The water absorption behavior of concrete is a critical indicator of its durability, and a comprehensive understanding of water transport characteristics can significantly enhance concrete performance. This study investigates the capillary water absorption properties of steel fiber-reinforced concrete across various strength grades by combining mercury intrusion porosimetry (MIP) and ¹H low-field nuclear magnetic resonance (¹H low-field NMR) techniques. Key findings reveal that the capillary water absorption of steel fiber-reinforced concretes occurs in the following two distinct stages: an initial rapid absorption phase (0 min to 6 h) and a subsequent slow absorption phase (1 day to 12 days). Modifications to the concrete matrix composition substantially reduce capillary water absorption rates, with ultra-high-performance concrete (UHPC) exhibiting exceptionally low absorption levels (the cumulative capillary water absorption of UHPC accounts for only 4.5–5.7% of that of C30 concrete). Additionally, for higher-strength concrete and extended absorption durations, the capillary water absorption rate deviates from the linear relationship with the square root of time. This deviation is attributed to the interaction of gel pore water with unhydrated cement particles, generating more hydration products, which refine the pore structure, reduce capillary pore connectivity, and increase pore tortuosity. Furthermore, steel fibers influence water transport through the following two primary mechanisms: interfacial interactions between the fibers and the matrix and a physical blocking effect that impedes water movement.

Water reducer and water-repellent admixture are very important in improving the workability and durability of cement-based materials. However, the synergistic effect of the two types of admixtures has not been well investigated. In this study, polycarboxylate ether-based superplasticizer (PCE) and octyltriethoxysilane (OTS) were adopted as water reducer and water-repellent admixture, respectively. Their synergistic effect on the fluidity, compressive strength, and water absorption rate of cement-based materials was investigated. Particularly, the pore structure and hydration state of cement paste were analyzed using ¹H Low-Field Nuclear Magnetic Resonance (¹H LF NMR). The result showed that the fluidity of cement paste containing different dosages of PCE was reduced by 5–10 mm by incorporating 1% OTS, and the compressive strength at the early age of 3 d of mortar containing high PCE dosage of 0.25% decreased up to 15% by using 1% OTS. In contrast, the compressive strength of mortar containing 0.20% PCE was slightly enhanced by the addition of 1% OTS. ¹H LF NMR analysis revealed that the combination of PCE and OTS would increase the pore size and total pore volume of cement paste, and more bleeding water would be generated at high PCE dosage. The intensity-weighted T₂ values of the main peak ($\overline{T_2}$) implied that both PCE and OTS produced a retardation effect on cement hydration. However, the water absorption rate decreased by 46.6% despite the increase in pore size and total pore volume. The conflict phenomenon powerfully revealed that the internal hydrophobic treatment by OTS has been successfully achieved. Overall, the combination of 0.20% PCE and 1% OTS exerted a positive synergistic effect in improving the compressive strength and water-repelling ability of cement-based materials, which is meaningful for improving their durability and service life.

Conventional synthesis of polycarboxylate superplasticizers (PCEs) typically relies on high-temperature processes, posing challenges for sustainable production. Ethylene glycol monovinyl polyethylene glycol ether (EPEG), characterized by the high reactivity of its vinyloxy double bond, offers a promising sustainable alternative for low-temperature synthesis. This study systematically investigates the aqueous free radical copolymerization of EPEG and acrylic acid, identifying a reaction temperature of 20 °C as the kinetic optimum that achieves a macromonomer conversion rate exceeding 95% under ambient conditions. Through the variation in five key process parameters, a clear “synthesis–structure–property” relationship was established, revealing that the weight-average molecular weight (M_w) acts as the pivotal regulator of performance. High-M_w PCEs exhibited superior initial dispersion driven by strong electrostatic repulsion and high adsorption but suffered from poor slump retention due to the rapid depletion of free polymers. Conversely, low-M_w variants, regulated by chain transfer agent dosage, significantly reduced the pore solution surface tension, thereby enhancing wetting ability and workability retention. The optimal synthesis conditions (20 °C, 4:1 acid-to-ether ratio, 2.5% initiator, 1.5% chain transfer agent) yielded PCEs with an ideal balance between initial dispersion and retention. Furthermore, the synthesis demonstrated excellent process robustness with a broad dosing window (>60 min). These findings provide a vital theoretical basis for the robust and low-temperature industrial production of EPEG-based PCEs for sustainable infrastructure materials.