Mechanical, Durability and Microstructural Performance of OPC-GGBFS-FGD Gypsum Ternary Concrete: Identification of an Operational Sulfate Activation Threshold.

Ordinary Portland cement (OPC) production contributes approximately 7-8% of global anthropogenic CO2 emissions, driving urgent demand for clinker-efficient binders utilizing industrial by-products. Flue gas desulfurization (FGD) gypsum and ground granulated blast-furnace slag (GGBFS) represent underutilized industrial by-products with documented potential as supplementary cementitious materials. This study investigates the mechanical, durability and microstructural performance of OPC-GGBFS-FGD gypsum ternary concrete mixtures incorporating untreated flue gas desulfurization (FGD) gypsum at 0-20% of total binder mass and ground granulated blast-furnace slag (GGBFS) at 25-50% of total binder mass in M30 structural concrete (w/b = 0.45). Compressive, split tensile and flexural strengths were evaluated at 7-90 days alongside rapid chloride penetration (RCPT), water absorption, strength efficiency index (SEI) and SEM-EDX analyses. Binary GGBFS replacement progressively enhanced long-term compressive strength, with T35F0 attaining 55.6 N/mm2 at 90 days (+33.7% relative to the OPC control). Moderate FGD gypsum contents (5-10%) further enhanced overall performance. Among all mixtures, T50F10 exhibited the best overall performance on the mechanical and durability indicators evaluated, achieving 54.2 N/mm2 compressive strength at 90 days together with a rapid chloride permeability value of 410 C, corresponding to 'Very Low' chloride ion penetrability. Beyond 10% FGD gypsum, progressive multi-parameter deterioration was observed, and mixtures containing 20% FGD gypsum failed to meet the M30 design requirement at 28 days. SEM-EDX confirmed that optimum sulfate activation produced a dense C-(A)-S-H-rich matrix, while excess sulfate caused matrix disruption. The findings establish 10% FGD gypsum by total binder mass as the optimum sulfate activation threshold for the investigated GGBFS and FGD gypsum sources at w/b = 0.45, and demonstrate the potential of untreated industrial FGD gypsum to produce durable, low-clinker structural concrete.