Early mechanical strength, hydration mechanism and leaching behavior of alkali-activated slag/fly ash paste filling materials

The enhancement of the early strength of the filling body is regarded as a critical technology for shortening the mining & filling cycle and achieving efficient production. It is a high-value utilization pathway to entirely replace Ordinary Portland Cement (OPC) with industrial waste residue. Hence, in this research, ground granulated blast furnace slag (GGBFS) and fly ash (FA) were selected as aluminosilicate precursors. The effects of single-component and binary activation of Na2SiO3, Na2SO4, Na2CO3, NaOH, NaAlO2 and triethanolamine (TEA) on the mechanical strength of backfill were systematically investigated. The microstructural evolution mechanism of hydration products was elucidated using XRD, TG-DTG, SEM and MIP analyses. The leaching characteristics and immobilization mechanism of heavy metals were further revealed. The experimental results indicated that the activator contributed an alkaline environment and active ions to the initial hydrolysis reaction of precursors and the polycondensation reaction of silica tetrahedra and aluminum tetrahedra with Ca2+ drove the hardening and development of the hydration products. Na2SiO3 exhibited the superior activation effect among single-component activators, while the binary activator presented notable "promoting and inhibiting characteristics". Moreover, the binary activator composed of 4% Na2SiO3 and 4% NaAlO2 conducted the highest compressive strength at 3 d and 7 d. The dominant hydration products of the backfill were ettringite, C-S-H gels, C-A-S-H gels and zeolite-like products. The density of microstructures and the crystallinity of hydration products were significantly enhanced ascribed to the synergistic and complementary effects of Na2SiO3 and NaAlO2. Heavy metals were immobilized by the hydration products through physical encapsulation and chemical bonding, and the leaching concentrations of all heavy metals (Pb, Cd, Cr(VI), Zn, Mn, As and Cu, etc.) were lower than the standard limits of Class III in GB/T 14848-2017.