Fly Ash and Bottom Ash from Biomass and Coal: Comparison of Properties in Grate and Fluidized Bed Combustion from the Perspective of Construction Applications

The cement industry seeks alternative raw materials to lower its environmental impact, and biomass ash represents a potential material for construction applications. This study evaluates biomass ashes (BMA) produced from grate and fluidized bed combustion, as well as co-combustion with coal, focusing on their chemical, mineralogical, and physical characteristics. The results reveal a substantial variability in BMA composition, influenced primarily by the fuel type and combustion method. This heterogeneity critically affects the reactivity and overall suitability of the BMA for use in construction materials. It was found that none of the 23 analyzed samples met the requirements of EN 450-1. This outcome is largely attributable to the combustion process and to sampling from the bottom part of the boiler, which typically yields material with properties outside the limits of the standard. Even when assessed directly against the specific limit values of EN 450-1, the ashes did not comply without further processing or modification. Despite these limitations, BMA show potential for use in accordance with EN 197-1, which permits the incorporation of up to 5 wt.% minor additional constituents. However, their practical application under this framework requires validation through tests performed on hydrated cement. These findings underline both the limitations and the promise of BMA as a supplementary cementitious material (SCMs) in sustainable construction.