Effect of sodium fluoride on the crystal regulation of high-strength α-hemihydrate gypsum from calcium sulphate dihydrate and phosphogypsum

The presence of impurities from raw materials directly affects the crystal shape control, process, and product performance of high-strength α-hemihydrate gypsum (α-HH,α-CaSO₄•0.5H₂O) from industrial by-product phosphogypsum (PG). In this work, effect of sodium fluoride on the crystal regulation of high-strength α-HH using calcium sulphate dihydrate(DH,CaSO₄•2H₂O) and PG as raw materials were investigated, respectively. The results showed that 0.6–1.8 wt% sodium fluoride can transform some α-HH crystals from fine needle shape to aggregate plate shape by changing the crystallinity, crystal habit and crystal diameter of α-HH. In the presence of 0.1 wt% citric acid and 0.6–1.8 wt% sodium fluoride, short hexagon-prism α-HH crystals can be obtained from DH or PG, sodium fluoride improved the content of α-HH in dehydration products, altered the dehydration and phase transition peak temperatures, transformed the crystallinity and crystal growth habit along the c-axis by the selectively chemisorption of F⁻, Na⁺ and citrate ions on the end surface of α-HH. With 0.1 wt% citric acid and 1.8 wt% sodium fluoride addition, the compressive strength of α-HH from DH and PG reached 59.8 MPa and 54 MPa, respectively, which were 54.3 MPa and 54.0 MPa higher than the blank samples.