Effect of fly ash content on concrete durability under magnesium sulfate attack and sustained load

This study investigates the effect of fly ash content (0–40%) on the durability of C30 concrete subjected to 8% magnesium sulfate corrosion (S) and its coupled action with 50% ultimate static loading (L-S). Through a combination of macroscopic mechanical testing and multi-scale microstructural characterisation (X-ray diffraction analysis, scanning electron microscopy, nuclear magnetic resonance, thermogravimetric–differential thermogravimetric analysis), the pozzolanic effect of fly ash, the load–damage mechanism and their coupled interaction were elucidated. Results indicate that under pure magnesium sulfate exposure, 30% fly ash content (FA30) exhibits optimal erosion resistance, with its refined pore structure significantly retarding ion penetration. In coupled conditions, microcracking induced by static loading accelerates deterioration rates to 1.6–1.7 times that of pure exposure, yet FA30 remains superior, whereas FA40 exhibited the most pronounced deterioration due to insufficient hydration and reduced calcium silicate hydrate (C–S–H) stability. A life prediction model based on relative compressive strength as a degradation indicator was established using a two-parameter Weibull distribution. Distribution verification confirmed this indicator’s applicability to both erosion environments studied. Fly ash incorporation flattened the reliability curve slope, with FA30 exhibiting the longest predicted service life under both conditions. This study provides theoretical foundations for durability design and admixture optimisation of concrete subjected to magnesium sulfate–static load coupling environments.