Mitigating water curing strength degradation, enhancing water resistance, and improving the properties of metakaolin geopolymer cement with CaO

Metakaolin (MK) geopolymers offer excellent sustainability and high compressive strength but exhibit water sensitivity, high shrinkage, and reduced durability due to their calcium-free nature. This study investigates the incorporation of calcium oxide (CaO) at 2.5–15% (in 2.5% increments) to overcome these shortcomings. Samples were cured under both air and water conditions to evaluate changes in workability, setting time, strength development, and microstructural evolution. Increasing CaO content reduced flowability and accelerated setting but significantly improved compressive strength, dimensional stability, and durability. Optimal performance was achieved at 10% CaO, forming a hybrid binder composed of coexisting N-A-S-H and C-(A)-S-H gels that produced a denser, more cohesive microstructure with reduced permeability and drying shrinkage. The results confirm that CaO incorporation is an effective strategy to enhance MK geopolymer cement performance and long-term stability. Overall, these findings highlight the potential of CaO modification to develop more durable, sustainable, and practical metakaolin-based construction materials.