Workability and mechanical performance of shield muck–slag-based engineered geopolymer composites modified with chemical admixtures

Engineered geopolymer composite (EGC) has been suggested to be an eco-friendly strain-hardening material with superior crack-control capability, which can effectively resist the erosive effects in marine environments. Herein, we proposed the novel EGC formulations synthesized from a blend of shield muck and slag (SS-EGC), addressing both the diminishing supply of geopolymer raw materials and the environmental challenges posed by accumulating shield muck stockpiles. A comprehensive experimental program evaluated the influence of various chemical admixtures—including borax retarders, naphthalene-based superplasticizers, BCH retarders (a composite consisting of 80 wt% barium chloride and 20 wt% zinc sulfate), melamine superplasticizers, and polycarboxylate-based superplasticizers—on the fresh and hardened properties of SS-EGC. We show that borax retarders effectively improve workability and maintain or enhance mechanical performance by moderating the hydration and setting process. In contrast, naphthalene-based superplasticizers and BCH retarders, while beneficial in improving flowability and extending setting time respectively, adversely affect the composite's mechanical strength. Meanwhile, melamine superplasticizers and polycarboxylate-based superplasticizers have limited effect in the highly alkaline environment of SS-EGC. Notably, a significant deterioration in mechanical properties is observed with polycarboxylate-based superplasticizers, contrasting sharply with their superior performance in conventional cement-based formulations. Additionally, key parameters derived from the viscosity–time curve effectively characterize SS-EGC workability. Specifically, flowability exhibits a linear negative correlation with η5 (viscosity at 5 min), while the initial setting time increases linearly with ln t (rate of viscosity growth). These findings provide valuable guidance and data support for further optimizing the engineering applications of SS-EGC.