INFLUENCE OF DIFFERENT ADDITIVES ON THE MECHANICAL PERFORMANCE OF α-HEMIHYDRATE GYPSUM FROM PHOSPHOGYPSUM

alpha-hemihydrate gypsum is synthesized from phosphogypsum and the influence of a CaO treatment, the solution pH and a maleic acid addition on the microstructure have been systematically investigated. The influence of the resolvable phosphorous on the microstructure of hemihydrates gypsum can be reduced by CaO treatment. The mid-diameter of the columnar crystals decreases and the crystal surface becomes smooth with the solution pH decrease. Moreover, the addition of maleic acid changed the crystal growth direction and equiaxed alpha-hemihydrates gypsum crystals were obtained in a pH=2 solution. Furthermore, the effect of different additives, such as Portland cement and circulating fluidized bed slag, on the morphology and mechanical properties of hemihydrates gypsum was also studied. It was observed that the compressive strength decreased after the addition of Portland cement and circulating fluidized bed slag.