Assessing the effect of particle characteristics on the rheological properties of mortar with recycled fine aggregate

This study systematically investigates the impact of recycled fine aggregates’ (RFA) particle characteristics on the rheological behavior of mortar mixtures by direct testing, theoretical calculations, and dynamic analysis. First, key parameters of RFA were quantitatively assessed, including time-dependent water absorption, packing density, and particle morphology. The rheological performance of mortar mixtures, containing varying RFA volume fractions and saturation levels, was examined according to these particle properties. Dynamic analysis, based on the relationship between rheological parameters and solid particle content, was employed to determine the dynamic packing fraction of RFA in shear flow. Additionally, the interactions between RFA and cement were considered, and the overall packing fraction of the particle system was evaluated using the compressible packing model (CPM). The results indicate that RFA exhibits high water absorption and rapid absorption kinetics, along with a distinct non-spherical morphology, leading to a low packing density. The water absorption of unsaturated RFA is strongly correlated with the water-to-cement ratio of the paste. Dynamic analysis effectively describes the influence of particle parameters on rheology, with the fitted packing fractions slightly higher than the measured values. The theoretical calculations based on CPM indicate that the packing fraction of the particle system exhibits a trend of increasing first and then decreasing with respect to the volume ratio of RFA to cement (A/C), reaching its peak when A/C is 1.3896. This relationship can be used to rapidly assess the impact of the particle system on the rheological behavior of the mixture. The methodology employed in this study provides new insights into the mix proportion design of cement-based materials and promotes the broader application of RFA.