Fresh concrete is a concentrated suspension of Portland cement and aggregates. Cement-based materials show a fluid-to-solid transition incorporating water, which allows us to cast and place a structure in field. Ease of construction requires a lower viscosity of cement-based materials, and it could be achieved by cement particle dispersion in water. In this study, a laser backscattering measurement identifies the cement dispersion in in situ samples, and their size distribution are parametrized using the occurrence rate of a Poisson process. The cement dispersion is unaffected by external shearing intensity while the concentration (or water-to-cement ratio) or the use of dispersant alters its degree. Finally, associating the dispersion measurement with the microstructural state determined by their viscosity curves makes it clear how the cement viscosity decreases. (c) 2020 The Society of Rheology.