Determination of utilization strategies for hemihydrate phosphogypsum in cemented paste backfill: Used as cementitious material or aggregate

Due to the prominent advantages of low cost and efficient utilization, cemented hemihydrate phosphogypsum (HPG) backfill is becoming popular in mine goaf treatments in China as a new promising branch of cemented paste backfill (CPB). The HPG gelling activity time dependence determines its role in CPB, that is, whether it is used as a cementitious material or aggregate. Laboratory and field experiments showed that the HPG gelling activity decreases with an increasing aging time due to the gradual HPG conversion from the hemihydrate to the dihydrate form. The HPG conversion can be described by the Avrami equation, and further divided into acceleratory and slow reaction periods. Soluble P₂O₅ formed insoluble brushite coatings, significantly inhibiting the HPG conversion. Increasing the Al₂O₃ content and reducing temperature further retarded the HPG conversion. Reducing the temperature inside the stacks by lowering their stacking height delayed the HPG gelling activity decay. At a stacking height of 1.5 m, HPG can be prepared into cementitious materials for common CPB methods within the first 137 h of aging, thereafter it can only be used as an inactive aggregate. Finally, an application case is presented to illustrate the effectiveness of the utilization strategies in guiding the use of HPG in CPB.