BACKGROUNDThis work aims to investigate the possibility of recycling by-product phosphogypsum (PG) as raw material for preparing -calcium sulfate hemihydrate (-CSH) in chloride-free salt solution under mild conditions. The dehydration of PG in a concentrated Ca(NO3)(2) solution at 97 degrees C under atmospheric pressure was analyzed. The effect of Al-2(SO4)(3) on the phase-transition kinetics as well as succinic acid concentration on the crystal morphology of -CSH were investigated in detail. RESULTS-CSH was successfully prepared from PG in 3.7 mol L-1 Ca(NO3)(2) solution at 97 degrees C under atmospheric pressure. Addition of Al-2(SO4)(3) accelerates dehydration rate of PG and decreases crystal size of -CSH. Moreover, succinic acid is an effective morphology modifier for -CSH. When the reaction is carried out with 5.84 mmol L-1 Al-2(SO4)(3) and 0.15 wt% succinic acid, the -CSH obtained has aspect ratio of 1.2, and the corresponding paste shows enhanced 3D bending/compressive strengths of 11.3/41.7 MPa, which are comparable with those of commercial -CSH. CONCLUSIONSThis chloride-free system provides an improved alternative for preparing high quality -CSH, and helps to promote value-added utilization of PG. (c) 2018 Society of Chemical Industry