Developing advanced construction materials with enhanced mech. performance and fire resistance is critical for applications in extreme environments, for example, Liquid Metal-Cooled Fast Breeder Reactors (LMFBRs).Alkali-activated cementitious materials (AACMs) exhibit promising properties for sodium fire-resistant concrete in such reactors.This study systematically investigates the effects of elevated temperatures, as well as combined elevated temperatures with NaOH exposure, on the compressive strength of fly ash (FA) and blast furnace slag (BFS) pastes activated by sodium fluorosilicate (SF) and sodium silicate solution (SSS).Key variables include the cementitious material-to-sodium silicate solution ratio (CM/SSS), sodium fluorosilicate-to-sodium silicate solution ratio (SF/SSS), and sodium silicate solution modulus (SSSM).The pore structure and microstructural changes of FA and BFS pastes were elucidated by MIP, SEM, EDS, and XRD.The results show that FA paste achieves compressive strength improvements of 23.5-37.4 % at 200 °C and retains strength between 800-900 °C, but melts at 1100 °C.Under combined high-temperature and NaOH exposure, FA paste maintains its shape at 500 °C but melts completely at 800 °C.In contrast, BFS paste withstands up to 900-1100 °C with compressive strength of 48 MPa at 900 °C and resists NaOH corrosion at 800 °C.These findings highlight the superior thermal and chem. stability of BFS paste, making it a viable candidate for LMFBR applications.By shedding light on the high-temperature and chem. resistance of AACMs, this research not only bridges existing knowledge gaps and offers a foundation for future studies on optimizing AACMs for extreme operational environments.