Effects of carbonation methods on mechanical behavior and carbonation mechanisms of coal gangue based geopolymer stabilized saline soil

This study systematically investigated the effects of different carbonation methods on the failure patterns, unconfined compressive strength (UCS), and strength growth rate of geopolymer stabilized saline soil. Carbonation mechanisms were examined using X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), thermogravimetric analysis (TG), scanning electron microscopy (SEM), and energy dispersive spectroscopy (EDS). The results show that the unconfined compressive strength (UCS) of carbonated stabilized soil is significantly improved, exceeding 4.1 MPa at a geopolymer content above 25% under mixed carbonation. For conventional carbonation curing and superimposed carbonation, the strength exceeds 3.8 MPa and 6.2 MPa, respectively, at geopolymer contents over 20%. Among the carbonation methods studied, the strength of 20–35% geopolymer stabilized soil decreases in the order: 2 h superimposed carbonation, 1 h superimposed carbonation, 2 h carbonation curing, 1 h carbonation curing, and mixed carbonation. With increasing geopolymer content, the failure mode changes from cracking to disintegration and peeling, with failure behavior shifting from ductile to brittle. All carbonation methods show obvious strengthening effects when geopolymer content exceeds 25%. Microstructural analysis indicates that carbonation promotes the formation of CaCO3 and C–A–S–H, N–A–S–H, C–N–A–S–H gels, resulting in a denser microstructure. Thermogravimetric analysis shows that carbonated geopolymer-stabilized soil decomposes in three stages: removal of free and bound water, decomposition of low‑crystallinity CaCO3, and decomposition of high‑crystallinity CaCO3. These results provide a theoretical basis and practical reference for the engineering application of carbonated geopolymer stabilized saline soil.