Flocculation and sedimentation in suspensions using ultrasonic wave reflection

This work was undertaken to help understand and interpret the ultrasonic wave reflection response of Portland cement paste as it transforms from a fluid-like suspension to a solid in the first hours after mixing. A high impact polystyrene buffer (delay line) was used to measure small changes in the P- and S-wave reflection coefficients. Two materials were studied: a non-hydrating colloidal alumina suspension whose microstructure was manipulated between dispersed and flocculated states by adjusting the pH and a coarse silica suspension that readily sedimented. The S-wave reflection coefficient clearly distinguished between dispersed and flocculated states. Sedimentation of particles in dispersed suspensions was distinguished using the P-wave reflection coefficient. Based on these findings, the observed P- and S-wave responses from hydrating Portland cement paste are interpreted in terms of flocculation and sedimentation processes.