Thixotropy refers to a strong dependence of the material behaviour on the previous flow history, which in particular means that the time to achieve complete structural rebuilding at rest increases with the previous shear intensity. In that case it is more or less implicitly assumed that particle-particle interactions play a role at rest (in the rebuilding phase) while viscous dissipations dominate as soon as flow starts. In this paper, we demonstrate that the rheological behaviour of sewage sludge is driven by a competition between solid interactions and viscous forces under shear. The latter effects dominate above a critical shear stress, TC, but below this value the solid structure progressively rebuilds even under shear as the shear rate decreases towards zero (which in particular generates dead zones in circular cups or tanks). We also show that the time-dependent phase is directly related to the flow history of the material: beyond x0 the transient phase is longer as the time of rest increases, while below xc the shear compliance, which reflects the brittleness of the solid network, decreases with the time of rest. Moreover, by scaling the experimental time by t" where tr and n are respectively the time of rest and an ageing index, all the compliance curves fall along a master curve, indicating some self-similarity of the behaviour of the material during aging for different flow histories.
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