Synergistic Effect of Metakaolin and Engineered Biochar on the Hydration Development of Ultra-High-Performance Concrete

This study investigates the combined effects of engineered biochar (EBC) and metakaolin on the hydration, shrinkage, and thermal stability of Ultra-High-Performance Fiber-Reinforced Concrete (UHPFRC). Isothermal calorimetry revealed that metakaolin accelerated hydration, advancing the main peak by nearly 10 hours due to its high reactivity and nucleation effect. In contrast, biochar sustained hydration, increasing cumulative heat release by approximately 10% over 14 days by gradually releasing stored moisture. Thermogravimetric analysis (TGA) confirmed enhanced hydration, with biochar-containing mixes showing greater mass loss in the 100–200 °C range and higher Degree of Hydration (DOH) values, sustaining reactions for up to 56 days. Shrinkage testing further demonstrated biochar’s internal curing capacity, reducing total shrinkage by 57% at 21 days and maintaining a 52% reduction at 202 days. Collectively, the results highlight that metakaolin and biochar significantly improve hydration efficiency, mitigate shrinkage, and enhance durability, though careful mix optimization is necessary to balance mechanical performance.