Chloride binding mechanism in seawater-mixed UHPC

The rapid development of marine concrete structures and the sharp shortage of freshwater resources contribute to the wide investigation of seawater-mixed ultra-high-performance concrete (SWUHPC). However, few studies have investigated the chloride ions (Cl-) binding mechanism of SWUHPC. Herein, the chloride binding experiments and molecular dynamics (MD) simulation were carried out to reveal the physically and chemically bound Cl- mechanisms of SWUHPC. The results of the experiments clearly demonstrate that the addition of silica fume (SF) led to a significant decrease in the capacity of Cl- binding. Conversely, the incorporation of metakaolin (MK) resulted in a marked increase in the content of chemically bound Cl-. Furthermore, it is revealed through MD simulations that the amount of physically bound Cl- heavily depends on the Ca/Si ratio of C-S-H. A higher Ca/Si ratio results in a stronger electrostatic effect of the C-S-H surface on Cl-, which increases the physical binding of Cl- via Ca-Cl bonds. In addition, it is found that Al[6] and Ca in the interlayer region of C-A-S-H formed the main structure layer (Ca4Al2(OH)122+) of Friedel's salt, and then chemically adsorbed Cl- in the pore solution. These findings provide novel nanoscale insights regarding the physically and chemically bound Cl- mechanisms of SWUHPC.