The expeditious setting time and poor workability of high-calcium fly ash have limited its feasibility as an eco-sustainable binder in cementitious systems, despite its off-setting cost and environmental benefits. This study focuses on the alteration of high-calcium fly ash mortar to achieve the ASTM standard and enhancement in performance through the alkali activation method. The methodology implied the variation of the ratio of sodium silicate to sodium hydroxide, sodium hydroxide molarity (8M and 10M), and the use of two types of plasticizers (polycarboxylate ether and polymelamine sulfonate). The findings revealed that the reduction of the Na₂SiO₃/NaOH ratio and the use of polycarboxylate ether enhanced the setting time for up to 190 min and achieved a 28-day compressive strength of 44.9 MPa. The flexural strength was also increased to 5.11 MPa, and the drying shrinkage was decreased to as low as 0.03%. All the optimized mixes complied with the thresholds of ASTM C191, C109, and C596. These results indicate that mortars based on high calcium fly ash can substitute ordinary Portland cement on a 1:1 basis for both structural and nonstructural applications, thereby decreasing CO₂ emissions and improving the sustainability of the materials used. The novelty of this study is that full cement replacement is possible in an HCFA-based geopolymer system, which is tailored from an optimized activator chemistry and plasticizer selection, and exceeds previous efforts that had to use as alternative to cement blending or compromise strength or workability.