Effect of Soluble P2O5 Form on the Hydration and Hardening of Hemihydrate Phosphogypsum

Abstract

The preparation of building gypsum from phosphogypsum is one of the ways to realize its resource utilization. However, the precondition is first to reveal the influence mechanism of the impurities in phosphogypsum on the properties of plaster. Thus, the influence of the form of soluble P2O5 (H3PO4, ${\text{H}}_2{\text{PO}}_4^{-}$ , and ${\text{HPO}}_4^{2-}$ ) that is considered the most significant impurity on the hydration and hardening of hemihydrate phosphogypsum was studied in this paper, which included the setting time, mechanical properties, hydration rate, crystal structure, and micromorphology of hydration products. The results showed that both H3PO4 and ${\text{H}}_2{\text{PO}}_4^{-}$ can accelerate the early hydration of hemihydrate phosphogypsum, shorten the setting time of paste, and improve the mechanical properties of hardened paste. In addition, H3PO4 had a more significant effect than ${\text{H}}_2{\text{PO}}_4^{-}$ . Different from H3PO4 and ${\text{H}}_2{\text{PO}}_4^{-}$ , ${\text{HPO}}_4^{2-}$ significantly delayed the early hydration reaction of hemihydrate phosphogypsum, significantly prolonged the setting time of the paste, and significantly reduced the mechanical properties of the hardened body. The test data of XRD, IR, and SEM showed that two negative effects can be caused when soluble P2O5 existed in the form of ${\text{HPO}}_4^{2-}$ . On the one hand, ${\text{HPO}}_4^{2-}$ can lead to the formation of Ca3(PO4)2 insoluble substance in the hydration reaction of hemihydrate phosphogypsum, which significantly hindered the hydration reaction and prolonged the setting time of the paste. On the other hand, ${\text{HPO}}_4^{2-}$ can change the crystallization habit of the hydration product of CaSO4·2H2O and make it change from needle-columnar to semiflake and irregular shape, resulting in a significant reduction in the mechanical properties of hardened paste.