Effect of combined flotation and ball milling modification on biomass fly ash for low-carbon cementitious materials

Biomass fly ash (BFA), a byproduct of bioenergy generation, offers sustainability benefits but shows low reactivity and limited applicability in cementitious systems due to its irregular morphology and residual carbon. To overcome these limitations, this study proposes a novel integrated flotation–ball milling pretreatment to simultaneously remove unburned carbon and activate BFA, thereby enhancing its physicochemical reactivity as a supplementary cementitious material (SCM). Its effectiveness is validated through comprehensive microstructural and mechanical analyses. Experimental data confirmed that flotation effectively removed unburned carbon, while moderate ball milling (30 min) refined particle structure, increased amorphous content, and improved flowability by approximately 10 % compared with untreated BFA. Mortars incorporating modified BFA-A achieved a 28 % higher 28-days compressive strength than those containing untreated BFA. In contrast, excessive milling (90 min) induced particle agglomeration and diminished performance gains. Furthermore, MIP revealed reduced total porosity and refined pore structures in treated samples. At 91 days, the modified A and B groups exhibited a higher proportion of fine pores (0.01–0.1 μm) and fewer macropores than the untreated counterparts, consistent with lower CH contents and higher amorphous phase fractions from XRD and corroborated by denser C-S-H networks observed in SEM, indicating enhanced pozzolanic reactivity. From an environmental perspective, the use of modified BFA enabled an estimated 30 % reduction in CO₂ emissions relative to pure cement. Overall, the flotation–milling pretreatment effectively improves the reactivity of biomass fly ash as a low-carbon SCM, promoting waste utilization and carbon reduction in sustainable construction.