Rice University (Houston, Texas) scientist Rouzbeh Shahsavari performed an atom-level computer analysis of tobermorite, a naturally occurring crystalline analog to the calcium-silicate-hydrate that comprises cement and holds concrete together.
By understanding the structure, researchers hope to make concrete stronger, tougher, and better able to deform without cracking under stress.
Tobermorite forms in layers, like paper stacks, that solidify into particles. These particles often have screw dislocations and shear defects that help relieve stress by allowing layers to slide past each other. Alternately, they can allow the layers to slip only a little before the jagged defects lock them into place.
Shahsavari’s team built computer models of tobermorite “super cells,” with dislocations either perpendicular to or in parallel with layers in the material, and then applied shear force. They found that defect-free tobermorite deformed easily as water molecules caught between layers helped them glide past each other.
But in particles with screw defects, the layers only glided so far before being locked into place by tooth-like core dislocations. That effectively passed the buck to the next layer, which glided until caught—relieving stress without cracking.
“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,” says 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.”
Source: Rice University, news.rice.edu.