Effect of alkali-activation pre-treatment of sintered aggregates from biomass fly ash on microstructural and mineralogical evolution

The increasing demand for lightweight construction materials and the depletion of natural aggregates highlight the need for circular solutions based on industrial residues. Co-incineration biomass ash (BA), despite its high availability, carbon content, and variable composition, remains underutilised in high-value applications. This study explores a previously unexamined valorisation route through the production of sintered alkali-activated aggregates using sodium-silicate-assisted pre-treatment. Two BA mixes with different Na 2 O dosages (7.57 and 5.44 wt% Na 2 O) were pelletized and thermally treated between 700– 1200 °C. The alkali activation pre-treatment simultaneously improved the granulation efficiency, enabled the formation of alkali-activated gel, and supplied Na 2 O as a flux, significantly influencing the crystalization, melting, and sintering behavior. Comprehensive characterisation using mercury intrusion porosimetry, dilatometry, X-ray diffraction, Fourier-transform infrared spectroscopy, thermogravimetry–differential thermal analysis, and scanning electron microscopy revealed a coherent thermal sequence: from gel deterioration and a macroporosity development below 800 °C, to the crystallisation of Ca–Mg silicates and the formation of an akermanite-dominated matrix at 800–1000 °C, followed by partial melting and sintering in the presence of a liquid phase above 1000 °C. A higher alkali content promoted earlier densification and strength development. Aggregates with higher Na 2 O content (BA1) exhibited an earlier onset and higher intensity of sintering shrinkage, reaching a compressive strength of 4.53 MPa at 1100 °C, corresponding to more than a fourfold increase compared to thermally untreated aggregates, whereas the lower-alkali mix (BA2) remained below 0.26 MPa at the same temperature. Open porosity of BA1 aggregtes increased to 78.8% after heating to 800 °C due to deterioration of the alkali-activated gel, followed by densification accompanied by akermanite-dominated crystallization and pore coalescence, resulting in 73.1% porosity and a bulk density of 1.28 g/cm 3 at 1100 °C. The results identify BA as a promising precursor for lightweight or dense SAA and demonstrate alkali-activation-assisted thermal treatment to be a technically applicable circular-economy pathway for converting co-incineration BA into value-added construction materials.