Revealing fly ash induced cement hydration retardation: The application of embedded integrated sensing element and coda wave analysis

Fly ash (FA) is a common supplementary cementitious material and it delays early cement hydration. This study applies a novel integrated sensing element (ISE) to monitor cement hydration and to characterize the influence of FA. The results show that signals evolve with hydration. Amplitude shows an early decrease followed by recovery. Energy exhibits a rapid drop and then a gradual increase, and the time to reach the minimum energy is delayed by 9.36%, 14.61%, 19.10%, 22.85%, and 33.08% for 5%, 10%, 15%, and 20% FA compared with 0% FA. The dominant frequency band remains within 140-170 kHz, while the peak magnitude in the frequency domain evolves more slowly as FA content increases. The coda wave is sensitive to microstructural development and shows slower evolution with higher FA, which indicates inhibition of early hydration. Coda wave analysis at multiple times confirms this slowdown. Setting time tests show reductions of 4%, 9%, 14%, 22%, and 28% in the initial setting time, and wave velocity measurements show decreases in shear (1018-862 m/s) and longitudinal (2175-2012 m/s) velocities with increasing FA. These consistent results demonstrate the feasibility of ISE for in situ detection of hydration retardation in cementitious materials that contain FA.