## 4 Conclusions  
The present work provides a comprehensive experimental study with respect to the effects of ultrafine blast furnace slag on hydration, microstructural formation, and chloride transport in cementitious specimens. The role of the degree of saturation in the resistance to chloride penetration is emphasized. The main conclusions can be summarized as follows.  
1. The hydration kinetics of ultrafine slag powders can be well predicted using Avrami's equation. With increases in slag addition, the amount of calcium hydroxide was decreased while the ettringe content was found to be increased. The dilution effect dominated the pore solution chemistry. By adding 85% slag, the pH value decreased to a level below 13.4 and tended to be reduced slightly in the long term owing to the continuous hydration of the slag.
2. The lowest chloride penetrability was found by incorporating 65% slag, whereby the pore structure was significantly refined, delaying chloride diffusion. Meanwhile, the pore solution chemistry, including OH\({}^{-}\) and SO\({}_{4}\)\({}^{2-}\), was diluted, together with a high amount of hydrotalcite, promoting chloride binding. The effect of slag addition on chloride binding behavior agrees well with the Langmuir isotherm.
3. Under drying-wetting conditions, a chloride peak presented in the skin layer of saturated specimens, while the peak was not obvious for unsaturated specimens. The specimens of lower saturated levels exhibited a higher chloride transport at early exposure to drying-wetting cycles. The effect of the initial saturation level on chloride transport became less pronounced as the exposure continuously proceeded.
4. Between slag and OPC binders, the difference in the chloride transport driven by capillary action gradually diminished with a decrease in the saturation level. The use of slag greatly enabled the promotion of the resistance to chloride ingression under drying-wetting cycles, in particular for long-term exposure.  
It is normally considered that higher fineness results in higher reactivity of slag powders. However, higher fineness will inevitably increase the amount of water needed to maintain fluidity, and slag powders react more drastically at an early age. Sudden hardening and cracking can then be expected in the cementitious system. A higher fineness of slag does not necessarily allow for a higher replacement level in cementitious systems. The hydration degree of slag tends to be decreased when more cement is replaced by slag, irrespective of the fineness. From a durability point of view, the hydration of slag to a higher degree should be emphasized when slag is used in marine concrete.  
Writing--original draft and investigation, W.L.; data curation and formal analysis, L.Y.; visualization and validation, W.J.; methodology and resources, H.D.; conceptualization, writing--review and editing and supervision, Y.Z. All authors have read and agreed to the published version of the manuscript.  
This research was funded by the Key Laboratory of Advanced Civil Engineering Materials (Tongji University), Ministry of Education (202102), the Qishan Scholar (Overseas Program) of Fuzhou University (GXRC-19023), the Natural Science Foundation of Fujian Province, China (2020)01482), the National Natural Science Foundation of China (51903047), and the Natural Science Foundation of Jiangsu Province, China (BK20191270).  
Not applicable.  
Data Availability Statement: The data presented in this study are available on request from the corresponding author.  
The authors are grateful to the Fujian Provincial University Research Center for Advanced Civil Engineering Materials and the Microlab of Delft University of Technology. Both provided guidance and support for the present work.  
The authors declare no conflict of interest.