Ca(OH) 2 -regulated AFt-gel intergrowth and strength development in low-clinker slag binders under fixed sulfate supply

The performance limitations of high-slag, low-clinker binders arise from restricted hydration and insufficient synergy between crystalline and gel phases. Under fixed sulfate supply, this study investigates how Ca(OH)2 regulates macroscopic properties, hydration-product evolution, and AFt/C-(A)-S-H formation in OPC-GGBFS binders. Macroscopic tests and multiscale characterization show that Ca(OH)2 accelerates setting and exerts a non-monotonic influence on strength. C40A8 (40% OPC, 55% GGBFS, 5% hemihydrate gypsum, and 8% Ca(OH)2 added by base-powder mass) achieves compressive strengths of 26.44, 38.34, and 49.23 MPa at 3, 7, and 28 d, respectively. Appropriate Ca(OH)2 promotes slag-network depolymerization and Si/Al release, facilitating coordinated AFt and C-(A)-S-H formation. Interpenetrating, coating, and bridging AFt needles and continuous gel form a dense crystal-gel network, whereas excess Ca(OH)2 increases residual CH, local Ca enrichment, and structural heterogeneity. Fixed sulfate stabilizes AFt formation, while suitable calcium-alkaline supply optimizes crystal-gel organization, enabling synergistic strengthening and guiding low-clinker, high-slag binder design.