<p>The efficiency of alkali-activated ground granulated blast furnace slag in stabilizing dredged sediments with high water contents is suboptimal because the activators become diluted. To improve stabilization efficiency, additives such as nano-CaCO₃ are proposed. However, some of the proposed additives may not be practical owing to their high costs. This study experimentally investigates the addition of Na₂CO₃ for the stabilization of dredged sediment with high water contents (i.e., 100%) using Ca(OH)₂-activated slag. Experimental results show the optimal content of Na₂CO₃ to obtain the highest 28-day unconfined compressive strength of stabilized sediments is 0.2% gravimetrically. Below the optimal content, the strength increases with Na₂CO₃ content. Above the optimal content, a decrease in strength is observed. By examining the reaction products and microstructure of the stabilized dredged sediments, it is observed that the coupling mechanism of cation exchange and calcite precipitation promotes the development of finer capillary pores, leading to a reduction in interpore connectivity and lower structural heterogeneity of the fine capillary pores. Experimental evidence from this study broadens the practical applications of sustainable soil stabilization using additives.</p>