Cement’s ‘genome’ could make concrete greener

“The heart of concrete is C-S-H—that’s calcium, silicate, and hydrate (water). There are impurities, but C-S-H is the key binder that holds everything together, so that’s what we focused on," says Rouzbeh Shahsavari. (Credit: Susan Brzozowski/Flickr)

By paying attention to concrete’s atomic structure, scientists say they could make it better and more environmentally friendly.

The team of researchers has created computational models to help concrete manufacturers fine-tune mixes for general applications.

Materials scientist Rouzbeh Shahsavari says the team created what it considers a game-changing strategy for an industry that often operates under the radar but is still the third-largest source of carbon dioxide released to the atmosphere.

The annual worldwide production of more than 20 billion tons of concrete contributes 5 to 10 percent of carbon dioxide, according to the researchers; only transportation and energy surpass it as producers of the greenhouse gas.

There are benefits to be gained for the environment and for construction by optimizing the process, says Shahsavari, an assistant professor of civil and environmental engineering at Rice University.

Calcium, silicate, hydrate

“The heart of concrete is C-S-H—that’s calcium, silicate, and hydrate (water). There are impurities, but C-S-H is the key binder that holds everything together, so that’s what we focused on.

“In a nutshell, we tried to decode the phases of C-S-H across different chemistries, thereby improving the mechanical properties of concrete in a material way.”

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The years-long study involved analysis of “defect attributes” for concrete, Shahsavari says.

One was in the ratio of calcium to silicon, the basic elements of concrete. Another looked at the topology of atomic-level structures, particularly the location of defects and the bonds between “medium-range” calcium and oxygen or silicon and oxygen atoms—that is, atoms that aren’t directly connected but still influence each other. The combination of these defects gives concrete its properties, he says.

Shahsavari notes a previous work by the team defined average chemistries of cement hydrates. (Cement is the component in concrete that contains calcium and silicon.)

“C-S-H is one of the most complex structured gels in nature, and the topology changes with different chemistries, from highly ordered layers to something like glass, which is highly disordered. This time, we came up with a comprehensive framework to decode it, a kind of genome for cement,” he says.

The calcium ‘sweet spot’

The team looked at defects in about 150 mixtures of C-S-H to see how the molecules lined up and how their regimentation or randomness affected the product’s strength and ductility.

The ratio of calcium to silicon is critical, Shahsavari says.

“For strength, a lower calcium content is ideal,” he says. “You get the same strength with less material, and because calcium is associated with the energy-intensive components of concrete, you use less rebar and you save energy in transporting the raw material. Also, it’s more environmentally friendly because you put less carbon dioxide into the atmosphere.”

Alternately, a higher ratio of calcium (indeed, there is a sweet spot) provides more fracture toughness, which may be better for buildings and bridges that need to give a little due to wind and other natural forces like earthquakes or well cement subjected to downhole pressure or temperature variation.

“This is the first time we’ve been able to see new degrees of freedom in the formation of concrete based on the molecular topology,” Shahsavari says. “We learned that at any given calcium/silicon ratio, there may be 10 to 20 different molecular shapes, and each has a distinct mechanical property.

“This will open up enormous opportunities for researchers to optimize concrete from the molecular level up for certain applications,” he says.

The findings appear in Nature Communications. Researchers from Massachusetts Institute of Technology (MIT) and Marseille University contributed to the work.

The Concrete Sustainability Hub at MIT, the Portland Cement Association, and the National Ready Mixed Concrete Association Research and Education Foundation supported the research.

Source: Rice University