Glass foam production from waste glass and recycled carbon fibers: Synergistic role of carbon microfibers as blowing agents and possible bubble wall reinforcement

Urban glass waste powder, a major by-product of residential glass recovery processes, and secondary waste carbon fiber, an industrial waste generated during the production of woven/non-woven fabrics, both present significant recycling challenges and are often disposed of in landfills. This research aims to develop a novel, sustainable, and eco-friendly optimized approach to produce light-weight glass foam (GF) by using these wastes. A critical challenge in GF production lies in optimizing its structural and functional properties, which are linked with its closed/open porous structure, to comply with evolving application-specific performance requirements. GF, characterized by its porous microstructure and low density, is widely utilized in industrial applications due to its excellent thermal insulation, mechanical properties, and environmental benefits.In this study, urban waste glass (powdered form) is mixed with 0.5 wt%, 1 wt%, and 1.5 wt% of powdered recycled carbon fiber (PRCF) as a foaming and reinforcing agent, while the foam structures were successfully obtained at 900 °C, 950 °C, and 1000 °C. The effects of sintering temperature, and foaming agent content using optical microscopy, scanning electron microscopy, laser thermal constant analysis, and a universal testing machine, respectively, as the key parameters influencing foam structure, pore distribution, compressive strength, thermal conductivity, bulk density, and total porosity were investigated systematically. Furthermore, by integrating heating microscopy with thermogravimetric analysis (TGA), this study offers a comprehensive understanding of carbon-fiber-induced foaming, establishing a direct correlation between carbon-fiber oxidation and gas evolution, the resulting temperature distribution, the viscous deformation of the softened glass matrix, and the formation of the final cellular microstructure. At 900 °C, foam glass exhibited predominantly closed porosity (22.4-42.8%), with minimal open porosity (1.6-2.0%), indicating restricted pore growth due to the high viscosity of the glass matrix. Increasing the processing temperature to 950 °C significantly enhanced both total and closed porosity, reaching 66.4% and 63.2%, respectively, at 1.5 wt% PRCF, while maintaining low open porosity (∼3.2%), suggesting efficient gas entrapment and stable pore formation. Further increasing the temperature to 1000 °C promoted pore coalescence, reduced bulk density, and increased open porosity, which may compromise mechanical integrity. Thus, the combination of 1 wt% PRCF and 950 °C sintering temperature was determined to be optimal, offering a favorable balance of porosity, low density (∼1.07 g/cm3), low thermal conductivity (0.13 W/m.K), and compressive resistance (1240.29 N). This optimized foam glass composition shows potential for construction and thermal insulation applications where low density, predominantly closed porosity, and sufficient mechanical response are required. This process also supports sustainable development by promoting waste valorization and advancing circular economy principles.