Chemistry, concrete and ceramic cement

Whatever you’re building, strength tends to be a prerequisite, so anything that makes things stronger and more reliable is worth knowing.

In two new studies in the US, chemists and engineers have proposed a solution to problems with an age-old material – concrete – and found a possible problem with a modern one – advanced ceramics used in everything from solar panels to biomedical implants.

Old world first. Because concrete is porous, “freeze-thaw cycles” can be an issue in areas with large ranges in temperature. Water gets in, then freezes and expands, building up pressure as the ice crystals grow, eventually popping the surface of the concrete.

The usual way to avoid such damage is to add tiny air bubbles to the concrete to act as pressure release valves, but bubbles weaken the concrete and make it even more porous.

Now Wil Srubar and colleagues from the University of Colorado Boulder have proposed a new approach based on prevention rather than compensation, and inspired, they say, by organisms that survive and thrive in sub-zero environments.

Writing in the journal Cell Reports Physical Science, they explain how anti-freeze proteins bind to ice crystals to inhibit their growth.

They developed polymer molecules (polyethylene glycol-graft-polyvinyl alcohol) that mimic the protein’s properties, added them to a concrete mix, and found this effectively reduced ice crystal size by 90%.

The new mix also withstood 300 freeze-thaw cycles while maintaining its strength, but the researchers stress there is still much work to be done to determine long-term resilience and economic viability of the approach in a real-world application.

The second study dealt with cement of a different kind – the one that holds together the microscopic crystalline grains that make up many advanced polycrystalline ceramics.

Materials science engineers from the University of Wisconsin-Madison say it has been assumed that these grain boundaries are very stable, but their new study suggests that might not always be the case.

Writing in the journal Nature Materials, they say that silicon carbide, which is used in nuclear energy, jet engines and other high-tech applications, may be just as susceptible to radiation-induced segregation as metal alloys are. Which may be good and bad.

It’s been known since the ‘70s, they say, that because metal atoms share electrons freely, they are able to mix and unmix easily. When bombarded by ion radiation, some atoms in the metals will pop out of place and move toward the grain boundaries. If different types of atoms move at different rates, the chemistry of the alloy can be altered.

Atoms in ceramics are more selective about which neighbours they bond with, however, and the bonds are much stronger than in metals. That’s why researchers believed these atoms weren’t subject to the same type of segregation.

However, painstaking work by Izabela Szlufarska and colleagues found that isn’t the case with silicon carbide – and likely other polycrystalline ceramics as well, they suggest.

And that is something of a double-edged sword. Ceramics may be subject to the same types of damage and deterioration at their grain boundaries as metal alloys, though at different temperatures, but the segregation could be useful in materials engineering to produce specialised versions of ceramics like silicon carbide.

“Maybe the radiation can be used as a tool to fine tune grain boundary chemistry,” says co-author Xing Wang.