Strength behaviour of stacked phosphogypsum incorporating dissolution–recrystallisation equilibrium

Large phosphogypsum (PG) stacks risk dam failure, with an insufficient consensus on the shear strength parameters for stability analysis. To this end, a combination of scanning electron microscopy (SEM) and triaxial tests was undertaken to investigate the underlying mechanism between crystal structure and shear strength of in situ and remoulded PG samples. The shear strength and deformation of PG were significantly affected by dissolution and recrystallisation. Dissolution weakened the cementation between particles, leading to a stabilisation of approximate 11 kPa under different confining pressures in the initial shear stage. The hardening phenomenon was related to the formation of cluster crystals under saturated conditions. An increase from 1.57 to 1.73 in the critical state stress ratio on remoulded samples occurred as the K0 consolidation time increased from 4 to 28 days. The compressive deformation of PG is accompanied by chemical consolidation, which is mainly impacted by the consolidation conditions (saturation) rather than the consolidation time. In the engineering design of the PG stacks, φ’ could be taken to a higher value at saturation and c’ could be higher when the dry density is higher than 1.2.