Influence of medium–high temperature exposure on the mechanical properties and pore-structure fractals of fly ash-blended cementitious composites

Cementitious composites in applications, such as nuclear dry storage facilities and other nuclear-related structures, are exposed to sustained medium-to-high temperatures during service. However, the effects of such thermal exposure on pore-structure evolution, mechanical performance, and durability, particularly in fly ash (FA)-blended systems, remain unclear. This study investigates the influence of high-temperature exposure on the microstructure, mechanical strength, and carbonation resistance of cementitious composites with varying FA replacement rates. Specimens with 0%, 20%, and 40% FA replacement were cured at 23°C for 14 days and exposed to 80, 100, and 120°C for 28 days. Pore structures were analyzed using mercury intrusion porosimetry, and fractal dimensions were calculated for mesopores (<100 nm) and capillary pores (≥100 nm). Thermogravimetric analysis and X-ray diffraction evaluated hydration products and chemically bound water. Mesopore structures became simpler with increasing exposure temperature, while capillary pore complexity increased in specimens containing 0% and 20% FA. High-temperature exposure increased compressive strength; however, it caused strength reduction with prolonged exposure and increased carbonation depth, particularly at higher FA replacement levels. The decomposition of ettringite and C–S–H occurred at high temperatures, reducing chemically bound water even at 80°C. Compressive strength correlated with capillary pore fractal dimensions, while carbonation depth was governed by the mesopore structure. At 40% FA replacement, these correlations weakened owing to limited hydrate formation. These findings provide fundamental insights into the design and durability evaluation of cementitious composites used under high-temperature exposure conditions and thermally demanding engineering applications.