Calcium Hydroxide Modification of Metakaolin Geopolymers: A Sustainable Strategy to Enhance Water Resistance, Inhibit Efflorescence, and Improve Corrosion Protection

This study presents a sustainable strategy to address the inherent limitations of metakaolin (MK)-based geopolymers—specifically their water sensitivity, high permeability, efflorescence, and susceptibility to corrosion—arising from their calcium-deficient composition. To mitigate these issues, calcium hydroxide (Ca(OH) 2 or CH) was incorporated into MK mixtures at dosages ranging from 1% to 15%. The influence of CH on mechanical and durability performance was evaluated under both air and water curing conditions. The results show that although a higher CH content reduced flowability and accelerated setting, it markedly enhanced compressive strength, water resistance, and corrosion protection. CH addition also effectively suppressed efflorescence and refined the transport properties of the geopolymer matrix. These improvements are attributed to the formation of C-(A)-S-H gel, which densified the microstructure and strengthened the overall matrix. Electrochemical assessments further confirmed the enhanced corrosion resistance of embedded steel reinforcement. Collectively, the findings demonstrate that CH supplementation is an effective approach for improving the performance and long-term durability of MK-based geopolymers, broadening their potential for structural and environmental applications.