This study aims to experimentally investigate the ultrasonic behavior of fresh cement focusing on the contribution of the entrapped air bubbles. Frequency dispersion and attenuation carry delicate information that is not possible to gather by traditional ultrasonic pulse velocity. This is measured by simple indicators that quantify the frequency dependence of propagation velocity of longitudinal waves through fresh cementitious media. It seems that dispersion shows much stronger sensitivity to the microstructural processes, since the presence of superabsorbent polymers in mortar induces a large difference in dispersion parameters when compared to reference cement mortar, while only marginal difference in threshold-based pulse velocity. To reach this aim, references are taken from, and comparisons are made to other liquids in order first in order to validate the reliability of the methodology and to better understand the contribution of the cavities in the obtained dispersion and attenuation curves. Ultrasonic dispersion assessment of cementitious media has the potential to bring a lot of information on the microstructure of materials, as well as the ongoing processes.

Ultrasonic monitoring of cementitious media during hydration is useful since parameters like wave velocity and amplitude are influenced by the developing stiffness of the medium, allowing evaluation of the curing stage and elastic modulus in real time. However, wave propagation in such a heterogeneous system is very complicated due to different constituents, as well as the several length scales of the heterogeneity, allowing only a rough interpretation behind the specific trends. In the present study, the ultrasonic velocity and attenuation are investigated in terms of the frequency. It is seen that the dispersion, being a result of the initial properties mismatch, is smoothened after setting for all the materials while simultaneously the attenuation significantly decreases. The model mixes include reference mortar, mortar with super-absorbent polymers and mortar with retarder. The effect of the admixtures is evident as SAPs delay the setting and the smoothening of the dispersion curve, while the retarder produces two types of phenomena: first the rapid "false setting" obvious by sudden change in dispersion and attenuation and delayed hydration. It is suggested that dispersive features of ultrasound can strongly enhance the characterization of fresh cementitious media.

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