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.

Polybenzimidazole (PBI), a high-performance polymer known for its exceptional thermal stability and chemical resistance, was processed by solution electrospinning to manufacture fibrous non-woven membranes. The process was repeated under different conditions by adjusting four main settings: the polymer solution concentration, the flow rate, the voltage applied between the needle and the collector, and the separating distance. To clarify the interplay between process parameters and material properties, a Design of Experiment (DOE) approach was used to systematically analyze the effects of said parameters on microstructural properties, including fiber diameter, porosity, and air permeability, pointing out that the increase in viscosity improves fiber uniformity, while optimizing the applied voltage and the needle–collector distance enhances jet stability and solvent evaporation, crucial for defect-free fibrous microstructures. Post-processing via calendering further refined the membrane texture and properties, for example by reducing porosity and air permeability without significantly altering the fibrous morphology, particularly at low lamination ratios. Thermal and mechanical evaluations highlighted that the obtained electrospun PBI membranes exhibited enhanced flexibility, but lower tensile strength compared to cast films due to the underlying open pore microstructure. This integrated approach—combining experimental characterization, DOE-guided optimization, and post-processing via calendering—provides a systematic framework for tailoring PBI membranes for specific applications, such as filtration, fuel cells, and molecular sieving. The findings highlight the potential of PBI-based electrospun membranes as versatile materials, offering high thermal stability, chemical resistance, and tunable properties, thereby establishing a foundation for further innovation in advanced polymeric membrane design and applications for energy and sustainability.

In heterogeneous fibrous networks, the void fraction measured by displacement porosimetry does not coincide with the fraction accessible to through-plane flow, and classical capillary-based permeability models, which take a single nominal porosity as input, cannot resolve this discrepancy. In the present work, a defect-aware structural framework is developed and applied to a set of 17 electrospun polybenzimidazole (PBI) membranes produced in triplicate across a broad processing window, covering fiber diameters from 83 to 174 nm, total porosities from 68 to 85%, and defect fractions from 0 to 52.2% as quantified by Scanning Electron Microscopy (SEM) image analysis. A defect-corrected effective porosity is introduced as a transport-oriented structural descriptor, defined as the fraction of nominal pore volume remaining accessible to through-plane flow after accounting for the obstruction imposed by morphological irregularities. Starting from this quantity, tortuosity is estimated through three complementary analytical models, Tomadakis, Bruggeman, and Koponen, and the corresponding Kozeny–Carman structural resistance constants are derived. All three models consistently show increasing tortuosity and amplified flow resistance as defect content increases, with model divergence becoming most pronounced below an effective porosity of approximately 65%, identifying a structural threshold beyond which heterogeneous fibrous networks deviate significantly from idealized pore-path descriptions. Experimental Darcy permeability, derived from Gurley air-flow measurements, does not exhibit a statistically significant bivariate correlation with the defect fraction alone (r_s = −0.38, p = 0.13), consistent with the multi-factorial nature of macroscopic transport in morphologically complex fibrous media. Kozeny–Carman-derived permeability, computed from the corrected effective porosity, still diverges from the Darcy reference by up to 2.7 orders of magnitude, but the deviation is not random: it varies monotonically with effective porosity and crosses unity at 0.81. This discrepancy is therefore interpreted not as a framework limitation but as a quantitative diagnostic of the structural conditions under which classical capillary-based permeability models become unreliable for heterogeneous electrospun mats. The framework is thus diagnostic rather than predictive: from routine SEM micrographs alone, it delimits the structural regime in which capillary-bundle descriptions lose validity, enabling resource-efficient screening of fibrous membranes for clean-technology applications.