## 6 Conclusions  
This study reveals the influence of salt concentration on the compression characteristics and microstructure of marine sedimentary soil through one-dimensional compression tests and SEM microscopic analysis of reconstituted clay from the Yellow Sea with different NaCl concentrations, which in the range of 0-5%. The main conclusions are as follows:  
(1) At a NaCl concentration of 2.5%, the compressibility of the soil is significantly enhanced, with the maximum compression coefficient of 0.609 MPa\({}^{-1}\), the maximum compression index of 0.17, and the minimum compression coefficient of 6.5 \(\times\) 10\({}^{-4}\)-7.5 \(\times\) 10\({}^{-4}\) cm\({}^{2}\)/s. The response becomes more pronounced with increasing compression pressure. In the secondary compression stage, the salt effect gradually weakens, and the rebound characteristics are unaffected by the NaCl concentration. (2) SEM scanning test results show that at a NaCl concentration of 2.5%, the soil particles are angular, the pore fractal dimension reaches its maximum value of 1.09-1.14, and the particle fractal dimension reaches its maximum value of 1.18-1.23. The pores and particles tend to be arranged in the same direction. The pore anisotropy rate reaches its minimum value of 43%, and the particle anisotropy rate reaches its minimum value of 40%, corresponding to the lowest pore and particle orientation probability entropy. When the pore solution concentration is less than 2.5%, the soil sample develops narrow pores and rounded particles during one-dimensional compression. When the concentration of the pore solution exceeds 2.5%, the microstructure shows reverse evolution, confirming that the particle structure of the soil sample conforms to the particle rearrangement mechanism dominated by surface-to-surface contact under medium salinity conditions. (3) At the threshold concentration of 2.5%, a balance between electrostatic forces and attractive forces enables stable surface-to-surface contacts, maximizing compressibility. This nonmonotonic behavior underscores NaCl's dual role in both compressing the double electric layer to facilitate particle rearrangement and inducing aggregation through ionic shielding. Variations in NaCl concentration significantly influence the compressibility of marine clays. It is recommended that the influence of salinity sensitivity on the compression characteristics of marine sedimentary soil be considered in the design of marine foundations, with particular attention to the critical threshold effect of 2.5% NaCl concentration in Yellow Sea marine sedimentary soils to enhance the safety and reliability of the project. (4) Conventional geotechnical testing using deionized water significantly underestimates the compressibility of marine soils under fluctuating salinity conditions. The lack of saline ions in such tests alters soil microstructure and fabric development, leading to substantial deviations in key compression parameters (e.g., compression index C\({}_{\text{c}}\)) from in-situ behavior. These artifacts compromise the reliability of marine clay assessments for engineering design. Therefore, maintaining in-situ salinity conditions during sample preparation and testing is crucial. Further studies should investigate marine soil behavior through additional testing protocols, particularly triaxial shear and isotropic compression tests. Conceptualization, Y.Z., X.W. and M.H. ; methodology, H.X., X.W., S.W. and M.S. ; software, H.X., T.L. and S.W. ; validation, Y.Z., X.W., M.H. and M.S. ; formal analysis, H.X., X.W. and T.L. ; investigation, M.S. ; resources, M.S. ; data curation, Y.Z. and M.H. ; writing--original draft preparation, H.X., M.H. and S.W. ; writing--review and editing, Y.Z. and T.L. ; visualization, H.X. and S.W. ; supervision, M.H. and M.S; project administration, M.S. ; funding acquisition, M.S. All authors have read and agreed to the published version of the manuscript. This research was funded by \"Pioneer\" R&D Program of Zhejiang (No.2022C03009). Not applicable.