Influence of bentonite clay on the rheological behaviour of fresh mortars

Fine mineral additives are often used in the formulation of ready-mix mortars as thickeners and thixotropic agents. Yet, these attributed fresh state properties are not clearly defined from the rheological point of view. In the present study, we consider the influence of bentonite (montmorillonite-based clay mineral) on the rheological behaviour of mortars, including in particular creep and thixotropy. The mortar pastes are subjected to different shear-rates and then allowed to creep under fixed shear stresses until reaching steady state, which corresponds to either rest if the applied stress is smaller than the yield stress or permanent flow otherwise. The evolution of the creep strain is investigated depending on shear history for different contents of bentonite. The microstructure rebuilding kinetics after shear (thixotropy) is considered by analysing the temporal evolution of the creep strain for different applied shear stresses (lower than the yield stress). As expected, bentonite is found to enhance the mortar creep (or sag) resistance. This enhancement consists of both an increase of the yield stress recovered after shear, and a diminution of the characteristic time for yield stress recovery (related to microstructure rebuilding).