Understanding the role of magnesium ions on setting of metakaolin-based geopolymer

Seawater-based geopolymers potentially capitalize on the abundant seawater and reduce the CO2 emissions for marine/offshore constructions. Unlike the well-documented seawater ions (e.g., Ca2+, Na+, and Cl−), existing studies on magnesium-ion (Mg2+)-induced changes of geopolymers, especially the non‑calcium mixtures, have been limited. In this study, Mg2+ was confirmed to retard the setting of sodium-silicate activated metakaolin from Vicat and calorimetry tests. Through chemical extraction and spectroscopy characterization, the retardation mechanism was found mainly due to the consumption of OH− via Mg2+ in the activating solution, which decelerated the dissolution and condensation during geopolymerization. Supplemental Vicat penetration under a controlled alkalinity indicated a further retardation via the "coating effect" of magnesium silicate hydrate (M-S-H) gels, which were produced by the interaction of Mg2+ and the aqueous silicates. These findings substantiate the tailorability of setting by Mg2+, an abundant marine resource, thereby paving the way to develop eco-friendly cementitious binders with balanced performance.