Gallic acid as a bio-based multifunctional admixture for alkali-activated slag

Alkali-activated slag (AAS) is a low-carbon binder, yet its rapid setting, poor workability, and high autogenous shrinkage restrict application. This study investigates gallic acid (GA), a plant-derived polyphenol, as a sustainable admixture for AAS. GA improves flowability and extends setting time in a dosage-dependent manner, while appropriate dosages maintain favorable strength development. At an optimal dosage of 0.05%, GA achieves the highest 28-day compressive strength and reduces autogenous shrinkage by up to 44.2%. Mechanistic analysis indicates that GA regulates hydration through a dual-stage mechanism. Early-stage Ca 2+ complexation and surface adsorption retard slag dissolution, whereas later-stage GA–Ca complexes promote C–(A)–S–H gel formation and microstructural densification. Consequently, GA enhances eco-efficiency and economic benefit by 6.6–20.9% and 6.7–21.7%, respectively. These findings demonstrate that GA is an effective green admixture for cleaner and more sustainable AAS design.