Due to the large amount of old mortar attached to the surface of recycled coarse aggregate (RA), its water absorption capacity is significantly higher than that of natural coarse aggregate. Consequently, the water saturation degree (S_W) of RA plays a crucial role in influencing the microstructure and resistance to Cl^- erosion of recycled aggregate concrete (RAC). However, the specific effects of different saturation states on RAC performance remain controversial. Some studies suggest that water released from surface-saturated RA weakens the newly formed interfacial transition zone (ITZ), reducing resistance to chloride penetration, whereas others argue that its desorption behavior acts as an internal curing agent that enhances RAC durability. Furthermore, the influence of S_W on the microstructure under different water control schemes-namely, constant mixing water supply (labelled as S1) and constant total water supply (labelled as S2)-has yet to be fully elucidated. Therefore, this study aims to systematically investigate the effects of varying S_W of RA on chloride diffusion and binding behavior in RAC under different control schemes, and to elucidate the underlying microscopic mechanisms.

The RA used in this study had a particle size range of 4.8-31.5 mm, an apparent density of 2650 kg/m³, a water absorption rate of 5.4%, a crushing index of 16.6%, and an old mortar content of 36.7%. The fine aggregate was river sand with a fineness modulus of 2.0, a moisture content of 1.0%, and a mud content of 0.8%. Ordinary Portland cement 42.5 R with a specific surface area of 345 m²/kg and a density of 3.12 g/cm³ was used. A polycarboxylate superplasticizer with a solid content of 40% and a water reduction rate of 25% was added to control the workability of fresh concrete. Under both S1 and S2, the effective water-to-cement ratio (W_e/C) and the total water-to-cement ratio of RAC were maintained at 0.55. The S_W of RA was adjusted to 0%, 50%, 80%, and 100% by controlling the soaking time.

After curing for 28 d, the rapid chloride migration coefficient (D_RCM) of RAC was determined using the rapid chloride migration method. Cubic specimens were then immersed in a 5% NaCl solution for one-dimensional natural chloride ingress tests. At exposure durations of 60, 120, 180 d, and 240 d, the concentration profiles of free Cl^- and total Cl^- were measured using potentiometric titration. Based on Fick's second law of diffusion, the effective and apparent chloride diffusion coefficients (D_a and D_e) were calculated. In addition, after 28 d of curing, the distribution of unhydrated cement content and the porosity in the new ITZ were analyzed using backscattered electron imaging (BSE), while the pore size distribution of the new mortar was determined by mercury intrusion porosimetry (MIP). Furthermore, scanning electron microscopy combined with energy-dispersive X-ray spectroscopy (SEM/EDS) and X-ray diffraction (XRD) were employed to characterize the microstructure and phase composition of chloride-binding products on the concrete surface layer after 240 d of exposure.

Under S1, as S_W of RA increases, the content of unhydrated cement content in the new ITZ decreases, while porosity increases. In contrast, under S2, the opposite trend is observed. MIP results further indicate that under S1, total porosity, effective porosity, and the proportion of macropores (>100 nm) in the new mortar all increase significantly with increasing S_W, whereas under S2, these parameters decrease. Under S1, the water absorption capacity of RA decreases with increasing S_W; moreover, RA with high S_W may release water into the fresh mixture, resulting in an increase in the actual W_e/C ratio of the cement matrix. Conversely, under S2, higher S_W corresponds to a lower effective W_e/C ratio during the early hydration stage, thereby suppressing both hydration and pore development.

Since higher porosity and improved pore connectivity lead to increased chloride diffusivity, under S1, the values of D_RCM, D_e, and D_a all increase with increasing S_W. In contrast, under S2, the chloride diffusion coefficients decrease as S_W increases. SEM/EDS and XRD analyses confirm the occurrence of chloride binding in RAC, revealing the presence of chloride-bearing C-S-H gel and Friedel's salt after chloride ingress. During the early exposure stage (60-120 d), RAC under S1 exhibits a stronger chloride-binding capacity due to a higher initial degree of hydration. Under S2, however, a larger amount of residual unhydrated cement continues to hydrate during prolonged immersion, resulting in superior long-term chloride binding capacity. Moreover, under S1, the chloride binding capacity decreases with increasing S_W, whereas under S2, the opposite trend is observed. The decay rate of the chloride diffusion coefficient increases with S_W under S1 but decreases under S2. This may be attributed to the fact that, compared with S2, higher S_W under S1 leads to a larger early-age chloride diffusion coefficient, which promotes the formation of more corrosion products and significantly refines the pore structure, thereby accelerating the time-dependent reduction in chloride diffusivity.

Under S1, increasing S_W reduces the water absorption capacity of RA, and highly saturated RA may release water into the fresh mixture. This results in an increased W_e/C ratio in the new ITZ and mortar, leading to higher porosity and a greater proportion of macropores, and consequently higher chloride diffusion coefficients. In addition, the extra moisture promotes the hydration reaction, reducing the content of unhydrated cement particles in the new mortar and the new ITZ. In contrast, under S2, higher S_W corresponds to a lower effective W_e/C ratio during early hydration, which significantly suppresses hydration, pore development and chloride diffusion. SEM/EDS and XRD analyses confirm that chloride binding in RAC involves both physical adsorption and chemical binding. During the exposure period of 60-120 d, RAC under S1 exhibits a stronger chloride-binding capacity due to a higher initial degree of hydration. Under S2, however, the continued hydration of residual unhydrated cement during long-term immersion provides superior chloride binding capacity after 180 d. Furthermore, under S1, increasing S_W is detrimental to chloride binding capacity, whereas under S2, it is beneficial. The decay rate of the chloride diffusion coefficient increases with S_W under S1 but decreases under S2. This behavior is attributed to the larger initial diffusion coefficient under S1, which promotes the formation of more corrosion products and leads to progressive refinement of the pore structure over time.