Durable gypsum composites from FGD β-gypsum enabled by hemihydrate blending and UV-resin treatment

Flue gas desulfurization (FGD) gypsum, a major industrial by-product generated by coal-fired power plants, presents significant disposal challenges and limited durability when utilized as β-hemihydrate (β-HH) gypsum in construction applications. This study investigates a two-step modification approach to improve the sulfate resistance and long-term performance of FGD-derived gypsum composites through blending of FGD-derived β-HH with α-hemihydrate (α-HH), followed by UV-curable resin surface treatment. Composite specimens with different β/α-HH ratios were prepared and exposed to a 5% Na 2 SO 4 solution for up to 121 days. Compressive strength, water absorption, ultrasonic pulse velocity (UPV), thermal resistance, dimensional stability, and microstructural characteristics were evaluated to assess durability performance. The results showed that uncoated pure β-HH specimens experienced severe deterioration, with compressive strength decreasing to 1.81 MPa and linear expansion reaching 5.8% after exposure. In contrast, increasing the α-HH content significantly improved structural compactness and durability under sulfate-rich exposure. Compared with uncoated specimens, UV-resin treatment reduced water absorption by 30–60%, increased the 121-day residual compressive strength of pure β-HH from 1.81 to 5.34 MPa, and reduced expansion by up to 70%. Notably, the 50:50 β/α-HH blend maintained a 121-day compressive strength of 27.3 MPa, which increased to 32.6 MPa after UV-resin treatment, while retaining more than 85% of its compressive strength after exposure to 200°C. Furthermore, strong correlations were observed among compressive strength, moisture transport behavior, and microstructural integrity, demonstrating the interdependence of these properties in governing the long-term durability of the hybrid gypsum system.