Ultrasound creates pressure changes in the cement-water-suspension that lead to cavitation, which means bubbles are formed that grow until they reach an unstable size and then implode.
Extreme conditions - high pressures and high temperatures - are created on the outer edges of the bubbles, which stimulate and accelerate the formation of C-S-H phases. The C-S-H phases are formed during the hydration of cement and are responsible for the hardening of the concrete.
On this untreated concrete there are only few C-S-H clusters.
The C-S-H formation after the sonocrete mixing can be clearly seen when compared to an untreated sample - here 2 hours into the hydration process. While the reference sample only exhibits a few C-S-H clusters, in the sonocrete sample many clusters of the C-S-H phases have formed and are spread evenly across the surface.