Impact Load Performance and Mesostructure Evolution of Alkali-Activated Fly Ash–Slag Concrete after Sustained High Temperature

Sustained high-temperature conditions (60°C–200°C) are the environment that concrete often encounters in the use process, but there are few reports on the performance evolution of alkali-activated concrete under long-term exposure to high temperatures. In this paper, the effects of sustained high temperature on the impact load performance and microstructure of alkali-activated fly ash–slag concrete (AAFS) are studied. The results showed that static and impact load compressive strengths exhibited similar patterns of change under sustained high temperatures; the pore variations and cracks within the AAFS concrete are highly synergistic at the mesoscale level. After 3 days, the water loss from the matrix accelerated, leading to an increase in microcracks, whereas both compressive strength and energy absorption density decreased under static and impact loads. The effects were observed at duration of 7 and 28 days. Hydration helped reduce pore and microcrack volumes, resulting in a denser internal structure for AAFS concrete, which enhanced its compressive strength and energy absorption density under static and impact loads. However, after 90 days of continuous exposure, gel decomposition occurred, increasing pores and microcracks, significantly degrading the mechanical properties of AAFS concrete. After sustained high-temperature action, the impact resistance of AAFS concrete was greatly improved, and the dynamic increase factor (DIF) increased by 40.7%. With the increase of duration, the changes of pores and microcracks in AAFS concrete were persistent, but did not produce an accumulation effect.