Self-foaming lightweight geopolymer using E-waste as a foaming agent: rheology-controlled microstructure and performance evolution

The growing accumulation of electronic waste (e-waste) has increased the demand for high-value recycling strategies. Foamed geopolymer materials offer a promising solution for energy-efficient building applications. This study proposes the use of waste printed circuit board (PCB) powder as a solid foaming agent in glass powder (GP)–ground granulated blast furnace slag (GGBS) alkali-activated geopolymers, where foaming is induced by the hydrogen gas from metallic aluminum in PCB. The fresh rheology, pore structure, and hardened properties are systematically investigated to elucidate the governing mechanisms. Results indicate that paste viscosity dominates gas retention, bubble stability, and pore topology, thereby controlling density, porosity, and transport-related properties. An appropriate viscosity promotes stable foaming with predominantly isolated pores, whereas excessive gas evolution or insufficient viscosity leads to pore interconnection and performance deterioration. The optimal mixture, with a PCB powder content of 10 wt% and an average paste viscosity of 3.5∼4.5 Pa s, achieves a density of 600∼700 kg/m³ and a thermal conductivity of 0.20∼0.25 W/(m·K). Leaching tests confirm compliance with regulatory limits. This study establishes a rheology-controlled foaming mechanism for high-value e-waste utilization.