Effect of alkali-activation pre-treatment of sintered aggregates from biomass fly ash on microstructural and mineralogical evolutionTesovnik, Anže (Avtor) Ducman, Vilma (Korespondenčni avtor) alkali-activated materialsbiomass ashsintered aggregateslightweight aggregatesThe 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 Na2O dosages (7.57 and 5.44 wt% Na2O) were pelletized and thermally treated between 700 and 1200 ◦C. The alkali activation pretreatment simultaneously improved the granulation efficiency, enabled the formation of alkali-activated gel, and supplied Na2O as a flux, significantly influencing the crystalization, melting, and sintering behavior. Comprehensive characterisation using mercury intrusion porosimetry, dilatometry, X-ray diffraction, Fouriertransform 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 crystallization 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 Na2O 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 aggregates 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/cm3 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.Elsevier20262026-05-06 11:24:43Neznano29286UDK: 54ISSN pri članku: 1873-3956DOI: 10.1016/j.ceramint.2026.03.041COBISS_ID: 271010307sl© 2026 The Authors