This image shows a screw dislocation disrupting the regular rows of atoms in tobermorite, a naturally-occurring crystalline analog to the calcium-silicate-hydrate that makes up cement. Image: Multiscale Materials Laboratory/Rice University.Concrete isn't thought of as a plastic, but plasticity at small scales boosts concrete's utility as the world's most widely-used material by letting it constantly adjust to stress, decades and sometimes even centuries after hardening. Rice University researchers are now a step closer to understanding why.
The Rice lab of materials scientist Rouzbeh Shahsavari performed an atom-level computer analysis of tobermorite, a naturally-occurring crystalline analog of the calcium-silicate-hydrate (C-S-H) that makes up cement, which holds concrete together. By understanding the internal structure of tobermorite, Shahsavari and his team hope to make concrete stronger, tougher and better able to deform without cracking under stress. They report their results in a paper in ACS Applied Materials & Interfaces.
Tobermorite, a key element in the superior concrete Romans used in ancient times, forms in layers, like paper stacks that solidify into particles. These particles often have screw dislocations, shear defects that help relieve stress by allowing the layers to slide past each other. Alternately, the layers can slip only a little before the jagged defects lock them into place.
The researchers built the first computer models of tobermorite ‘super cells’, with dislocations either perpendicular to or in parallel with the layers in the material, and then applied shear force. They found that defect-free tobermorite deformed easily, due to water molecules caught between the layers helping them to glide past each other.
But in particles with screw defects, the layers only glided so far before being locked into place by the tooth-like core dislocations. That effectively passed the buck to the next layer, which glided until caught, and so on, relieving the stress without cracking.
This ‘step-wise defect-induced gliding’ around the particle's core makes it more ductile and able to adjust to stress, said Shahsavari, an assistant professor of civil and environmental engineering and materials science and nanoengineering.
"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 acts 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."
Shahsavari said he hopes this work will provide design guidelines for developing stronger, tougher concrete, as well as other complex materials.
This story is adapted from material from Rice University, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.