Wood and paper biomass fly ash physicochemical properties: focus on the amorphous fraction

To achieve zero net carbon dioxide (CO₂) emissions by 2050, the search for new alternative materials to replace Portland clinker is urgent. Biomass fly ash (BFA) is one of the possible materials. Given the extremely high variability in the mineralogical, chemical and microstructural properties of these ashes, a large number of samples must be explored. This study presents a physicochemical characterisation of wood and paper BFA, offering guidelines for sample preparation and data analysis. Four BFAs were characterised using particle size analysis, specific gravity, Brunauer–Emmett–Teller (BET)-surface area analysis, total carbon analysis, scanning electron microscopy, X-ray fluorescence, thermogravimetric analysis and X-ray diffraction (XRD). The studied biomass ashes were found to contain more than 15 phases, with 30 wt.% of potentially reactive phases, including both crystalline and amorphous components. The chemical composition of the amorphous phase is estimated to lie between that of calcareous fly ash and blast-furnace slag. The correlation between the XRD amorphous hump position and the calcium oxide/(silicon dioxide + aluminium oxide) (CaO/(SiO₂+Al₂O₃)) ratio of the amorphous phase is further refined within the calcareous fly ash domain.