Charge reversal at C-S-H surface/electrolyte interfaces: A mean-field molecular theory approach

Understanding the sorption of corrosive ions in cement requires a complete comprehension of charge reversal at the C-S-H/electrolyte interfaces. However, this charge phenomenon remains incompletely understood. We develop a mean-field molecular theory to revisit charge reversal behaviors by investigating how ions relax at the interfaces – whether bound or mobile – while considering ion–surface and ion–ion interactions. As a feature of our theory, we allow divalent calcium ions to adopt two binding configurations – bridging and non-bridging modes – with the ionized silanol sites, highlighting the necessity of multivalent ion condensation for charge reversal. Conversely, we demonstrate that the product of bulk concentration and the exponential of the electrosteric energies governs the accumulation of mobile ions at the interfaces, where a cancellation between them causes nonmonotonic behaviors for the mobile ions. In short, comprehending how ions compact the interfaces enables our theory to capture published experimental and simulation results, facilitating a deeper understanding of the charge reversal phenomenon.