This research investigates the relationship between mineralogical composition and compressive strength in alkali-activated cement–sand mortars incorporating palm oil fuel ash (POFA) as a partial replacement of Portland cement. POFA was introduced at 5 wt.% and 10 wt.% of the binder, and activation was achieved using a NaOH–Na₂SiO₃ solution (3:1 mass ratio). Compressive strength and bulk density were evaluated at 7 and 28 days, while phase evolution was analyzed by X-ray diffraction (XRD) coupled with Rietveld refinement. The results demonstrate that POFA incorporation significantly modified the CaO–SiO₂–Al₂O₃ balance of the system, promoting the consumption of portlandite and the formation of Na- and K-rich aluminosilicate phases such as albite and muscovite. The control mixture exhibited the highest compressive strength values, whereas increasing POFA content reduced both strength and density due to calcium dilution, lower gel compactness, and increased porosity. Nevertheless, all mixtures exhibited progressive strength development over time, indicating continued hydration and geopolymerization reactions associated with the formation of hybrid C–(N,K)–A–S–H gels. These findings demonstrate that POFA can effectively participate in alkali-activated hybrid binders when applied at controlled replacement levels, highlighting its potential as a sustainable supplementary material for lower-carbon cementitious systems.