Optimization of mix proportions and early-age hydration mechanism of alkali-activated FA-GGBS cementitious materials

To address the empirical challenges of complex mix design and the limited understanding of early-age hydration mechanisms in alkali-activated materials (AAMs), this study systematically optimized a fly ash (FA)-ground granulated blast furnace slag (GGBS) binary system using Response Surface Methodology (RSM). A Box-Behnken design (BBD) was employed to evaluate the interactive effects of precursor content, activator modulus, and alkali content on both the fresh workability and mechanical properties. Furthermore, the macroscopic hydration kinetics and pore structure evolution were elucidated utilizing low-field nuclear magnetic resonance (LF-NMR), isothermal calorimetry, and multi-scale microstructural characterizations. The results indicate that an optimal multi-objective balance was achieved at a 70% GGBS content and an activator modulus of 1.2, yielding a peak compressive strength of 85.5 MPa without compromising practical construction workability. Reaction heat analysis revealed that increasing alkalinity and GGBS content significantly accelerated the exothermic reaction and shortened setting times, primarily due to the rapid cleavage of Ca-O bonds. Continuous LF-NMR monitoring demonstrated the consistent transformation of free water into bound water, confirming that high-calcium systems facilitate the refinement of capillary pores into microscopic gel pores. Microstructural evidence further substantiated that the optimized mix proportions promote the synergistic formation of interlocking C-A-S-H and N-A-S-H gel networks, resulting in a substantially denser matrix compared to low-calcium systems. Finally, a three-stage hydration mechanism characterized by a “dual activation” effect is proposed, elucidating how the dynamic water balance between dissolved FA and GGBS the coexistence of C/N-A-S-H gel networks. This provides a solid theoretical foundation for the precise performance tuning of AAMs.