Effects of air entrainment on rheology

The effects of air entrainment on rheological parameters were studied using cement paste. The addition of air-entraining agent increased the air content up to a saturation level, above which no further increase in air content was observed. With increasing air content, the yield stress increased and the plastic viscosity decreased. The increase in yield stress was an unexpected result because increasing air is well known to cause an increase in slump, and yield stress and slump are known to be negatively correlated (as yield stress increases, slump decreases). Two competing effects are proposed to explain the effects of entrained air bubbles on rheological parameters: the attraction of cement particles and bubbles to form bubble bridges, and a fluid response of air bubbles due to their deformability. Bubble bridges are proposed to dominate in the yield stress and the fluid response is proposed to dominate when the sample is flowing.