Cement becomes stronger when it has molecular defects

Scientists have found a way to turn weakness into strength

Scientists have found a way to turn weakness into strength

Researchers at Rice University in Houston are trying to make concrete stronger, tougher and better able to deform without cracking under stress. To do this they have studied the internal structure of tobermorite, a naturally occurring crystalline analog to the calcium-silicate-hydrate (C-S-H) that makes up cement, which in turn holds concrete together. Using advanced microscopy techniques, they noticed twisting imperfections in its layers – the so called screw dislocations. When applying force, they observed that defect-free tobermorite deformed easily as water molecules caught between its layers allowed them to glide past each other. In contrary, in particles with screw defects, each layer only glided so far before being locked into place by the tooth-like core dislocations, and as the buck was passed to the next layers, stress was effectively guided out to the edge, instead of being absorbed and creating a crack. Even more incredible, the imperfections would spread to neighboring molecules, increasing the material’s flexibility.

“What first comes to mind is that defects are bad for material,” says Rouzbeh Shahsavari, an assistant professor of civil and environmental engineering and materials science and nanoengineering at Rice University. But this was certainly not the conclusion that his team, who experimented with computer models of tobermorite featuring such imperfections, came to. Their findings were recently published in the American Chemical Society journal ACS Applied Materials and Interfaces. 

“The insight we get from this study is that unlike the common intuition that defects are detrimental for materials, when it comes to complex layered crystalline systems such as tobermorite, this is not the case,” said Shahsavari. “Rather, the defects can lead to dislocation jogs in certain orientations, which act as a bottleneck for gliding, thus increasing the yield stress and toughness. These latter properties are key to design concrete materials, which are concurrently strong and tough, two engineering features that are highly desired in several applications. Our study provides the first report on how to leverage seemingly weak attributes — the defects — in cement and turn them to highly desired properties, high strength and toughness’’. The team believes that by optimizing these screw dislocations, the concrete produced could be twice as strong.

A screw dislocation disrupts the regular rows of atoms in tobermorite, a naturally occurring crystalline analog to the calcium-silicate-hydrate that makes up cement. Rice University scientists simulated tobermorite to see how it uses dislocations to relieve stress.

Courtesy of the Multiscale Materials Laboratory