Effect of Sodium Fluoride on the Properties of α-Hemihydrate Gypsum from Phosphogypsum

The presence of impurities directly affects the properties of α-hemihydrate gypsum (α-HH) prepared from phosphogypsum (PG) as a raw material. However, the effect of soluble fluorine impurities on the properties of α-HH by autoclaving remains insufficiently understood. This study investigated the influence of sodium fluoride on the morphology, hydration, and hardening properties of α-HH, using XRD, XPS, SEM, MIP, and tests of setting time, evolution of hydration temperature increase, and strength. The results showed that during the preparation of α-HH, some F^- reacted with Ca²⁺ to form CaF₂, which adhered to the surface of the α-HH crystal, hindering the growth and development of the crystal and resulting in small crystals with rough surfaces. When α-HH hydrated, sodium fluoride caused the early, rapid nucleation of dihydrate gypsum (DH) crystals, accelerating the crystallization process of DH. The introduction of sodium fluoride inhibited the early hydration of α-HH and promoted its later hydration. The increase in sodium fluoride content caused the initial setting time of α-HH hydration to first increase and then decrease, while the final setting time continued to decrease. In the absence of sodium fluoride, the average pore diameter of the hardened paste was approximately 617.99 nm. When the NaF content was 0.2%, the DH crystals were prismatic and densely packed, which resulted in a decrease in the average pore diameter to 449.35 nm. When the NaF content was 0.6%, the DH crystals exhibited a plate-like morphology and were loosely interlocked, leading to an increase in the average pore diameter to 1169.58 nm. Based on these results, the sodium fluoride content in PG should be controlled below 0.2%.