The purpose of this study was to investigate the influence of fine powder (SF fines) obtained from recycled concrete aggregate on the hydration kinetics and mechanical–microstructural properties of hydrated cement through experiments and three-dimensional microstructural modelling using CEMHYD3D. Mixtures prepared with different substitution levels ranging from 0 % to 30 % by volume were examined. The experimental results indicated that the presence of calcite (CaCO3) and residual hydrates in SF powder accelerated the hydration of cement phases, particularly the C3A and C3S phases, at early ages. Increasing the SF fines substitution level led to an increase in the degree of hydration (DoH) of cement compared to that of the reference. The compressive strengths of mortars decreased with SF powder substitution levels; however, a substitution rate of 10 % led to a strength comparable to that of the reference. The modelling results also indicated that increasing the SF fines substitution level increased the DoH of cement. However, at very early ages, the hydration retardation of in-dividual cement phases increased with increasing SF powder substitution levels, particularly for the C3A and C4AF phases. The findings of the study suggest that SF fines can act as fillers, increasing the nucleation zones for cement hydration reactions, and as silico-aluminous sources for pozzolanic reactions.