Mechanism of air-entraining agent to improve the properties of coal-based solid waste backfill

Fly ash exhibits volcanic ash activity and can be utilized as a cementitious material to reduce filling costs. The limited flow characteristics of mine solid waste filling slurry can impact backfill efficiency. Instead of using a large amount of cement, we employ fly ash as a cementing material to prepare filling slurries with varying air-entraining agent dosages. The results indicate that as the air-entraining agent dosage increases from 0 to 8%, the fluidity of the filling slurry improves, and the strength increases within the first 5 days of curing. The strength of the filling increases initially from 5 to 28 days of curing with the increase in air-entraining agent dosage, reaching a peak at 4% dosage, with an 11.35% higher strength compared to the filling body without the air-entraining agent. The microscopic pore structure and hydration product analysis reveal that introducing an appropriate amount of bubbles results in an even distribution of cementitious material and aggregates, improving fluidity and strength. However, excessive addition of air-entraining agents inhibits the growth of C–S–H and the filling ability of AFt for primary pores, ultimately impacting the long-term strength growth of the filling body.