Multiscale pore optimization of UHPC with microcapsules: From nanoscale to macroscale shrinkage regulation

Superabsorbent polymers (SAP) are widely utilized as internal curing (IC) agents to effectively mitigate the autogenous shrinkage of ultra-high-performance concrete (UHPC). However, the macropores left behind by SAP can pose significant risks to the mechanical properties of UHPC. To address this challenge, this study developed double-emulsion microcapsules with an alkali-sensitive shell, designed to simultaneously achieve reduced shrinkage and enhanced mechanical strength in UHPC. The effects of these microcapsules on hydration, mechanical properties, shrinkage, and the micro- and nano-scale structure of UHPC were systematically investigated. The results revealed that the incorporation of microcapsules substantially reduced autogenous shrinkage while promoting hydration without compromising compressive strength. Specifically, at an optimal microcapsule content of 1.0 %, the autogenous shrinkage rate decreased by 49.5 % compared to UHPC without microcapsules, while the compressive strength remained unaffected. Additionally, the IC process was significantly improved, as evidenced by enhancements in the interfacial transition zone (ITZ) and the mitigation of microcrack development, validated through nanoindentation and X-ray microcomputed tomography (X-ray μCT) analyses. A quantitative method evaluating the fragmentation degree of internal cracks in UHPC is proposed to systematically characterize the impact of microcapsules on IC efficiency, providing theoretical insights into leveraging microcapsule technology to achieve UHPC with low shrinkage and high strength.