Microstructure characterization in cement paste using backscattered diffuse ultrasound

In this paper, the quantitative ultrasound technique (QUS) technique is applied to characterize voids in cement paste. Experiments are conducted on two series of cement paste specimens: a pair of regular samples with two levels of capillary porosity and a pair of air entrained samples with two concentrations of bubbles. The specimens are placed in a water tank and scanned at 5 MHz in pulse-echo mode. Velocity, attenuation, transmission coefficient and backs cattering cross section are measured. Then, several models are used to estimate the parameters of the microstructure from the backscattered energy. Finally, the microstructure is analyzed from digital images. In the air- entrained samples, the mean scatterer size is correctly estimated and a clear difference in the acoustic concentration is observed in both samples, although this difference is higher than expected. In regular cement paste, the ultrasonically estimated scatterer size is too high to correspond to capillary pores. In this case, the scattered energy may originate from clumps of flocculated cement paste.