Impact of Na and K Activators on Properties of Class F and C Fly Ash‐Based Geopolymers

ABSTRACT This study investigates the mechanical and microstructural evolution of geopolymers synthesized from Class F and Class C fly ash using two distinct activator systems: sodium‐based (NaOH + Na 2 SiO 3 ) and potassium‐based (KOH + K 2 SiO 3 ). A constant liquid‐to‐solid ratio of 0.33 was maintained, followed by optimized thermal curing at 75°C for 24 h. The experimental results reveal a fundamental kinetic disparity: Class F systems achieved a peak compressive strength of 52.41 MPa at 28 days, supported by high ultrasonic pulse velocity (UPV) and structural stiffness. In contrast, Class C specimens suffered from “flash setting” due to rapid calcium hydration, which inhibited long‐term strength development and resulted in a severely diminished elastic modulus. Although FTIR analysis confirmed successful chemical transformation in all series, evidenced by the Si–O–T band shift to 1000 cm −1 and high transmittance values at the 1640 cm −1 band (87.21%–89.02%), in Class C geopolymers revealed a failure in structural moisture integration. This microstructural incoherence, further supported by SEM–EDX findings, led to a fragmented “honeycomb” morphology. While many studies focus on individual activator effects, this study is necessitated by the need to understand how the interaction between alkali cation types (Na + vs. K + ) and fly ash calcium content dictates the microstructural evolution and long‐term integrity of alkali‐activated binders. By investigating the range from low‐calcium geopolymeric networks to high‐calcium hydration‐dominant systems, the research demonstrates that mechanical performance is ultimately determined by the kinetic harmony between the precursor and the activator. This approach proves that rapid calcium‐based hydration in high‐calcium precursors can undermine geopolymerization, thereby compromising the continuity and stability of the structural framework.