Crystallization evolution, microstructure and properties of sewage sludge-based glass-ceramics prepared by microwave heating

A Microwave Melting Reactor (MMR) was designed in this study which improved the microwave adsorption of sewage sludge to prepare glass-ceramics. Differential scanning calorimetry (DSC), X-ray diffraction (XRD), and scanning electron microscopy (SEM) were used for the study of crystallization behavior and microstructure of the developed glass-ceramics. DSC and XRD analysis revealed that crystallization of the nucleated specimen in the region of 900-1000°C resulted in the formation of two crystalline phases: anorthite and wollastonite. When the crystallization temperature increased from 900 to 1000°C, the tetragonal wollastonite grains were subjected to tensile microstresses, causing the cracking of crystal. Al ions substituted partially Si ions and occupied tetrahedral sites, giving rise to the formation of anorthite. The relationship between microwave irradiation and crystal growth was studied and the result indicated that the microwave selective heating suppressed the crystal growth, giving apparent improvements in the properties of the glass-ceramics. The glass-ceramics products exhibited bending strength of 86.5-93.4MPa, Vickers microhardness of 6.12-6.54GPa and thermal expansion coefficient of 5.29-5.75×10 ^-6/°C. The best chemical durability in acid and alkali solutions was 1.32-1.61 and 0.41-0.58mg/cm ², respectively, showing excellent durability in alkali solution.