Effect of soluble fluorine on the hydration, strength and microstructure of hemihydrate phosphogypsum

Preparing beta-hemihydrate phosphogyspym (HPG) from phosphogypsum (PG) is a promising and effective method to utilize PG massively. However, the influence mechanism behind the harmful impurities on the properties of HPG is unclear. In this paper, the effect of soluble fluorine (F) on the hydration, mechanical strength, and microstructure of HPG plaster was investigated. The results show that soluble F accelerated the CaSO4 & sdot;0.5 H2O dissolution and CaSO4 & sdot;2 H2O precipitation. The higher soluble F content resulted in a more accelerated degree. During the hydration of HPG, CaF2 and CaSO4 & sdot;2 H2O co-precipitated in the liquid phase at the beginning of the dissolution stage, which shortened the setting time of HPG. The proportion of free F was higher in the liquid phase, and F species on CaSO4 & sdot;2 H2O surface contained a higher F- and CaF2 proportion resulting from the Ca2+ active site. The adsorption of free F adversely affected the 2 h and dry strength by weakening the electrostatic attraction among CaSO4 & sdot;2 H2O, thereby increasing the porosity. Moreover, soluble F selectively combined with CaSO4 & sdot;2 H2O on the c{111} facet, changing the morphology of hydration products from needle-like to plate-like as well as a loose structure