Insight into the microstructural evolution and fluorine migration mechanism of lightweight ceramsite derived from fluorite tailings

Fluorite tailings (FT), a byproduct of fluorite ore processing, pose significant environmental risks due to improper disposal. This study presents a sustainable approach to repurpose FT into lightweight ceramsite through pelletizing and sintering. Systematic investigations of microstructural evolution revealed that the effects of raw material ratios (FT:clay=6:4–9:1), sintering parameters (1150–1250 °C, 10–50   min), and SiC addition (0–1%) were governed by feldspar formation and pyroplastic mass development. Thermodynamic calculations and differential thermal analysis further elucidated phase transformation pathways. Optimal conditions (70% FT, 30% clay, 1210 °C, 30   min, 0.5% SiC) yielded ceramsite with a compressive strength of 4.63   MPa, apparent density of 0.890   g/cm 3 , and 1-hour water absorption of 0.21%. Microstructural characterization (SEM/ICT) confirmed a dense shell encapsulating uniform spherical pores (0.4–0.6 sphericity, 0–2   mm diameter), explaining the exceptional water resistance. Crucially, fluorine migration mechanisms were identified: Fluorine speciation evolved from cubic CaF 2 to hexagonal CaF 2 /Mg 3 SiO 4 F 2 during sintering, with subsequent encapsulation in the feldspar-pyroplastic matrix reducing fluoride leaching to 0.87   mg/L (7.05   mg/L in raw FT). This work demonstrates a viable strategy for transforming FT into high-performance construction materials while immobilizing fluorine, addressing dual waste management and contamination challenges.