Regulating Silicate Speciation and Geopolymerization Kinetics through Crystalline Water in Solid Sodium Silicate Activator

The performance of one-part alkali-activated materials (OP-AAMs) is strongly influenced by the hydration state and silicate speciation of solid alkali activators. In this study, the role of crystalline water in solid sodium silicate activators was systematically examined by varying the ratio of sodium metasilicate hydrate (Na2SiO3·9H2O) to anhydrous sodium silicate, while maintaining a constant Na2SiO3 dosage and effective water-to-binder ratio. Fresh and hardened properties were characterized using in situ fourier transform infrared spectroscopy (FTIR)–rheometry, isothermal calorimetry, 29Si MAS NMR, X-ray diffraction (XRD), mercury intrusion porosimetry, and scanning electron microscopy. Reducing crystalline water increased the proportion of reactive Q1 species from 24.7% to 86.0%, enhanced workability with a spread diameter of 235 mm, and accelerated early reaction kinetics, shifting the main exothermic peak from ∼20 min to 3–5 min. An optimal 50/50 blend achieved a compressive strength of 73.8 MPa at 28 days, a nanopore fraction of 83.86%, and a highly polymerized Q4/Q4(1Al)-rich gel network. These results highlight crystalline water as a key regulator of reactivity and gel structure in OP-AAMs.