Microstructural and mechanical response of fly ash–bentonite modified fiber-reinforced concrete under coupled sulfate attack and wet–dry cycles

Concrete infrastructures located in coastal areas are frequently subjected to coupled sulfate corrosion and wet–dry cycling (WDC). The durability behavior of concrete incorporating bentonite and fly ash (FA) was evaluated under the simultaneous influence of WDC. Concrete containing nine mixtures of different amounts of bentonite and FA were exposed to 150 WDC in Na 2 SO 4 (aq), MgSO 4 (aq) and water, respectively. Physical and mechanical properties, for instance, compressive strength along with the relative dynamic modulus of elasticity (RDEM) were tested to assess the degree of deterioration of the concrete during WDC. In addition, the concrete stress–strain characteristics were analyzed to determine the linkage among modulus of elasticity, peak stress, and peak strain in different sulfate solutions and the frequency of WDC. Furthermore, environmental SEM and industrial computed tomography (CT) were employed to examine the product composition and pore structure evolution at various erosion phases. Results show that concrete incorporating fly ash and bentonite exhibited up to a 33.2% increase in compressive strength in Na 2 SO 4 and a 61.5 MPa peak strength in water during WDC. However, RDEM decreased by up to 53.5% in high–fly ash mixtures after 150 cycles. CT analysis further revealed that pores larger than 0.1 mm 3 increased by 120.11% in the control group, whereas the addition of bentonite and fly ash reduced macropore volume by 49.21%, demonstrating their synergistic role in refining the pore structure and enhancing durability. The peak stress followed the same increase–decrease pattern in sulfate environments but continued to rise in water with more WDC cycles. This study introduces a novel multiscale evaluation of fiber-reinforced concrete incorporating bentonite and fly ash under coupled sulfate attack and wet–dry cycling, a combination rarely addressed in previous research. By integrating mechanical testing, stress–strain modeling, SEM, and 3D CT pore analysis, the work reveals the synergistic roles of bentonite and fly ash in refining pore structure and governing deterioration mechanisms. These findings provide new insights for designing durable concrete for coastal environments.