Sustainable hybrid activation: Synergistic effects and optimization of one-part alkali-activated slag using sodium sulfate and sodium carbonate

The development of one-part alkali-activated materials (AAMs) using near-neutral and moderately alkaline salts offers a promising, low-carbon alternative to hazardous two-part systems. However, overcoming delayed reaction kinetics and setting anomalies remains a critical challenge. This study systematically investigates the fresh state, mechanical, and microstructural evolution of one-part slag mortars activated by sodium sulfate (Na2SO4), sodium carbonate (Na2CO3), and their hybrid combinations, adopting a hybrid binder approach through modification with calcium-rich additives (Portland cement, hydrated lime, and quicklime). Results demonstrate that while Portland cement ensures a controlled dissolution rate and balanced setting times across all environments, the incorporation of 7.5% quicklime in pure carbonate media triggers an instantaneous flash set. This extreme reactivity leads to a severe structural collapse, plunging the 90-day compressive strength to 16.11 MPa. Microstructural analyses (TGA and FTIR) suggest a link between this mechanical failure and a massive accumulation of unreacted portlandite (4.51% mass loss) rather than cohesive C-(A)-S-H gel formation, a degradation further corroborated by a drastic attenuation in ultrasonic pulse velocity (UPV). Conversely, the synergistic implementation of hybrid sulfate-carbonate activators effectively mitigates these anomalies. Sulfate ions buffer early-age kinetics to prevent flash setting-recovering the collapsed strength up to 29.16 MPa while carbonate ions facilitate sustained late-age matrix densification. Ultimately, tailoring specific calcium boosters within a hybrid activator framework presents a highly robust and potentially low-carbon framework for developing sustainable binders for practical on-site construction applications.