Incorporation of calcium in glasses: A key to understand the vitrification of sewage sludge

Abstract

The quantity of sewage sludge generated daily by wastewater treatment plants represents a major environmental problem and a financial burden for plant operators. Valorization strategies focusing on reusing sewage sludge as a raw material are currently developed. Vitrification can help us reduce the volume of waste and binds the components in the structure of chemically stable glasses and glass-ceramics. In this study, the vitrification of sewage sludge inside a basaltic rock has been simulated by producing glasses and a glass-ceramic from basalt enriched in calcium that lie between the stability fields of pyroxene and melilite in the system CaO–MgO–SiO2–Al2O3. CaO addition causes the oxidation of the melt at above the liquidus, increases the crystallization temperature, decreases the melting temperature, and improves the microhardness of the glasses. Glass-ceramic processes improves the properties of the Ca-doped basalt glass. The microhardness of the glass (8.2 GPa) and the glass-ceramic (8.6 GPa) and leaching tests (in the ppb range) place both the glass and the glass-ceramics at the high end of the mechanical properties and chemical resistance of ceramic tiles for the building industry.