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Choice of alkali cation in mix design of geopolymers is critical for their thermal performance. However, the influence of alkali cation type on strength endurance of geopolymers subject to high temperature exposure and the underlying governing mechanisms have not been studied. This study investigated the effects of alkali cation type on high temperature response of fly ash geopolymers aiming towards structural applications. In-depth investigation was carried out to discover the underlying mechanisms governing the strength endurance of fly ash geopolymers subject to high temperature exposure and its correlation with their volume stability and chemical stability. Results showed that potassium geopolymer exhibited significant strength enhancement (30–40%) and sodium geopolymer displayed reduced strength (10%), and the strength of mixed sodium and potassium geopolymer remained unchanged after exposure to elevated temperature. While the binders were chemically stable without deterioration and formation of new crystal phases after high temperature exposure, the volume stability varied with the type of alkali cation used. Formation of cracks and gel densification due to shrinkage, and healing of micro-cracks and change of pore sizes due to sintering were identified as responsible mechanisms at different temperature ranges. While crack development and enlargement of pores lowered the strength, densification of matrix and healing of micro-cracks favored strength gain. These competing mechanisms determine the strength endurance of geopolymers depending on the type of alkali cation used. Overall, it shows that geopolymers can be tailored, such that stable (or even enhanced) strengths upon thermal exposure are realized, for structural applications. [ABSTRACT FROM AUTHOR]