Internal curing mechanisms of recycled fine aggregate with different pre-wetting degrees in UHPC: Insights into water migration and hydration

Severe self-desiccation makes ultra-high performance concrete (UHPC) susceptible to early autogenous shrinkage and cracking. Recycled fine aggregate (RFA) can serve as an internal water reservoir, but how its pre-wetting degree regulates water migration, hydration, and dimensional stability in coarse-aggregate UHPC remains insufficiently understood. this study systematically evaluated the effects of RFA with different pre-wetting degrees (0%-100%) on the mechanical properties, shrinkage behavior, water migration, hydration, and microstructural evolution of UHPC. The underlying mechanisms were examined using ¹H low-field nuclear magnetic resonance, isothermal calorimetry coupled with the Krstulović-Dabić model, scanning electron microscopy, thermogravimetric analysis, and mercury intrusion porosimetry. The results showed that increasing the RFA pre-wetting degree reduced flowability but progressively enhanced the internal curing. Compared with UHPC containing non-pre-wetted RFA, fully pre-wetted RFA increased the compressive and flexural strengths by up to 12.73% and 13.54%, respectively. It also reduced the 56-d autogenous shrinkage by 35.96% relative to the control mixture, although the incorporation of RFA increased drying shrinkage because of its lower stiffness and higher porosity. The fully pre-wetted RFA initiated active water release at approximately 6 h and reached the maximum release rate at 8 h, thereby sustaining water availability, promoting hydration, refining the pore structure, and improving matrix and interfacial integrity. These findings contribute to a better understanding of the feasibility of converting RFA into a valuable internal curing material, offering a sustainable and efficient solution for mitigating autogenous shrinkage in UHPC.