Performance and environmental assessment of biomass ash-based lightweight aggregate concrete produced via different cold-bonded routes

Cold-bonded methods offer a low-temperature route for converting biomass ashes into lightweight aggregates, but the performance and environmental trade-offs of different bonding routes remain unclear. This study compared municipal woody biomass ash (MWBA) and palm oil fuel ash (POFA) using cementing and one-part alkali-activation routes. Four types of cold-bonded lightweight aggregates (CBLWAs) were used as the sole coarse aggregate in lightweight aggregate concretes (LWACs), and their mechanical and functional performance, interfacial characteristics, cradle-to-gate environmental impacts and heavy metal leachability were evaluated. The LWACs achieved oven-dried densities of 1682–1878 kg/m 3 and 28 d compressive strengths of 25.0–33.1 MPa. The MWBA-based alkali-activated aggregate concrete exhibited the highest compressive strength and the best elevated-temperature resistance, retaining 40% of its initial strength at 800 °C, whereas the POFA-based alkali-activated aggregate concrete showed the lowest density-normalized thermal conductivity of 0.49 W/m·K per 1000 kg/m 3 . BSE–EDS observations indicated the most continuous interfacial contact in the MWBA-based alkali-activated aggregate concrete and the clearest interfacial separation in the POFA-based cementing aggregate concrete, consistent with their strength and water absorption behavior. Stepwise life-cycle assessment showed that upstream CBLWA production dominated LWAC impacts. Electricity consumption dominated cumulative energy demand, whereas global warming impact was more strongly influenced by binder and solid activator production. The alkali-activation route provided selected mechanical or thermal advantages, but these benefits were accompanied by higher energy demand from solid activator production and oven curing. Leaching tests showed effective heavy metal immobilization, with all mixtures satisfying non-hazardous waste criteria.