Effect of PFDTS/TiO₂ Coating on Microstructure and Wetting Behavior of Phosphogypsum

Phosphogypsum (PG) constitutes a form of solid byproduct emanating from the manufacturing process of wet-process phosphoric acid. The fabrication of one metric ton of wet-process phosphoric acid entails the generation of approximately five tons of phosphogypsum, a highly prolific and economically viable waste stream. If we can effectively solve the problem of poor hydrophobicity of phosphogypsum, it is possible to replace cement and other traditional cementitious materials. In this way, we can not only improve the utilization rate of phosphogypsum but also obtain significant economic and environmental benefits. In the present investigation, hydrophobic surface coatings were synthesized and applied onto the surface of α-hemihydrate phosphogypsum (α-HPG) utilizing sol-gel processing and impregnation techniques. After hydroxylating α-HPG with alkaline solution (OH-α-HPG), titanium dioxide nanoparticles (TiO₂) hybridized with perfluorodecyltriethoxysilane (PFDTS) were grafted on its surface. The assessment of the hydrophobic properties of the coatings was conducted through water contact angle measurements, Fourier transform infrared (FTIR) spectroscopy, X-ray photoelectron spectroscopy (XPS), and scanning electron microscopy (SEM) analyses. The contact angle remained above 124.2° after strong acidic and alkaline immersion and 50 tape adhesion experiments with good chemical stability and durability, and the mechanism of surface hydrophobicity modification was discussed. The experimental outcomes demonstrated a notable increase in the hydroxyl group concentration on the α-HPG surface following hydroxylation, significantly enhancing the attachment rate of PFDTS and TiO₂ onto the HPG surface. PFDTS and TiO₂ can undergo chemical interaction with hydroxyl groups, facilitating their robust adsorption onto the surface of OH-α-HPG through chemisorption mechanisms. After bonding the OH-α-HPG surface with PFDTS and TiO₂ via hydrogen bonding, the otherwise hydrophilic α-HPG surface acquired excellent hydrophobicity (OH-α-HPG-PT, contact angle (CA) = 146.7°). The surface modification of α-HPG through hydroxylation and hydrophobicity enhancement significantly augmented the compatibility and interfacial interplay between α-HPG and PT. This research successfully enhanced the hydrophobic properties of α-HPG, profoundly showcasing its immense potential within the construction industry and the realm of comprehensive solid waste utilization.