Synergistic activation of GGBS–FA–titanium gypsum ternary geopolymer by Ca(OH)₂ and Na₂CO₃: Workability, rheological characteristics, and geopolymerization mechanisms

Geopolymers, recognized as low-carbon cementitious materials, offer a viable pathway for reducing carbon emissions and enhancing the utilization of industrial by-products. To improve the processability and mechanical performance of conventional GGBS–FA binary geopolymers, this study introduced titanium gypsum (TG) as a supplementary precursor, and employed an eco-friendly composite activator, composed of Ca(OH)₂ and Na₂CO₃ in a 1:1 molar ratio, to develop a GGBS–FA–TG ternary geopolymer system. The effect of Na₂O equivalent, TG content, and water-to-binder (w/b) ratio on the setting behavior, rheological properties, compressive strength, and microstructure were systematically investigated. Rheological analysis using the Herschel–Bulkley (H–B) model accurately captured the yield stress and shear-thinning behavior of the fresh pastes. The prolonged setting time induced by the mild alkaline activator was effectively shortened with increasing TG content. Both compressive strength and flowability initially improved with the addition of TG, but began to decline when the TG content exceeded 6 %. Under the optimal mix proportion (5 % Na₂O equivalent, 6 %TG, w/b of 0.41), the 28-day compressive strength reached 43.5 MPa, and the fluidity increased by 62.5 % compared to the binary GGBS–FA geopolymer. Microstructural analysis (XRD, TG–DTG, SEM) confirmed the coexistence of gel products and needle-like ettringite (AFt) in the ternary system. The incorporation of TG facilitated AFt formation, which contributed to pore-filling and strength development. However, excessive AFt formation led to volumetric expansion and microcracking, ultimately degrading the mechanical properties. The results provide a practical parameter window and mechanistic insight for designing safer, low-alkali geopolymer binders with tunable setting and rheological performance. © 2026 Elsevier Ltd.