Relationship between the evolution of organic impurities and properties of β-hemihydrate phosphogypsum

Abstract

Transforming phosphogypsum (PG) into β-hemihydrate gypsum plasters is one of the most effective and high-value-added pathways to consume and utilize PG. However, the relationship between the evolution of organic impurities and the properties of gypsum plaster is still unclear. In this paper, the composition and evolution of organic impurities, including pure organic matter (PO) by flotation and polyacrylamide (PA), were characterized, and their influence on the dehydrate behavior of PG and water requirement for normal consistency, setting time, mechanical strength, microstructure, and appearance of gypsum plasters at the calcination temperature of 150 °C and 200 °C were explored. The results showed that at 150 °C, PA and PO could increase the water requirement for normal and setting time of gypsum plaster and reduce the mechanical strength due to the reduced wettability and bonding strength causing by their intact structure and covering on HPG surface. At 200 °C, decomposed products of PO containing P-O-P and OH- groups could interact with Ca2+ and inhibit the dissolution of hemihydrate gypsum, resulting in coarsened crystals, thereby reducing the compressive strength. PA could partially decompose into the micro-molecular polymer and cover on the hydration products, leading to an increased densification compared to 150 °C. Besides, organic impurities could cause blackening, spots, and mildew on the gypsum plaster surface.