Efficiency of Superplasticizers in Low‐carbon Alkali‐activated Slag–Silica Fume Binders Governed by Polymer Solubility

ABSTRACT Workability control of alkali‐activated binders (AABs) remains an obstacle to producing low‐carbon clinkerless ultra‐high‐performance concrete, as their formulation usually necessitates low water‐to‐binder (W/B) ratios, high binder content, and concentrated activator solutions. To address this challenge, a greener and more manageable composite K 2 CO 3 –KOH activation strategy was adopted, and the performance of lignosulfonate (LS) and polycarboxylate ether (PCE) superplasticizers was systematically compared in K 2 CO 3 –KOH‐activated slag, with and without silica fume, across a range of W/B ratios. The results indicate that the performance differences between PCE and LS are primarily governed by their solubility. PCE exhibits an apparent solubility threshold near W/B ≈ 0.35 (0.74 M K 2 CO 3 and 1.22 M KOH), below which large‐scale aggregation suppressed effective dispersion, coarsened the pore‐entry structure, and reduced later‐age strength. In contrast, LS retained a stable soft‐colloidal state over the investigated activator concentrations, enabling more robust dispersion, pore refinement, and improved later‐age strength despite stronger early‐age retardation. These findings provide mechanistic guidance for admixture selection in ambient‐cured, low‐carbon AABs by identifying solution stability as a prerequisite for effective dispersion and strength development.