Leaching and hydrating mechanisms, economic benefits of backfill materials by using coal fly ash-slag-based binder for environmentally sustainable production

Herein, the blast furnace slag (BFS), steel slag (SS), desulphurisation gypsum (DFG) and mechanically modified fly ash (MFA) were used as a coal fly ash-slag-based binder (FSD) for the solidification/stabilisation (S/S) of non-ferrous metal tailings and to prepare a coal fly ash-slag-based backfill materials (FST). The results revealed that the optimal ratio for FST was G9I-2, which consisted of 20% MFA, with an appropriate fluidity (240 mm) and setting time (13 h) that satisfied the backfill pipeline transportation requirements, and compressive strengths with 7 and 28 d curing of 2.43 and 8.47 MPa, respectively. The diffusion coefficient (Do) of FST ranged from 2.13 x 10(-11) to 2.50 x 10(-10) cm(2)/s, with a leachability index (LI) of 10.08-10.54. These values indicated that the FSD effectively encapsulated Pb and Zn by S/S, with G9I-2 exhibiting the lowest risk of leaching and the leaching mechanism of Pb and Zn was dissolution. The solidification of lead and zinc primarily occurred through the formation of hydration products such as C-(A)-S-H gel, possibly forming a complex salt phase. The addition of an appropriate amount of MFA (20 wt%) increased the content of functional hydration products (15%, ettringite; 14%, complex salt mineral phase) and the degree of polymerization. An increase in the Al/Si ratio (0.11087 +/- 0.00948 similar to 0.38838 +/- 0.01935) in the gel region of G9I-2 indicated the continuous transformation from C-S-H to C-A-S-H phase. Life cycle assessment (LCA) and economic benefit analysis (EBA) demonstrated that FST materials significantly reduced CO2 emissions (0.45 kg/t) and generated significant economic benefits (2.76 USD/t) for mine backfill applications. These findings highlight the significance of MFA in achieving sustainable and high-performance construction materials, with potential applications in various construction projects.