Valorization of biomass bottom ash in alkali-activated GGBFS-fly ash: Impact of biomass bottom ash characteristic, silicate modulus and aluminum-anodizing waste

Abstract

Growing production of green energy from biomass has led to a surge in biomass bottom ash (BBA) generation, often relegated to landfills due to high leachable heavy metal content. This study proposed solidifying BBA in alkali-activated granulated ground blast furnace slag (GGBFS) and class F fly ash (CFA) blended binders. Investigations into the influence of BBA content, fineness, silicate modulus of activators, and aluminum-anodizing waste (AAW) on solidification performance were conducted, focusing on reaction mechanisms, phase assemblages, mechanical properties and leaching behavior. Results indicate relatively low reactivity of BBA and high leachable chloride (Cl-), sulphate (SO42-), chromium (Cr), molybdenum (Mo), lead (Pb) and zinc (Zn) contents from BBA. Increased BBA substitution leads to strength deterioration, while ground biomass bottom ash (GBBA) improves the mechanical performance slightly. This improvement is attributed to the increased reactive aluminosilicate content and an enhanced packing system resulting from finer particles. However, GBBA induces an increased heavy metals leaching. Higher silicate modulus activators enhance mechanical properties and reduce the leaching of Mo and Cl-, while lower silicate modulus activator accelerates the reaction process, promoting the formation of hydrotalcite-like phases and reducing the leaching of Cr and SO42-. Incorporating 0.5 wt% aluminum-anodizing waste improves immobilization efficiency of toxic ions, but further increases in dosage lead to higher leaching due to weakened mechanical performance. Overall, hybrid binders with 10 wt% G/BBA exhibit desirable mechanical properties, along with sufficient immobilization of leachable heavy metals, enabling their use as potential construction materials.