Activation energy of chloride ion diffusion in C-S-H

Chloride diffusion through C-S-H, the primary binding phase in cementitious materials, plays a critical role in controlling corrosion initiation. The temperature dependence of the diffusion rate, described by activation energy E a , is essential for accurate service-life prediction, but remains poorly understood in C-S-H because the complexity of cement paste composite makes it difficult to isolate the role of C-S-H. In this study, C-S-H samples with controlled Ca/Si were prepared from hydrothermal synthesis and tricalcium silicate hydration. The diffusion activation energies were determined from results of chloride diffusion tests in the temperature range of 5–60 °C on chemically pure C-S-H monoliths with well-defined pore structure, where the effects of pore structure, Ca/Si and surface charge reversal were explored. The experimental results demonstrate that a finer pore structure with a higher fraction of nanopores produced by 800 MPa-compaction, results in a higher activation energy, whereas pore tortuosity shows little influence. The effect of the Ca/Si ratio is explained by the relative contributions of interfacial partitioning and intrapore diffusion, as described by the partition-diffusion model, in which chloride transport involves interfacial partitioning followed by diffusion within the pore: for C-S-H with lower Ca/Si, the more negatively charged surface increases the partition energy through stronger electrostatic repulsion at the pore entrance, whereas after surface charge reversal, intrapore ion-surface interactions become dominant. The overall activation energy is therefore determined by the balance between these two processes.