BACKGROUNDThis work aims to investigate the possibility of recycling by‐product phosphogypsum (PG) as raw material for preparing α‐calcium sulfate hemihydrate (α‐CSH) in chloride‐free salt solution under mild conditions. The dehydration of PG in a concentrated Ca(NO3)2 solution at 97 °C under atmospheric pressure was analyzed. The effect of Al2(SO4)3 on the phase‐transition kinetics as well as succinic acid concentration on the crystal morphology of α‐CSH were investigated in detail.RESULTSα‐CSH was successfully prepared from PG in 3.7 mol L‐1 Ca(NO3)2 solution at 97 °C under atmospheric pressure. Addition of Al2(SO4)3 accelerates dehydration rate of PG and decreases crystal size of α‐CSH. Moreover, succinic acid is an effective morphology modifier for α‐CSH. When the reaction is carried out with 5.84 mmol L‐1 Al2(SO4)3 and 0.15 wt% succinic acid, the α‐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 α‐CSH.CONCLUSIONSThis chloride‐free system provides an improved alternative for preparing high quality α‐CSH, and helps to promote value‐added utilization of PG. © 2018 Society of Chemical Industry

Phosphogypsum (PG), a by-product of phosphoric acid industry, causes environmental issues. In this paper, PG was effectively transformed into alpha-calcium sulfate hemihydrate (ɑ-CSH) via a semi-liquid method. The phase evolution and the effects of different organic acids and pH values on crystal morphology of the α-CSH were investigated by SEM, XRD and EDS. The properties of the prepared α-CSH with different aspect ratio were also compared. The results indicate that PG can be successfully transformed into α-CSH by the semi-liquid method at 140°C for 3.0 h. The morphology of the produced α-CSH is significantly influenced by the concentrations of organic acid and pH value. α-CSH crystal with well-morphology is obtained under the condition of 0.1 wt.% maleic acid and pH = 7.0. The obtained α-CSH with uniform particle size has the aspect ratio of 1.2. The 2 h and 3 d bending/compressive strengths of the hardened paste prepared from the α-CSH reach 6.4/20.6 MPa and 10.5/43.7 MPa respectively, which are comparable to those of the commercial α-CSH. Moreover, the lower aspect ratio of ɑ-CSH results in more compact microstructure and higher mechanical strengths of the hardened paste.

Impurity is an important factor that determines the crystallization process of α-calcium sulfate hemihydrate (α-CSH), but the effects of phosphorus impurities in waste phosphogypsum on the α-CSH prepared using the salt solution method are still not well known.