Abstract Mixing time is one of the operational parameters that can affect the rheological and mechanical behaviors of cemented backfill. The objective of this study was to investigate the effect of mixing time on the cemented backfill process. The backfill mixture consisted of a solid waste phosphogypsum (PG), a hydraulic binder, and water. The fresh slurry of backfill was sampled at seven different mixing times (ranging from 5 to 240 min), and its physical properties including apparent viscosity, electrical conductivity, bulk density, air content, bleeding rate, and setting time were measured. The slurry samples were then filled into molds to prepare hardened backfill samples. The pore structure, microstructure, and unconfined compressive strength (UCS) of the hardened backfill samples were determined at different curing ages. The results indicate that mixing time significantly affects the complete process of backfill, from the fresh slurry to the hardened structure. The optimum mixing time was found to be 60 min, under which the backfill slurry exhibited the best properties, including the lowest apparent viscosity, which enabled easier transport. Furthermore, the optimal slurry also showed the lowest air content and the highest bleeding rate, both of which resulted in a low porosity and consequently a high mechanical strength of the hardened backfill structure. On the other hand, outside of the optimum mixing time, either insufficient mixing or overmixing may lower both the fluidity of the slurry and the strength of the hardened backfill. This study therefore demonstrates that the mixing time is an important factor in the design of an easy-delivery and high-strength backfill system.