Influence of water–binder ratio on the optimal percentage of metakaolin in metakaolin-blended cement concrete

The incorporation of metakaolin (MK) in blended cement concrete enhances mechanical and durability properties by refining the microstructure. However, the optimal MK content remains uncertain across studies. This research investigates the influence of water–binder ratio (w/b) on the ideal MK replacement level. Two concrete grades, with w/b ratios of 0.6 and 0.3, were designed for target strengths of 30 MPa and 50 MPa, respectively. Cement was replaced with MK at 5%, 10%, 15%, 20%, and 25% by weight. Mixes were evaluated for workability, compressive strength, tensile strength, and sulfate resistance. Each result represents the average of three specimens, with a total of 234 samples tested—90 for compressive strength, 90 for tensile strength, and 54 for sulfate resistance. Increasing MK content reduced workability due to its fine particle size and high surface area. Strength tests showed that optimal MK content depended on the w/b ratio: at 0.6, 5% MK gave the best performance, exceeding control strength by 8.0% at 28 days; at 0.3, 15% MK was optimal, surpassing the control by 19.4%. Microstructural analysis confirmed improved packing density and pore refinement. Sulfate resistance assessment showed that 5–15% MK replacement improved performance, with strength loss after 90 days of MgSO₄ exposure below 3.6%, compared to 4.1% for the control and 4.4% for 20–25% MK mixes. Furthermore, statistical analysis indicates that MK addition improves concrete strength and durability; highest compressive strength correlation was observed at 15% and 25% for samples with w/b of 0.6 and 0.3 respectively. This study provides new insight into the coupled effect of MK content and water–binder ratio, establishing a quantitative basis for optimizing MK use in performance-based concrete design.