Environmental Performance of 3D-Formed Recycled Fiber-Reinforced Foamed Concrete: Leaching, Thermal Stability, and Microbiological Assessment

The increasing adoption of additive manufacturing in the construction sector has intensified the demand for lightweight, recyclable cement-based composites with low environmental impact, suitable for automated 3D printing. Foamed concrete reinforced with dispersed fibers and incorporating recycled constituents represents a promising class of multifunctional materials. However, its environmental performance remains insufficiently characterized. This study provides a comprehensive evaluation of thermal stability, leaching behavior, and microbial resistance of 3D-printable fiber-reinforced foamed cement composites produced with recycled components. Thermogravimetric–Fourier transform infrared (TG–FTIR) analysis confirmed a characteristic three-stage thermal decomposition pathway typical of hydrated cementitious systems. All composites exhibited high thermal stability, with residual masses of 88.56–90.17% at 900 °C. The binder type exerted a stronger influence on decomposition behavior than atmospheric exposure or freeze–thaw conditioning. Leaching tests revealed strongly alkaline eluates (pH 11.0–11.4), low total organic carbon (<0.6 wt.%), and only trace concentrations of BTEX (35–41 μg/kg), PAHs, and PCBs. Alkali activation increased the release of chromium (3.6–4.0 mg/kg), arsenic (1.2 mg/kg), copper (4.5 mg/kg), antimony (0.26 mg/kg), and sulfates (2700–4700 mg/kg), accompanied by elevated total dissolved solids (~18,000 mg/kg). Nevertheless, all environmentally relevant constituents remained well below the waste acceptance criteria (WAC), confirming effective immobilization of hazardous species within the hardened matrix. The results provide new insights into the relationships among material composition, porous microstructure, and environmental safety, demonstrating that the developed 3D-printable foamed composites exhibit robust performance, suitability for safe and durable applications, and favorable environmental performance.