Cemented backfill using Ca(OH)2-pretreated phosphogypsum as aggregate: Hydration characteristics, structural features and strength development

•Ca(OH)2-pretreated PG was used as aggregate for cemented PG backfill.•Hydration characteristics and structural features were studied.•Ca(OH)2 pretreatment promoted the early-age hydration and strength development.•Inappropriate Ca(OH)2 content was harmful to late-age strength development. Impurities present in phosphogypsum (PG) limited its use as an aggregate in underground mining construction materials. This study pretreated PG with varying Ca(OH)2 contents, and investigated the effect of pretreated PG on the strength development of cemented PG backfill from the perspective of hydration characteristics and structural features. The results show that although a higher Ca(OH)2 content promoted the early-age hydration and therefore increasing unconfined compressive strength (UCS) after 7 days of curing, inappropriate Ca(OH)2 content was found adversely effected the UCS with the extension of curing age. After 28 days of curing, UCS exhibited a decrease over the 2.2–3.2 wt% Ca(OH)2 content range and an increase over the 3.2–6.0 wt% Ca(OH)2 content range. Interestingly, with the extension of curing age to 28 days, little differences were observed in hydration degree between backfill samples, which was incompatible with UCS results. The differences in UCS gain occurred was primarily due to pretreatment-induced structural features changes rather than hydration characteristics influences. Among the various Ca(OH)2 contents, 3.2 wt% Ca(OH)2 resulted in the poorest structural features and therefore lowest UCS (0.30 MPa) after 28 days of curing. These findings provided valuable insights for the optimization of PG pretreatment in cemented PG backfill.