Reuse of phosphogypsum as hemihydrate gypsum: The negative effect and content control of H3PO4

Phosphogypsum can replace natural gypsum to produce hemihydrate gypsum. In order to effectively reutilize phosphogypsum, the influence mechanism of the impurities in phosphogypsum on the properties of plaster should be firstly revealed. This study systematically investigated the effect of H3PO4 on the hydration and properties of plaster. The results indicates that H3PO4 accelerated hemihydrate gypsum dissolution and dihydrate gypsum precipitation, which included route I, involving Ca2+ and SO42-, and route II, involving [CaSO4]0. However, when the content of H3PO4 was ≥0.5%, the effects decreased as H3PO4 increased. During hemihydrate gypsum hydration, the proportion of free P was higher, which mainly existed as H3PO4 and H2PO4-, but P species on dihydrate gypsum surface contained a higher HPO42- proportion, which was caused by Ca2+ weakening the binding of P-O-H to H+. The adsorption and deprotonation of free P promoted HPO42- to substitute SO42- in dihydrate gypsum and affected setting time and 2-h strength by enriching free P and HPO42- on dihydrate gypsum surface. H3PO4•1/2H2O on dihydrate gypsum surface largely degraded dry strength and porosity by transforming short-range electrostatic attraction among dihydrate gypsum crystals into hydrogen bonds. The content of H3PO4 should be controlled to less than 0.1% to prevent excessive loss of dry strength.