Small defects known as screw dislocations help account for the superiority of the Roman Empire’s concrete. These defects provide small-scale plasticity that allows hardened concrete to adjust to stress over time.
Materials scientists at Rice University in Texas made this discovery by studying tobermorite, a naturally occurring crystalline analog to the calcium-silicate-hydrate (C-S-H) that makes up cement, which in turn holds concrete together.
The American Ceramic Society recognizes that aluminum substituted tobermorite is a key ingredient in the longevity of Roman concrete. However, scientists had not understood the mechanism by which the mineral contributed to longevity. They did know that this substance often contained defects.
Tobermorite forms in layers that solidify into particles. These particles sometimes incorporate screw dislocations, which are defects that help relieve stress by allowing layers to slide past each other. The Rice team used atom-level computer analysis to build computer models of tobermorite “super cells.” Models with no defects deformed easily.
The researchers then built models that contained either perpendicular or parallel screw dislocations and applied shear force. They discovered that the defects prevented the material layers from sliding too far past each other; a defect “catches” an adjacent layer and prevents further slippage. This process works through other layers; each layer stops sliding when it catches on one of the dislocations.
This mechanism, called step-wise defect-induced gliding around the tobermorite particle core, increases the material’s ability to adapt to stress. The defects increase the material’s yield stress and toughness.
Rice materials scientist Rouzbeh Shahsavari says that this new knowledge paves the way for designing stronger concrete and other complex materials.