Alkali-activated Porous Materials from Biomass Bottom Ash for the Production of Insulation Panels

Abstract This study explores the feasibility of employing bottom ash obtained from biomass combustion as the exclusive precursor for the synthesis of alkali-activated foamed materials with potential applications in thermal insulation. The bottom ash, sourced from a biomass-fired power plant in Italy utilizing woody feedstock, was subjected to comprehensive chemical and mineralogical characterization. The analyses revealed a calcium-rich composition, indicating its suitability for activation using sodium silicate and sodium hydroxide solutions. The foaming behavior of the system was investigated using hydrogen peroxide (H₂O₂) and aluminum powder as foaming agents. Among these, H₂O₂ demonstrated superior effectiveness, particularly in the presence of highly alkaline activating solutions (e.g., 12 M and 15 M NaOH). Curing temperature was found to play a pivotal role in determining the microstructure and physical properties of the resulting materials. Samples cured at 60 °C exhibited enhanced porosity and lower thermal conductivity, thus favoring insulation performance. In contrast, those cured at 40 °C showed higher compressive strength, attributed to reduced shrinkage during the curing process. X-ray diffraction (XRD) and infrared (IR) spectroscopy confirmed the formation of hydrated calcium silicate phases, while scanning electron microscopy (SEM) revealed a more homogeneous and compact microstructure in samples activated under high alkalinity conditions. Notably, thermal conductivity values below 0.1 W/m·K were achieved in samples with high open porosity. Several formulations were identified that offer an optimal balance between mechanical strength and thermal insulation, thereby demonstrating the potential of these materials for structural insulation applications. The findings underscore the viability of utilizing biomass bottom ash as a sustainable and innovative raw material for the development of advanced insulation solutions in the construction sector.