Synergistic Mechanical–Chemical Activation of MSWI Bottom Ash: Microstructural Evolution and Reaction Mechanisms

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This study investigates how mechanical–chemical synergistic activation enhances the reactivity of municipal solid waste incineration bottom ash (MSWI BA). Characterization shows that MSWI BA has a strength activity index (68.3%) between fly ash (78.0%) and inert filler (61.2%), indicating potential reactivity. After 50 min of ball milling, the D50 value of bottom ash particles decreased from 17.04 to 9.76 μm (a reduction of 42.7%), specific surface area (SSA) increased to 3,990.6 m 2 / kg (an increase of 19.0%), and Fourier-transform infrared spectroscopic (FTIR) analysis revealed significantly reduced silica network polymerization. Six chemical activators were introduced to the physically activated material, with calcium formate (1%) showing the best performance, increasing the 28-day compressive strength by 18.21%. Thermogravimetric–derivative thermogravimetry (TG-DTG), X-ray diffraction (XRD), and scanning electron microscopy (SEM) analyses demonstrated that calcium formate primarily promotes C─S─H gel formation and optimizes its structure; calcium sulfate enhances structural density by forming needle-like ettringite (AFt) crystals; while triethanolamine promotes early hydration but inhibits later pozzolanic reactions. The activation mechanism involves ball milling, first destroying the passivation layer on the bottom ash surface and breaking Si─O─Si networks, followed by chemical activators facilitating the dissolution of reactive components for hydration reactions, forming a multiscale cross-linked network structure. This synergistic strategy not only provides a new approach for MSWI BA resource utilization but also establishes a theoretical foundation for activating other low-reactivity solid waste materials.