In-situ remediation of phosphogypsum in a cement-free pathway: Utilization of ground granulated blast furnace slag and NaOH pretreatment

In-situ remediating phosphogypsum (PG) for cemented paste backfill (CPB) in the contaminated site is economic management for promoting sustainable developments in the phosphate industry. This study concerns the combined use of NaOH pretreatment and ground-granulated blast furnace slag (GGBFS) additives to promote the solidification/stabilization of PG with a lower carbon footprint pathway. According to physico-chemical analyses, the NaOH pretreatment effectively removed approximately 95% of F within the PG, which may originally be present as sparingly soluble fluorides or coexisting with silicates. The micro mineralogical characterization illustrates that the pretreatment can accelerate the early age hydration, with more hydration products observed, including calcium silicate hydrates and ettringite, effective F and P retention candidates. Whereas the incorporation of GGBFS plays an essential role in promoting the generation of additional cement hydrates at the following stages. The macro mechanical performance analysis indicates that the mixtures of pretreated-PG-OPCGGBFS exhibit an excellent mechanical performance satisfying the design criteria. Subsequent elemental mapping and toxicity characteristic leaching procedures demonstrate that this combined approach has a competitive F and P immobilization ability compared to the typical OPC binder and individual GGBFS addition. The newly formed phases effectively controlled the concentration of F and P through adsorption, incorporation, or encapsulation. Objectively, the proposed methodology can be a promising candidate pathway for extrapolating the in-situ immobilization of PG. This study opens up new perspectives for synergetically recycling PG and GGBFS in a profitable and low carbon footprint way.

Automated summary

In this study, a combination of NaOH pretreatment and ground-granulated blast furnace slag (GGBFS) additives was used to solidify phosphogypsum (PG) for cemented paste backfill (CPB) with a lower carbon footprint. The NaOH pretreatment effectively removed approximately 95% of F within the PG, resulting in more hydration products and effective F and P retention candidates. The addition of GGBFS promoted the generation of additional cement hydrates. The macro mechanical performance analysis showed that the mixtures of pretreated-PG-GGBFS exhibited excellent mechanical performance. Elemental mapping and toxicity characteristic leaching procedures demonstrated competitive F and P immobilization ability compared to typical OPC binder and individual GGBFS addition. This study opens up new perspectives for synergistic recycling of PG and GGBFS in a profitable and low carbon footprint way.