This study investigates the partial replacement of Portland cement with locally sourced fly ash at 0–60 wt.% to evaluate its effects on hydration, densification, microstructure, and compressive strength. Cement pastes were prepared using water contents determined by the Vicat standard-consistency procedure and tested at 1, 3, 7, 28, and 90 days. XRD, FTIR, TG, and FE-SEM were used to interpret phase evolution and microstructural development. Within the investigated design, the 50 wt.% fly-ash mixture (Mix 6) showed the highest overall performance. At 90 days, its compressive strength was 49.92 MPa, 11.9% higher than the control (44.62 MPa), while bulk density increased by 52.0%, apparent porosity decreased by 39.0%, and the hydration index increased by 71.8%. The 60 wt.% mixture showed a subsequent decline, indicating a performance maximum within the investigated range rather than a universal optimum. Because the water-to-binder ratio decreased with increasing fly-ash replacement, the observed improvements reflect combined effects of mixture composition, water demand, particle packing, and hydration. The results demonstrate the potential of the selected local fly ash for high-volume cement replacement while highlighting the need for controlled w/b and statistical follow-up studies.