Effects of SAC-GGBS composite slurry coating on coal gangue aggregate morphology, interfacial structure, and concrete mechanical properties

The large-scale utilization of coal gangue as concrete aggregate is hindered by its high porosity, low crushing strength, and inherent bedding defects. Conventional coating modification based on ordinary Portland cement exhibits relatively slow early-age strength development and is therefore unsuitable for rapid construction applications, such as slope rehabilitation in mining areas. In this study, coal gangue aggregate (CGA) was modified using a composite slurry coating composed of sulphoaluminate cement (SAC) and ground granulated blast-furnace slag (GGBS), taking advantage of the rapid hardening and slight expansive characteristics of SAC and the pozzolanic reactivity of GGBS. The effects of the SAC-GGBS proportion on aggregate morphology, cylinder crushing strength, water absorption, concrete mechanical properties, stress-strain behaviour, and interfacial microstructure were systematically investigated. The underlying strengthening mechanisms were further elucidated using XRD, FTIR, TG-DTG, and SEM-EDS. The results showed that the SAC-GGBS composite coating formed a continuous hardened shell on the CGA surface, filled surface pores and microcracks, blunted sharp edges, and alleviated local stress concentrations induced by the irregular particle morphology. The G-20 group (SAC:GGBS=4:1) performed the best. Compared to the JZ group, its aggregate cylinder compression strength increased by 43.04%, water absorption decreased by 23.55%, and the corresponding concrete compressive and splitting tensile strengths rose by 45.03% and 68.71%, respectively. The elastic modulus went up from 1.81 GPa to 3.36 GPa. The stress-strain curves showed a markedly more gradual post-peak descending branch for the G-20 group. The failure mode changed from predominantly brittle interfacial fracture to progressive damage involving the cooperative load-bearing action of the aggregate and surrounding matrix. The Guo Zhenhai constitutive model provided satisfactory agreement with the experimental results, with coefficients of determination exceeding 0.98. Microstructural analyses further demonstrated that the Q 3 /Q 2 ratio in the G-20 group reached 0.71. The silicate network evolved from the low-connectivity structure represented by Q 2 to the high-connectivity structure represented by Q 3 .SEM-EDS observations confirmed the formation of a denser and more continuous ITZ between the aggregate and cementitious matrix. Therefore, the characteristics of SAC-GGBS composite coating in reinforcing aggregates, densifying interfaces, alleviating stress concentration, and regulating crack propagation provide a feasible solution for the application of coal gangue concrete in rapid repair and mining area engineering.