Synergistic treatment of gold tailings and low-activity fly ash: Particle modification, mechanical properties and microstructural evolution.

The incorporation of fly ash (FA) into cemented tailings backfill (CTB) reduces clinker consumption; however, the low reactivity of Class-F FA constrains early strength development. This study proposes a co-milling pretreatment in which anatase nano-TiO2 (NT) and FA are mechanically processed to produce an NT-FA composite powder for cemented gold tailings backfill. Mechanical activation of FA was performed to enhance the properties of the backfill material. The findings indicate that the composite powder refines the microstructure of hydration products, significantly augmenting the initial strength of the backfill. Relative to the OPC reference group, NT-modified FA-based cemented tailings backfill (NFCTB) exhibited an increase in unconfined compressive strength (UCS) of 17.36% to 80.99% at 3 days, thereby meeting the early-strength criteria for mine backfill. The UCS values showed maximum enhancements of 59.97% at 28 days and 73.44% at 120 days. The composite powder promoted the uniform crystallization of key hydration phases (C-S-H gels and AFt), thereby reducing the macropore fraction from 64.38% to 50.45% and contributing to a more cohesive structure. Additionally, digital image correlation analysis revealed that the modified specimens developed a more homogeneous strain distribution under load, delayed the initiation of macrocracks, and improved energy dissipation performance. NFCTB emitted 693.67-987.67 kg CO2-eq/t binder, with S13 achieving the lowest value and a 16.43% reduction relative to SPC. These results demonstrate the feasibility of using a co-milled NT-FA composite powder to improve the mechanical and microstructural performance of low reactivity FA-based backfill, thereby providing a scalable approach to developing more environmentally sustainable backfill materials.