**4. The influence of fly ash on the strength of backfill mass**

Several pieces of research show that adding fly ash improves the aggregate formation strength of coal gangue-fly ash backfill. In a study, the response surface method, multi-objective multi-verse optimization, and the desirability function approach were used [29]. The target of the study was to improve aggregate formation strength. The ideal combination was produced at mass concentrations of 79.65%, 57.19% for fine gangue, and 15.67% for fly ash as a percentage of the total mass. In addition, findings showed that the newly developed mixing method increases the fly ash's activity to encourage the early synthesis of calcium silicate gel and calcium silicate hydrate gel, strengthening aggregate formation.

Paste backfilling's rising popularity necessitates alternative binder optimization and bulk waste disposal. The effectiveness of fly ash as a partial replacement for regular Portland cement (OPC) for paste backfill application in underground mines is examined by Behera et al. [30]. The impact of fly ash addition on paste backfill's cohesion and uniaxial compressive strength (UCS) is demonstrated. According to the study, when fly ash was used in place of OPC, paste backfill's rate of strength development slowed down. However, with binder dosages of 8 wt% OPC, 7 wt% OPC, 6 wt% OPC, 7 wt% OPC +1 wt% fly ash, and 6 wt% OPC +2 wt% fly ash, the desired 28 days UCS of 1.1 MPa for the backfilling stope of the lead-zinc mine was reached. Therefore, fly ash is suitable for binders and can replace up to 25% of OPC (8 wt%). In the early curing stages, calcium silicate hydrate (C-S-H) did not form in paste backfills based on the OPC-FA binder. Gypsum was only identified in samples with OPC as the only binder, according to the analysis of microstructural evolution in paste backfill. The UCS development is more sensitive to fly ash replacement, according to the results of the multiple linear regression analysis on the interaction effects of OPC, curing time, and fly ash replacement percentage for 8 wt% and 5 wt% binder groups. The results would aid a better comprehension and paste backfill design for underground lead-zinc mines.

Fly ash for underground mining backfilling can effectively utilize solid waste, increase the backfill's strength, and lower cost, resulting in positive social and economic outcomes. Using a scanning electron microscope and the hydration properties of cement and fly ash, the causes of the variance in backfill strength were examined by Chang et al. [31]. When the concentration of the filling slurry was 74%, the cement content was 5%, the mass ratio of waste rock-tailings-fly ash was 6:2:3, and the CaO substance was 6:3, the strength of the backfill was substantially more significant than the existing strength of the mine's backfill. When the filling slurry contains tailings, however, the excessive amount of fly ash is likely to cause many fine particles to obstruct the hydration. The hardening of the slurry also reduces the porosity of the backfill.

The backfill strength steadily increased with the rise in fly ash to cement mass ratio, and both grew about linearly, when the filling aggregate was completely waste rock. The strength increased less from 56 to 90 days during the maintenance phase, yet it was 2.24 times as strong as at 56 days. The maintenance period from 28 to 56 days was when the strength development of backfill containing fly ash was mainly concentrated, and the growing tendency slowed. The backfill strength abruptly

decreased after the fly ash-cement mass ratio exceeded 3 (the fly ash-tailings mass ratio at this time was about 3:2). Fly ash had a significant negative impact on the strength at this time. As tailing particles were fine, the continuous increase in fly ash content resulted in more fine particles in the backfill, hampered the hydration of cementitious materials, and affected the growth of the backfill strength. In general, the increase of fly ash content promotes the strength of backfill in the middle and late stages. Fly ash has a two-way impact on the backfill's late strength when tailings are present. Therefore, determining the proper mixture of fly ash and tailings is crucial.
