Enhancing the Performance of Ultra-High Performance Concrete Using Expansive Agent and Pre-Wetted Biochar to Produce a Synergistic Effect

This study explores the impact of an expansive agent (EA) on the performance of internally cured ultra-high-performance concrete (UHPC) using pre-wetted biochar to achieve a synergistic effect in mitigating autogenous shrinkage. Here, the biochar provides internal curing water to mitigate self-desiccation, while the EA generates expansive products to compensate shrinkage, collectively improving hydration. The results revealed that a portion of the internal curing water was consumed by the EA to form expansive products, thereby shortening the prolonged setting time typically observed in UHPC with pre-wetted biochar. The incorporation of pre-wetted biochar markedly increased the internal relative humidity of UHPC, reducing its 7 d autogenous shrinkage by 35.7%. Furthermore, the addition of 1–4% EA further decreased the autogenous shrinkage by 6.8–30.3% compared to the internally cured baseline. Notably, the inclusion of pre-wetted biochar slightly enhanced the 7 d and 28 d compressive strengths of UHPC, with further improvements achieved by adding EA (up to 2%). This demonstrates the effectiveness of this internal curing approach, which maintained or improved the compressive strength of UHPC.