Rheology of Gypsum Foams

Mineral foams are obtained by foaming in the fresh state of a concentrated suspension of mineral binder. The hydration of the binder leads to the solidification of a highly porous media. Controlling the formation of such porous structures in the fresh state requires the identification of a rheological model capable of combining the rheology of the aqueous foam with the rheology of the concentrated suspension. The focus here is solely on the yield stress considered as a coupled function involving the yield stress of the concentrated suspension, the air bubbles volume fraction and dimensions. The studied mineral foams are gypsum foams obtained by direct mixing in the presence of a surfactant. The mixing time is a process adjustment parameter varying from 2 min to 20 min. It determines the pore structure of the mineral foam. The rheologies of aqueous foams, concentrated suspensions and mineral foams in the fresh state are studied using a rheometer equipped with a vane geometry. The fit of conventional rheological models on the results obtained for aqueous foams and concentrated suspensions leads to propose a rheological model for mineral foams. For each air volume fraction, a clear link appears between the yield stress of the mineral foam and the foam structure (size of bubbles). Different experimental results are successfully confronted to the proposed rheological model, which can be used to optimize the formulation - foaming condition pairing.