  Metakaolin is a broadly used industrial raw material, with applications in
the production of ceramics and geopolymers, and the partial replacement of
Portland cement. The early stages of the manufacturing of some of these
materials require the preparation and processing of a foamed metakaolin-based
slurry. In this study, we propose to investigate the rheology of a foamed
metakaolin-based fresh paste by performing well-controlled experiments. We work
with a non-reactive metakaolin paste containing surfactant, in which we
disperse bubbles of known radius at a chosen volume fraction. We perform
rheometry measurements to characterize the minimum stress required for the
foamed materials to flow (yield stress), and the dissipation occurring during
flow. We show that the yield stress of the foamed samples is equal to the one
of the metakaolin paste, and that dissipation during flow increases
quadratically with the bubble volume fraction. Comparison with yielding and
flow of model foamed yield stress fluids allows us to understand these results
in terms of coupling between the bubbles' surface tension and the metakaolin
paste's rheology.
