Reaction mechanisms of one-part and two-part slag-based binders activated by sodium carbonate and lime

Abstract

One-part alkali-activated slag (AAS) is a safer and more manageable alternative to a two-part formulation. This work compares the reaction mechanism, phase formation, microstructure and properties developments between one-part and two-part AAS pastes prepared by a combined lime (CaO) and sodium carbonate (Na₂CO₃) activator. The results show that the CaO-Na₂CO₃ combination effectively accelerates slag reaction, resulting in 3–6 times higher compressive strength in AAS than blended slag-OPC binder at 1 d. Initially, two-part AAS demonstrates a slightly greater accelerating effect due to rapid generation of a strong alkaline condition, characterized by a hydroxyl ion concentration ([OH⁻]) in pore solution that is twice that of one-part AAS. This elevated alkalinity in two-part AAS enhances early-age hydration of slag and promotes phase formation, resulting in increased strength and refined microstructure. However, after 28 d, the strength of one-part AAS approaches and even surpasses that of two-part AAS, attributed to a more stable and progressive reaction between Ca(OH)₂ and dissolving Na₂CO₃, which produces NaOH. This steady reaction maintains a stable pH and allows for the gradual release of alkalis, resulting in increased degree of hydration (DOH) of slag, mean chain length (MCL), Al/Si and Q²/Q¹ ratios of C-A-S-H, as well as enhanced Al linkage in C-A-S-H of one-part AAS. In addition, the one-part AAS activated by CaO-Na₂CO₃ demonstrates up to 93 % reduction in CO₂ emissions while maintaining comparable strength to OPC counterparts, highlighting its great potential as a green binder for sustainable construction applications.