Yield stress of aerated cement paste

TL;DR

This study investigates how aeration affects the yield stress of cement paste, revealing that surfactant properties influence whether bubbles weaken or strengthen the material, with implications for material design.

Contribution

It demonstrates the role of surfactant adsorption in controlling bubble effects on cement paste yield stress, introducing a new understanding of aeration's impact based on surfactant chemistry.

Findings

Bubbles decrease yield stress with low-adsorption surfactants, governed by the Bingham capillary number.

High-adsorption surfactants increase yield stress due to grain hydrophobization.

Air incorporation effects are comparable to adding solid particles.

Abstract

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.