Carbonation of lightweight alkali-activated aggregates based on biomass fly ash: Effect on microstructure and leaching behavior

Artificial aggregates offer a sustainable solution to large-scale waste utilization and the increasing demand for limited natural aggregates. This study extends the understanding of the production of artificial lightweight aggregates with a variable rotation speed approach based solely on biomass fly ash (BFA) alkali-activated materials (AAMs). Systematic variation of alkali content and solution density at a constant water-to-solids ratio showed that alkali concentration significantly influences granulation beyond what can be explained by water availability. The interplay between alkali activation and carbonation was investigated using different mix designs and curing conditions, as well as comparing simultaneous curing carbonation with post-cure carbonation. The results were evaluated with regard to the effects on the macro- and microstructural properties as well as on the leaching behavior. Prolonged carbonation initiated after aggregate formation resulted in premature depletion of Ca, limiting the development of C-A-S-H gels and increasing microporosity, leading to a reduction in mechanical properties. In contrast, post-curing carbonation maintained a compressive strength of over 1 MPa while still allowing carbonation benefits, resulting in compressive strengths comparable to lightweight expanded clay aggregates. Carbonation also proved to be an effective leaching mitigation strategy by stabilizing heavy metals through both physical encapsulation and chemical pH regulation. These results underline the importance of carbonation timing in high Ca AAMs and highlight lightweight aggregates as a viable pathway for BFA valorization, CO₂ sequestration and sustainable construction applications. This approach offers an alternative valorization strategy for BFA facing regulatory restrictions for direct use in cement, while contributing to carbon capture and circular economy initiatives.