Thixotropy

Thixotropy is the property of a material whose viscosity falls reversibly while it is being sheared and recovers once it is left at rest, without any change in composition. In a printable mortar the effect comes from a flocculated network of cement particles and fine additions that is broken down by pumping and mixing and rebuilds within seconds to minutes after deposition. The recovery is partly reversible physical flocculation and partly irreversible early hydration, and the two are difficult to separate experimentally.

In practice

Thixotropy is what allows one mix to be both pumpable and stackable: fluid enough in the hose and at the nozzle, stiff enough on the wall. It is characterised in a rheometer, typically as the rate at which static yield stress rebuilds after shearing, sometimes reported as a structural build-up rate in pascals per second. Clays, fine limestone, silica fume and viscosity-modifying admixtures are the usual means of increasing it. Too little thixotropy and the layers spread; too much and the material stops flowing evenly and the extruder pressure rises.

Related terms

• Yield stress — the threshold stress whose recovery thixotropy describes.
• Viscosity-modifying admixture — an additive used to tune thixotropic behaviour.
• Rheology — the study of flow and deformation that thixotropy belongs to.