built world. Concrete is the spine of skyscrapers, the span of bridges, and the bulk of dams. It is also one of the single biggest contributors to carbon dioxide emissions on Earth. The cement industry in the US contributes about 3.5 percent of the country's emissions. Worldwide, the industry contributes to about 5 percent of all emissions.

So, even though concrete mixing facilities don't blight the landscape like smoking oil refineries or jammed freeways, they are ripe for sustainable innovations. Like this one: Use less concrete by making it stronger. Ironically, this strength could come by making cement—the chemically-reactive powder that forms a brittle paste when exposed to water—with more molecular defects.

“What first comes to mind is that defects are bad for material,” says Rouzbeh Shahsavari, a materials engineer at Rice University in Houston. But in a new paper published this week in Applied Materials and Interfaces, he and two co-authors found that when when it comes to concrete, this is not quite the case.

They started by looking for clues to what made cement so strong. Using advanced microscopy techniques, they peered into layers of tobermorite, a cement mixture used by the Romans. They noticed twisting imperfections in the layers of material—in engineering parlance, they're called screw dislocations. Intrigued, Shahsavari started playing with computer models of tobermorite riddled with these imperfections.

“These are not the kind of models where you can fiddle with variable parameters and stuff like that,” he says. Unlike weather models, which are too complex to yield exact predictions, these chemical reaction models use quantum calculations to derive specific interactions between individual molecules. When Shahsavari applied pressure to the modeled tobermorite, the imperfections would guide the stress out to the edge, instead of absorbing it—and creating a crack. Even more incredible, the imperfection would spread to neighboring molecules, increasing the material's flexibility.

Concrete is mostly known for being really strong. And while it is also tough, concrete is also known for developing spiderwebbing cracks. “These two parameters, strength and toughness, are often contradictory,” says Shahsavari. He believes he could make concrete that is twice as strong by optimizing these screw dislocations.

Every time anyone mixes concrete, they will create imperfections. This is partly because the process itself is messy. But it's also because it's so easy to contaminate the mix. Cement is mostly calcium, silica, and aluminum. But if you put the right impurities in that dry cement—things like sodium, magnesium, and iron—and in certain percentages, then they may increase the number of these desirable imperfections.

Shahsavari wants to play around with impurities. He also thinks he might be able to induce imperfections by messing around with the ambient temperature during controlled situations like pre-cast concrete—huge slabs manufactured in factories and shipped to construction sites. Shahsavari admits that this would be nearly impossible at traditional outdoor pours, where concrete is delivered and mixed in trucks in all sorts of weather.

He and other researchers also need to play around with other cement mixtures. There's no standard chemical definition for cement: People change the ratio between calcium and sodium to tweak the properties of their final mix. Tobermorite has a calcium-sodium ratio of 1:1, making it easy to identify the basic behaviors of screw dislocations and apply them to other cement compositions.

It could be years before all that research is done and Shahsavari's Miracle Ready Mix is standard at every Home Depot. Even then, it won't solve issues like rust corrosion—currently causing consternation for commuters crossing Northern California's new Bay Bridge. “That is a durability problem, because bad chemicals are penetrating the pores into the cement,” he says. “But, once you have stronger concrete, your surface area is half, and less exposure to water, so chance of getting damaged would be half.” That's a pretty strong argument for a stronger material.

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