Alkali activated slag-metakaolin binders: A 720-day study on phase evolution, gel chemistry, and screening sustainability

This study investigates the long-term phase evolution, reaction mechanisms, and sustainability performance of alkali-activated blast furnace slag (BFS)-metakaolin (MK) binders over 720 days, aiming to link gel/phase evolution to long-term performance and screening sustainability indicators. Pastes with BFS/MK ratios of 80/20, 50/50 and 20/80 were activated with sodium silicate solutions (silicate modulus Ms=SiO 2 /Na 2 O=1.5) at 5, 10 and 15 wt% Na 2 O and cured under ambient dry storage conditions for up to 720 days. Mechanical performance and binder chemistry were assessed by compressive strength, XRD, FTIR, SEM-EDS, isothermal calorimetry and 29 Si MAS NMR. Reaction kinetics depended strongly on precursor chemistry: BFS-rich binders showed rapid, single-peak calorimetry profiles associated with early precipitation of Ca-rich C-(A)-S-H-type gels, whereas MK-rich blends exhibited slower multi-peak behavior, consistent with progressive aluminosilicate network formation. The BFS/MK ratio governed the dominant phase assemblage and long-term nanostructural evolution, with 80/20 systems dominated by chain-like C-(A)-S-H-type structures, 20/80 binders by highly cross-linked N-A-S-H-type frameworks, and 50/50 mixtures forming hybrid (C,N)-A-S-H-type gels. The combined SEM/EDS, FTIR, and ²⁹Si MAS NMR results further indicate that gel chemistry remained actively evolving at 720 days, consistent with continued structural reorganization rather than early-age stabilization. BFS-rich binders achieved the highest strengths (up to 95 MPa), but showed greater carbonation signatures by FTIR, whereas MK-rich binders displayed higher polymerization yet lower strength. A screening-level sustainability assessment identified 80/20 formulations as the most viable compositional window, outperforming a reference blended Portland cement, with ∼37% lower CO 2 -eq emissions, ∼22% lower energy demand, ∼31% lower cost, and approximately double the compressive strength. • Phase evolution of alkali-activated slag/metakaolin binders studied for 720 days. • Slag/Metakaolin ratio governs hybrid phase assemblage C-(A)-S-H ↔ N-A-S-H • Gel nanostructure continues evolving up to 720 days, indicating ongoing polymerization. • 80% slag binders cut screening CO 2 -eq by 37% and costs by 31% versus Portland cement. • High metakaolin contents rise embodied energy and reduce net sustainability gains.