Role of NaF on the performances of β-hemihydrate gypsum plaster

Abstract

Phosphogypsum can be used to prepare β-hemihydrate gypsum (β-CaSO4·1/2H2O) plaster. A prerequisite for any effective reuse of phosphogypsum is to understanded the act by which impurities in phosphogypsum influence the performance of β-CaSO4·1/2H2O plaster, and the role NaF was explored in this study. The results indicate that during β-CaSO4·1/2H2O hydration, the percentage of free F decreased considerably owing to its adsorption and the rapid precipitation of CaF2, and the percentage of free Na was very higher due to its strong solvation. NaF enhanced β-CaSO4·1/2H2O dissolution and CaSO4·2H2O precipitation, which included two pathways involving Ca2+ and SO42− as well as [CaSO4]0. The acceleration of NaF can be ascribed to the nucleation of CaF2 whose acceleration was slight and the catalysis of Na + whose acceleration was remarkable due to the transformation from [NaSO4]- to [CaSO4]0. The evolution of the zeta potential was one of the reasons for the faster initial setting at the lower hydration degree. The lower bonding force among the crystal grains of CaSO4·2H2O, the coarsened CaSO4·2H2O crystal grains and the quick hydration rate all contributed to the drop in 2-h and dry strengths, and when the dosage of NaF was 0.5%, the dry strength increased due to the optimized morphology of CaSO4·2H2O crystal grains. The dosage of NaF should be limited to ≤1.0% to avoid excessive strength loss.