The high-temperature behavior of alkali-activated fly ash (AAF) geopolymers is evaluated. The physical and phase changes in the material are determined to be a function of temperature of exposure. Thermal diffusivity and compressive strength are determined to be a function of temperature. The compressive strength after high-temperature exposure is related to the phase composition changes in the sodium aluminosilicate hydrate (NASH) gel and the porosity in the AAF. Exposure to 200°C produces an increase in the NASH gel content by up to 5% and a reduction in mass due to moisture loss. The physical damage in the material on heating up to 200°C is produced by vapor pressure generated inside the material, and it results in the creation of porosity in the 100-nm range. The level of damage is higher in AAF with a lower water content because of its finer pore structure. Up to 200°C, the combined influence of additional geopolymerization and physical damage produces an increase in strength in AAF with a higher water content and a decrease in strength at lower water content. There is an increase in the thermal diffusivity with moisture loss in geopolymer pastes with high water content. There is a consistent loss of strength in the range of 25-30 MPa between 200°C and 600°C, which is produced by changes in the NASH gel. There is a reduction in the content and changes produced by silica enrichment and dehydroxylation of the NASH gel between 200°C and 600°C. There is a strength gain of 3-6 MPa between 600°C and 1,000°C, which is produced by the conversion of the amorphous NASH gel to a more crystalline form. There are no additional changes in the porosity and thermal diffusivity at temperatures higher than 200°C. The results are significant for the fire design of concrete made with geopolymers. © 2022 American Society of Civil Engineers.