Heavy metals incorporated-alkali-activated waste glass binder and foamed glass: phase composition, physicomechanical performance, and reaction mechanism

This study investigates the immobilization of hazardous witherite (BaCO₃) and zaratite (Ni₃(CO₃)(OH)₄·4 H₂O) as heavy-metal carbonates within alkali-activated systems subjected to thermal treatments. The objective was to compare alkali-activated waste-glass (WG) binders cured at 60 °C with foamed glass (FG) produced at 800 °C in terms of heavy-metal stabilization, phase evolution, physicomechanical performance, and thermal insulation. WG was replaced with 10 or 30 wt% carbonate; specimens were cured at 60 °C for 3 days and subsequently heated at 800 °C for 2 h. TG/DTG, XRD, FTIR, physicomechanical, thermal-conductivity, pore-structure, and acid-leaching analyses were performed. Witherite caused only a modest strength reduction, whereas 10 and 30 wt% zaratite reduced the compressive strength of cured binders by approximately 37% and 84%, respectively. Despite immobilization efficiencies exceeding 99%, Ba²⁺ and Ni²⁺ concentrations in the leachates remained above the permissible thresholds. At 800 °C, a 10 wt% dosage promoted foaming, producing approximately six-fold volume expansion and thermal conductivities of 0.09–0.12 W·m⁻¹·K⁻¹, whereas 30 wt% inhibited foaming. Unlike the cured binders, witherite and zaratite in FG transformed into acid-resistant barium and nickel silicates, principally sanbornite (BaSi₂O₅) and liebenbergite (Ni₂SiO₄). Consequently, Ba²⁺ and Ni²⁺ concentrations released from all FG samples were below the regulated limits and 300–500 times lower than those from the corresponding cured binders. This dual function of high-load heavy-metal carbonates as foaming constituents and precursors of stable silicates enables hazardous-metal stabilization and the production of lightweight thermal-insulation materials from waste glass.