Synergistic Effects of Carbonated and Hydrophobically Modified Municipal Solid Waste Incineration Fly Ash on Mortar Performance and Heavy-Metal Immobilisation

Municipal solid waste incineration (MSWI) fly ash contains soluble salts and heavy metals, which may cause leaching risks and durability deterioration when directly used in cement-based materials. This study aimed to investigate the synergistic effects of carbonated and hydrophobically modified municipal solid waste incineration fly ashes on the engineering performance and heavy-metal immobilisation of mortar. Mortars containing modified fly ashes were evaluated in terms of hydration behavior, compressive strength, water absorption, electrically accelerated corrosion resistance, heavy metal leaching, and microstructure. Carbonated fly ash promoted hydration through the nucleation and filling effects of CaCO3, shortened setting time, increased cumulative hydration heat, and improved compressive strength by up to 4.5 MPa. Hydrophobic fly ash reduced particle wettability and capillary water transport, thereby reducing water uptake and mitigating visible corrosion-induced deterioration under accelerated conditions, although excessive dosage delayed hydration and reduced strength. The combined modification showed a clear synergistic effect, reducing water absorption by up to 39.9%. In particular, the C3H3 specimen, containing 75 kg·m−3 carbonated MSWI fly ash and 75 kg·m−3 hydrophobically modified MSWI fly ash, exhibited the lowest water absorption of 3.92% and effectively suppressed crack propagation and corrosion-product migration. The leaching concentrations of Cr, Cu, Zn, As, Cd, and Pb were below the GB 18598—2019 limits. X-ray diffraction (XRD), Fourier-transform infrared spectroscopy (FTIR), thermogravimetric analysis (TG), and low-field nuclear magnetic resonance (NMR) results indicated that the improved performance originated from a composite barrier involving carbonate filling, hydrophobic interfacial blocking, and heavy metal solidification/stabilization.