Blandine Feneuil: Cement foams: yield stress and stability
Title: Cement foams: yield stress and stability
Abstract: Cement foam advantages compared to normal concrete are its low density, low material need and thermal insulation capacity. To better understand how the morphology of cement foam affects its properties, we need to create solid samples with well controlled structure. The crucial issue to create well controlled cement foams is the stabilization of bubble shape until cement hardening. We focus on the relation between the cement paste consistency, i.e. its yield stress, and the bubble stability.
First, we measure the yield stress of cement paste without bubbles to study how it is affected by the presence of surfactant molecules. Some surfactants adsorb on cement grain surface, which modifies interactions between cement grains and consequently the yield stress of the cement paste. At low surfactant concentration, cement grain surface becomes hydrophobic and yield stress increases due to hydrophobic attraction between cement grains. At high surfactant concentration, adsorbed micelles create a steric repulsion between cement grains and make cement paste yield stress drop.
Second, we study the effect of bubbles on the yield stress of aerated cement paste, at air content below 40%. For a surfactant with low affinity to cement grains surface, results are consistent with literature, whereas dimensionless yield stress is much higher than expected when surfactant strongly adsorbs on cement grain surface. This effect is attributed to the change of the bubble surface properties due to the adsorption of hydrophobic cement grains at the air-liquid interface.
Last, we focus on cement foam stability, at air content 83%. Unexpectedly, the best foam stability is obtained for relatively low cement paste yield stress. Rheological measurements on the fresh cement foams allowed us to understand this improved stability as a consequence of the reorganization of cement grains into denser packing. Then, we define a stability criterion from the bubble size and the interstitial cement paste yield stress.