The accumulation of construction waste from building demolition and natural disasters has necessitated the repurposing of this waste into recycled aggregate (RA) for new structures. However, recycled aggregate concrete (RAC) generally exhibits low mechanical properties due to the low quality of RA. Previous studies introduced a multi-stage mixing approach to improve the properties of recycled concrete by sequentially adding binders to form a low water/binder ratio slurry on the surface of RAs. However, the improvement is not evident at an early age due to the delayed initiation of the pozzolanic reaction of fly ash. The present study aims to promote early-age improvement by incorporating fly ash and calcium silicate hydrate (C-S-H) seeds. Four different RACs with a water/binder ratio of 0.40 were prepared: control RAC, with 4 wt% C-S-H seeds, with fly ash and 2 wt% C-S-H seeds, and with fly ash and 4 wt% C-S-H seeds (fly ash replacement is 20%). Compressive strength tests and scanning electron microscope (SEM) analysis were conducted. The results demonstrated that RAC with fly ash and 4 wt% C-S-H seeds exhibited higher compressive strength than the control RAC. Notably, its early strength was more than 83% of RAC with only 4 wt% C-S-H seeds. This enhancement was attributed to the synergistic effect of C-S-H seeds on cement hydration and the fly ash reaction, as confirmed by the SEM analysis of the microstructure surrounding the RA.