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.
Tesovnik et al. (Sun,) studied this question.