Experimental and numerical insights into the degradation mechanism of hybrid fibers-reinforced alkali-activated materials under coupled sulfate attack and dry-wet cycles

This study investigates the degradation mechanisms of hybrid fiber-reinforced alkali-activated slag/fly ash mortar under the coupled effects of sulfate attack and dry–wet cycling. Polyethylene (PE) and steel fibers were incorporated to enhance toughness and crack resistance, with the optimal formulation determined as a 1:1 vol ratio of PE to steel fibers (a total of 2 vol%). Experimental results revealed that at this ratio, the compressive, flexural, and tensile strengths increased by 26.4 %, 33.7 %, and 42.5 %, respectively, compared with fiber-free specimens. Under cyclic exposure to 3 % and 5 % Na₂SO₄ solutions for up to 25 dry–wet cycles, both compressive and flexural strengths initially increased relative to the reference sample. This improvement was attributed to continued hydration and pore filling by crystalline products. Subsequently, progressive degradation occurred, characterized by mass loss (up to 3.8 %), increased porosity, and visible cracking in later stages. Microstructural analyses revealed the formation of expansive ettringite and gypsum, which contributed to crack propagation. Ion penetration profiling showed that sulfate concentration stabilized beyond a depth of 10 mm. A coupled numerical simulation using COMSOL Multiphysics was developed to model sulfate ion transport during cyclic exposure, showing excellent agreement with the experimental results (R > 0.98, RMSE < 0.01). The findings provide both systematic understanding and predictive modeling of durability performance, offering valuable guidance for the design of alkali-activated materials composites in aggressive environments. • In the early stages of sulfate exposure, specimens exhibited improved mechanical strength due to pore-filling by reaction products. • Long-term exposure to sulfate ions led to the accumulation of expansive ettringite and gypsum to increase porosity. • A dynamic balance between pore-filling and dissolution was observed with continued cycling. • The COMSOL-based transport model accurately reflected ion transport behavior and degradation progression.