Abstract: This study aims to clarify the influence mechanism of soluble phosphorus impurities on the preparation,hydration,
and properties of α-hemihydrate gypsum (α-HH),a key product for the high-value utilization of phosphogypsum. An integrated
process simulation strategy was adopted,where KH2PO4 was added to natural gypsum at different contents (0,0. 1%,
0. 3%,0. 6%) to simulate the soluble phosphorus impurities. Their effects throughout the autoclave preparation and subsequent
hydration processes were systematically investigated. The results indicated that soluble phosphorus impurities significantly altered
the crystal habit of α-HH,transforming its morphology from regular hexagonal prisms to short-prismatic,irregular rodlike,
and granular particles,accompanied by crystal refinement. Meanwhile,the impurities markedly accelerated the hydration
process,with the most pronounced accelerating effect observed at the content of 0. 3%,showing the shortest initial and final setting
time (3 min and 5 min,respectively) and the earliest hydration exothermic peak. However,this acceleration led to the deterioration
of the dihydrate gypsum crystal morphology in the hardened paste,evolving from dense,thick rods to loose,porous
fine rods,flakes,and irregular blocks,with weakened interlocking between crystals. Consequently,the macroscopic mechanical
properties were compromised:the absolute-dry compressive strength decreased continuously,and the 2 h flexural strength decreased
significantly after a slight increase at low impurity content. In summary,to achieve high-performance α-HH (meeting
strength grade α40 or above),the soluble phosphorus impurity content in phosphogypsum must be strictly controlled below
0. 1%. This research provides a crucial theoretical basis for the high-value utilization of phosphogypsum.

Key words: phosphogypsum,soluble phosphorus impurity,α-hemihydrate gypsum,hydration process,microstructure,mechanical
property