Effect of Phosphoric Acid on the Preparation of α-Hemihydrate Gypsum Using Hydrothermal Method

The effects of phosphoric acid (H₃PO₄) in pressurized aqueous solution on the dehydration of CaSO₄·2H₂O to form α-hemihydrate gypsum (α-HH) phase and the regulation of crystal shape were studied in this paper in order to provide guidance for the low-cost and high-value utilization of phosphogypsum. The results showed that H₃PO₄ can significantly accelerate the formation rate of the α-HH phase and that it did not participate in the formation of the α-HH phase in the form of eutectic phosphorus during crystalline phase transformation. In terms of crystal shape regulation, H₃PO₄ can impact the effect of a citric acid crystal regulator on α-HH crystal shape regulation. The more H₃PO₄ added, the greater the aspect ratio of α-HH. Accordingly, the water consumption and 2 h dry compressive strength of α-HH products were gradually increased and decreased with an increase in H₃PO₄ content, respectively. Despite this, the compressive strength of α-HH can still meet the requirements of the α20 grade in JC/T 2038-2010 "α High Strength Gypsum" in China when the H₃PO₄ content was limited to less than 0.4%.