Combined Chemical–Physical Modification of Coal Gangue Concrete: ITZ Densification and Enhanced Wetting–Drying Resistance

To investigate a combined chemical–physical modification strategy for improving the wetting–drying resistance of coal gangue concrete (CGC), CGC was prepared by replacing 25 wt.% of natural coarse aggregate with coal gangue coarse aggregate. Sodium silicate (8 wt.%) was used for chemical activation, while waterborne epoxy resin (3, 6, and 9 wt.%) was introduced to achieve physical sealing. The modification mechanism was investigated through mechanical tests, relative dynamic modulus measurements, X-ray diffraction (XRD), and scanning electron microscopy (SEM). The results showed that the combined effects of chemical activation and physical sealing contributed to improved resistance to deterioration under wetting–drying cycles, while the modification performance was also dependent on the resin dosage. Among the tested mixtures, CSM–6–8, representing the combined-modification group treated with a 6 wt.% aqueous epoxy modifying solution and an 8 wt.% sodium silicate solution, exhibited the best overall performance, with compressive and flexural strengths of 50.5 MPa and 10.9 MPa, respectively, after 60 wetting–drying cycles. After 120 cycles, its compressive and flexural strengths remained 11.92% and 33.70% higher than those of SM–0–8, respectively, indicating a sustained improvement in wetting–drying resistance. Its relative dynamic modulus reached a peak value of 1.05 after 28 cycles and decreased by only 3.4% relative to this peak after 90 cycles. Microstructural analyses suggested that sodium silicate modification may promote the formation of additional gel-like hydration products, while the cured epoxy resin may contribute to the development of a relatively continuous surface coating, with these effects jointly contributing to the densification of the pore structure and interfacial transition zone (ITZ), thereby reducing moisture transport and delaying deterioration. The better performance of the 6 wt.% resin dosage among the tested levels was associated with a favorable balance between coating development and the tendency for resin agglomeration at higher dosages. This study provides support for the design of coal gangue concrete with improved wetting–drying resistance and promotes its low-carbon utilization.