Plasticity at small scales boosts concrete's utility as the world's most-used material by letting it constantly adjust to stress, decades or centuries after hardening. To find out why, Rice University researchers performed an atom-level computer analysis 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. By understanding the internal structure of tobermorite, they hope to make concrete stronger, tougher, and better able to deform without cracking under stress.

A screw dislocation disrupts the regular rows of atoms in tobermorite. Rice University scientists simulated tobermorite to see how it uses dislocations to relieve stress when used in concrete. (Rice University Multiscale Materials Laboratory)

Tobermorite 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, they can allow the layers to slip only a little before the jagged defects lock them into place.

The researchers 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. The defect-free tobermorite deformed easily, as water molecules caught between layers helped them glide past each other. 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 it 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. Rather than defects being detrimental to materials, when it comes to complex layered crystalline systems such as tobermorite, this is not the case. 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 properties are key to the design of concrete materials, which are concurrently strong and tough — two engineering features that are highly desired in several applications. The work shows how to leverage seemingly weak attributes — the defects — in cement, and turn them into highly desired properties of high strength and toughness.

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