Study on the effect and mechanism of fly ash on the leachability resistance of cement mortar

To enhance the erosion resistance of cement mortar in alternating wet and dry environments, this study investigates the mechanism by which fly ash influences cement mortar. Accelerated erosion tests were conducted to evaluate the effects of fly ash content (0%–40%) on the macroscopic properties (erosion depth, calcium ion leaching, mechanical properties, porosity) of mortar subjected to continuous immersion in 3 M NH₄Cl solution and wet-dry cycling conditions. Microstructural and phase compositions were analysed using X-ray diffraction（XRD）and scanning electron microscopy（SEM）. Findings indicate that wet-dry cycling significantly exacerbates erosion, with calcium ion leaching approximately 46% higher than under continuous immersion. Fly ash addition effectively inhibits erosion progression, with 20% dosage yielding the most pronounced pore structure optimisation—reducing porosity by 7.4% after 72 days of erosion compared to the control group. whilst a 30% fly ash content yielded optimal mechanical properties, reducing calcium ion leaching by 36.8% and achieving the highest compressive strength retention post-erosion. Microstructural analysis indicates that fly ash consumes Ca(OH)₂ through pozzolanic reactions, generating more stable C-S-H gel and acting as a microfiller to refine pores, thereby retarding the penetration and dissolution processes of erosive media.