Experimental study on the effect of fly ash with ammonium salt content on the properties of cemented paste backfill

The wide disposal of fly ash (FA) that contains ammonium is an essential method to realize green backfill mining and resource utilization of solid waste. In this paper, FA containing ammonium with different ammonia contents was simulated by adding NH4HSO4 to the FA. The law and mechanism of influence of ammonia content on the performance of FA-based cemented paste backfill (FA-based CPB) were explored using thorough experimenta-tion, and a maximum ammonia content of FA containing ammonium used in backfill mining was proposed. The results showed that the fresh FA-based CPB slurry could be described by the Herschel-Bulkley model (R2 >= 0.991). With the increase in the ammonia content of FA, the yield stress and viscosity of the FA-based CPB slurry increased and the slump value decreased, due to the formation of specific firm bubble bridges between the cement particles, FA, and tiny bubbles and due to the fact that tiny bubbles need more water due to their large specific surface area. In addition, the uniaxial compressive strength (UCS) and elastic modulus of FA-based CPB decreased and the peak strain increased as the ammonia content of FA increased. Specifically, the 28-d UCS of FA-based CPB with ammonia contents of 239, 639, 1139, and 2139 ppm decreased by 12.4 %, 39.5 %, 42.4 %, and 47.0 %, respectively, compared to that of FA-based CPB with ammonia content of 139 ppm. Finally, MIP, SEM, and XRD were utilized to investigate the microstructure of FA-based CPB. Overall, the ammonium-containing FA adversely affected both the flowability and mechanical properties of FA-based CPB. The results provide a reference for the green and efficient use of FA containing ammonium salts.