Optimizing phosphogypsum utilization: The role of steel slag pretreatment in a sustainable ternary backfill material

The utilization of phosphogypsum (PG) as an aggregate in mine backfilling offers a promising approach for sustainable solid waste management. However, the inherent impurities in PG, including fluorides, phosphates, and residual acids, adversely affect hydration and long-term stability, necessitating pretreatment. This study investigated a ternary backfill system comprising PG, steel slag (SS), and ground granulated blast furnace slag (GGBFS), with emphasis on how SS pretreatment enhances PG's potential as an aggregate. First, PG was pretreated with SS, utilizing its alkalinity to neutralize residual acids (pH increased to 8.5) and immobilize impurities via chemical precipitation (90.9 % F - removal as CaF 2 , 99.8 % PO 4 3- -P removal as Ca 5 (PO 4 ) 3 OH). Subsequently, the ternary backfill was prepared, and its properties, including bleeding rate, porosity, chemical composition, and microstructure, were analyzed. Compared to direct mixing of the three raw materials, PG+SS pretreatment reduced the bleeding rate of the backfill slurry by 67.7 %, leading to denser particle packing and accelerated hydration kinetics. These microstructural enhancements corresponded to a 28-day unconfined compressive strength (UCS) increase of 10–130 %, varying with the specific mix design. Notably, the mixing ratio of the three raw materials emerged as a crucial factor influencing the composition of hydration products and consequently the backfill strength. The optimized 80–12.5–7.5 formulation (PG-SS-GGBFS) achieved 1.5 MPa strength through effective synergistic interaction between ettringite networks and C-(A)-S-H gel matrices. Leaching tests on 28-day samples confirmed that contaminant levels remained below China’s GB 8978–1996 standard, ensuring environmental safety. This study provides a novel framework for PG-based backfill systems, advancing industrial byproduct utilization while addressing environmental challenges. This study presents a novel PG-based backfill system suitable for mine backfilling and underground construction requiring environmentally friendly materials with appropriate strength. • 100 % industrial solid waste utilization in PG-SS-GGBFS ternary backfill system. • SS pretreatment neutralizes acidity and immobilizes impurities of PG. • PG+SS pretreatment reduces bleeding rate by 67.7 % and enhances structural density. • 42 % UCS improvement via optimized ettringite and C-(A)-S-H networks. • Leaching tests confirm compliance with GB 8978–1996 Class I standards.