Sodium sulfate, silicate, or carbonate? Activator-specific pathways, kinetics, and strength development in ultra-low-clinker hybrid binders

This study investigates hybrid binders (25% Portland cement, 50% calcined clay, 25% limestone) activated with sodium sulfate, sodium carbonate, and sodium metasilicate at different alkali levels. Mechanical performance, hydration kinetics, and microstructure were evaluated using a multi-technique approach. Sodium sulfate at 1.5% Na₂O eq achieved the highest early strength with only a minor reduction at 28 days and was associated with stable ettringite formation. This performance was sensitive to mixing conditions, as the use of Na₂SO₄·10H₂O required controlled storage to avoid pre hydration of the cement. In contrast, ettringite formation was suppressed in both sodium carbonate and sodium metasilicate hybrid binders despite having the total same sulfate content with the references. Sodium metasilicate accelerated hydration and was characterized by the formation of Na-substituted AFm (U-phase). Sodium carbonate system was characterised by limited carbonate-AFm formation detected only in fresh samples, with no persistence after hydration stoppage. Porosity analysis showed that, unlike the reference systems, the hybrid binders developed strength with hydration time without substantial refinement or redistribution of the coarser part of the MIP-accessible pore network. Sodium metasilicate hybrid binder exhibited H₂O-BET values comparable to sodium sulfate activation and a smaller critical pore diameter yet developed substantially lower compressive strength. Sodium carbonate hybrid binder showed the smallest critical pore diameter among all systems but significantly lower strength. These results demonstrate that in these hybrid binders, porosity refinement, critical pore diameter, and accessible surface area are not sufficient descriptors to explain strength development, challenging a common assumption in blended cement systems.