  Yield stress of aerated cement paste is studied. Samples are prepared by
mixing aqueous foam with cement paste, which allows controlling bubble size,
gas volume fraction and yield stress of the cement paste. Two distinct
behaviors are observed depending on the surfactant used to prepare the
precursor aqueous foam: (i) For a surfactant with low adsorption ability with
respect to cement grains, bubbles tend to decrease the yield stress of the
paste with magnitude governed by the Bingham capillary number, which accounts
for bubble deformability. (ii) For a surfactant with high adsorption ability,
bubbles increase significantly the yield stress. This behavior is shown to
result from the surfactant-induced hydrophobization of the cement grains, which
adsorb at the surface of the bubbles and tend to rigidify them. Within this
regime, the effect of air incorporation is comparable to the effect of added
solid particles.
