Transport kinetics of internal curing water from pumice in ultra-high performance concrete: The impact of ink-bottle pore size

Self-desiccation shrinkage is one of the primary factors contributing to cracking in ultra-high-performance concrete (UHPC), and internal curing has been identified as a highly effective solution to mitigate this issue. The internal curing efficiency is closely linked to the pore size of internal curing material, particularly ink-bottle structure, which plays a crucial role in moisture retention and release. This study investigates how the pumice ink-bottle pore size affects moisture transmission kinetics in UHPC. Using low-field nuclear magnetic resonance, scanning electron microscopy, and nanoindentation, the impact of pore characteristics on the absorption-desorption behavior of internal curing water and its correlation with UHPC microstructural evolution were investigated. The results reveal that ink-bottle hysteresis effect is more pronounced in finer pumice, with the hysteresis index of 0–0.6 mm pumice increasing by 42 % compared to 0.6–1.23 mm pumice. Consequently, fine pumice exhibits a delayed water release time (by 1 h) and a lower water release rate, with 38 % less water being released over 24 h in UHPC. When fine pumice content reaches 20 %, it reduces autogenous shrinkage by 71 % and forms a hydration product gradient layer than 45 μm in the interface transition zone of UHPC, demonstrating a more effective internal curing effect. Absorption process in ink-bottle pores is driven by capillary forces and osmotic pressure, while cavitation occurs via bubble nucleation under low vapor pressure. A dynamic moisture transfer model based on vapor pressure theory confirms that smaller ink-bottle pores amplify hysteresis effects and thereby enhance the efficiency of internal curing.