Potassium as a Promising Approach for Eliminating Efflorescence in Alkali-Activated Slag

Alkali-activated slag (AAS) has attracted increasing attention as a sustainable alternative to Portland cement due to its significantly lower carbon footprint and promising mechanical and durability performance. However, despite these advantages, the widespread application of AAS is still hindered by several challenges. Among them, efflorescence―manifested as white deposits on material surfaces—remains one of the most common and persistent issues, adversely affecting both the aesthetic quality and long-term durability of materials. Mitigating efflorescence is therefore critical for the practical implementation of AAS binders. This study systematically investigated the influence of K on the efflorescence of AAS pastes prepared using NaOH and KOH solutions. Incorporating K in the activator significantly suppressed efflorescence: a 25% substitution decreased it by 59%, while substitutions above 50% achieved reductions exceeding 98%. The underlying mechanisms were elucidated through analysis of phase assemblage, gel chemistry, pore structure, and carbonate formation. K exhibited higher binding strength and capacity than Na within the gel, effectively decreasing the concentration of free alkalis available for efflorescence. Moreover, saturated K 2 CO 3 solutions showed much lower equilibrium relative humidity than Na 2 CO 3, rendering K-based systems more hygroscopic and less prone to efflorescence under ambient conditions. A K substitution of approximately 40% was predicted to substantially suppress efflorescence. These findings highlight the pivotal role of K in enhancing the durability of AAS binders.