Microstructural Evolution and Mechanical Performance of Concrete Incorporating Palm Oil Fuel Ash as a Partial Cement Replacement

The incorporation of supplementary cementitious materials derived from agro-industrial residues has emerged as a promising strategy to reduce the environmental impact associated with Portland cement production while promoting circular economy principles. This study investigates the influence of palm oil fuel ash (POFA) as a partial cement replacement on the mechanical and microstructural properties of concrete. Concrete mixtures containing 0%, 10%, 15%, and 20% POFA by mass of cement were produced using a constant water-to-binder ratio of 0.47. Compressive strength was evaluated at 7, 14, 28, and 56 days, while microstructural characterization was performed using scanning electron microscopy coupled with energy-dispersive spectroscopy (SEM–EDS) to assess hydration products and the interfacial transition zone (ITZ). The incorporation of POFA resulted in lower compressive strength at early curing ages, with reductions of approximately 16–21% compared with the control mixture, reflecting the slower kinetics of pozzolanic reactions. However, prolonged curing promoted significant strength development. At 56 days, concretes containing 10% and 15% POFA exhibited compressive strengths 3.7% and 9.2% higher, respectively, than the control concrete, whereas the 20% replacement level resulted in a 14.6% reduction. SEM observations revealed a denser cementitious matrix, improved aggregate–paste bonding, and a more refined ITZ in mixtures containing 10–15% POFA. EDS analysis showed Ca/Si ratios of 1.22 and 1.08 for the 10% and 15% POFA mixtures, respectively, indicating the formation of silica-rich C–S–H gel associated with effective pozzolanic activity. The results demonstrate that POFA can be successfully utilized as a supplementary cementitious material in concrete. While the 15% replacement level produced the highest 56-day compressive strength, both the 10% and 15% mixtures exhibited favorable microstructural characteristics and effective pozzolanic activity, indicating that both replacement levels are suitable for sustainable concrete production.