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[Display omitted] • A pressure-assisted sintering treatment was developed for treating MSWI fly ash; • The mechanical properties and heavy metal leaching behavior were greatly improved; • A high-strength product was produced from fly ash sintered at a low temperature; • Surface diffusion and grain boundary diffusion were the main sintering mechanisms. Municipal solid waste incineration (MSWI) fly ash disposal is an urgent task with some technical bottlenecks. In this study, a novel pressure-assisted sintering method was employed to treat the MSWI fly ash. A series of pressure-assisted sintering experiments were carried out by varying mechanical pressures and sintering temperatures, and their properties of compressive strength, density and heavy metals leaching behavior were determined to screen out the optimal conditions. Instrumental analysis of XRF, SEM, XRD and TEM-EDX and calculation kinetics were conducted to explore the enhancement mechanism of pressure-assisted sintering. With the help of mechanical pressure, a high-strength ceramic product was produced from MSWI fly ash sintered at a low temperature (400 °C), which never occurred in the conventional low-temperature sintering process. Maximum compressive strength of 218.30 ± 4.08 MPa was obtained at 400 °C and 100 MPa, which was much higher than conventional construction materials of brick and cement. In addition, the leaching concentrations of heavy metals obtained from pressure-assisted sintering process were lower than the standard limitation. The SEM and XRD results revealed that the increased mechanical properties and the decreased heavy metals leaching concentration were mainly attributed to the increased density and crystalline degree. The kinetics calculation results indicated that the sintering activation energy was much lower than the sintering process without pressure, suggesting surface diffusion and grain boundary diffusion were main sintering mechanisms in the pressure-assisted sintering process. These findings proved that pressure-assisted sintering could be a promising method to treat fly ash together with producing high-value building materials. [ABSTRACT FROM AUTHOR]

[Display omitted] • The chemical species and transformation process of Cl in fly ash were investigated; • The distribution of chloride was affected by microstructure transition; • Chloride transformed into a stable form during pressure-assisted sintering. High chloride content in municipal solid waste incineration (MSWI) fly ash is a critical restriction for the recycling and re-utilization due to the acceleration of the corrosion process in construction material. Therefore, the investigation of chloride distribution and transition behavior in fly ash is of great significance. In this study, the distribution and chemical species of chloride in fly ash were first analyzed through varying mechanical pressures (10, 30, 50, 70, and 100 MPa) and sintering temperatures (300 °C, 400 °C and 500 °C) during the pressure-assisted sintering process. Instrumental analyses of SEM-EDX, element mapping, XPS, XRD and chloride leaching concentration were conducted to explore the chloride transition behavior. The results suggested that chlorides were homogeneously distributed on the surface of fly ash particles, which was significantly influenced by the microstructure change during the pressure-assisted sintering process. In addition, the chloride migration was enhanced by the pore shrinkage under different mechanical pressures and the increased atomic mobility at various temperatures. The mineral results showed that the main crystals in raw fly ash were NaCl, KCl and CaClOH, and the minerals were transformed into more complex and stable crystal phases, such as Ca 10 (Si 2 O 7) 2 SiO 4 Cl 2 (OH) 2 and Ca 10 (SiO 4) 3 (SO 4) 3 Cl. The transformation of chloride from soluble salts to stable Cl-containing minerals is beneficial to the immobilization of chloride, which suggested that pressure-assisted sintering could effectively decrease the chloride leaching concentration. This study provides a further understanding of chloride transition and distribution behavior during pressure-assisted sintering treatment. [ABSTRACT FROM AUTHOR]

Thermal behavior of municipal solid waste incineration (MSWI) fly ash is extremely complicated due to the simultaneously occurred reactive processes and the products with different chemical compositions, therefore, the investigation of chemical compounds transition behavior and mechanism during the integrated thermal process is of great significance. In this study, the macro-thermal treatment of fly ash and thermo-gravimetric analysis via non-isothermal methods were carried out and Málek method was firstly introduced to explore the mechanism of multi-step reaction for fly ash. The mineral transition results suggested that the halite, sylvite in the raw fly ash transformed to more complex crystals in treated samples, such as chlorellestadite, polyhalite and enstatite during the thermal process. And the heavy metals leaching concentration decreased with the temperature increased from 300 °C to 1200 °C, and the leaching values were lower than the standard limitation after thermal treatment. In addition, three major steps of fly ash reactions (300–380 °C, 650–750 °C and 890–1130 °C) during the thermal process were observed and expressed by first order reaction for the first step, three-dimensional diffusion for the second step and three dimensions of limiting surface reaction between both phases for the third step, respectively. The kinetic study revealed that the mineral transition process was in well accordance with the simulated reaction mechanism during the thermal treatment. The obtained results are expected to provide the research basis for studying detailed thermal characteristics and reaction mechanism during the thermal treatment of MSWI fly ash. Image 1 • The integrated thermal behavior of MSWI fly ash thermal treatment was investigated. • Reaction mechanism determined by Málek method was identified as F1, D3, and R3. • The results provided integrated thermal behavior and reaction mechanism of fly ash. [ABSTRACT FROM AUTHOR]