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

Abstract

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.