From slurry to hardened matrix: Quantifying stage-resolved fluoride solidification/stabilization by biochar in phosphogypsum cemented backfill

Abstract

Soluble fluoride in phosphogypsum (PG) threatens the safety of mine backfill systems by retarding cement hydration and releasing contaminants. However, its solidification/stabilization (S/S) mechanisms in PG-based cemented paste backfill (PCPB) remain insufficiently understood. This study screened three biochars and identified coconut shell biochar (CSB) as the most effective fluoride adsorbent, owing to its greater abundance of oxygen-containing functional groups as revealed by XPS and DFT analyses. Upon incorporation into PCPB, CSB suppressed fluoride release through a three-stage pathway: surface adsorption in fresh slurry (Stage I), Ca-assisted precipitation during hydration (Stage II), and physical encapsulation within the hardened matrix (Stage III). Stages I and II together accounted for more than 90% of the total S/S efficiency. Stage II dominated at low CSB dosages, whereas Stage I became more pronounced at higher dosages. In contrast, the Stage III contribution remained relatively stable at 7.03% to 8.85% and showed no significant dependence on CSB dosage. Regarding mechanical performance, 4% CSB produced the most favorable hydration environment. This dosage yielded the highest Ca(OH)₂ content, the lowest porosity, and a 21.46% increase in 28-day compressive strength relative to the control. By contrast, excess CSB suppresses hydration through dilution and competitive water absorption, leading to increased porosity and weakened matrix densification. These findings provide mechanistic insight and practical design parameters for sustainable, resource-efficient PCPB systems in PG-based mine backfill operations.

Keywords

cemented paste backfill / coconut shell biochar / fluoride solidification/stabilization / hydration / phosphogypsum

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