The chloride (Cl^-) binding capability of calcium silicate hydrate (C-S-H) plays an important role in the chloride resistance and, consequently, the service life of concrete used in marine and deicing environments. The synergistic regulation between geometric confinement and electrostatic interactions and its dependence on the Ca/Si ratio remain insufficiently resolved although the existing studies examine either pore structure (accessibility and transport) or interfacial electrostatics (electrical double layer, EDL). This gap is addressed via establishing a coupled pore structure-EDL framework calibrated against a controlled C-S-H series at different calcium-silicon (Ca/Si) ratios, thereby transforming qualitative descriptions into the mechanism-based quantification of chloride adsorption. In this study, systematic measurements and mathematical descriptions were performed to clarify the C-S-H pore structure, surface potential distribution, and chloride binding capacity. Effects of the Ca/Si and chloride concentrations were investigated. The dynamic competition mechanism between pore-scale accommodation and EDL-mediated electrostatic partitioning was quantitatively characterized.

C-S-H gels with different targeted Ca/Si ratios (i.e., 0.6, 1.0, 1.4, and 1.8) were prepared via chemical precipitation with sodium silicate and calcium nitrate in deionized water. After homogenization, the suspensions were adjusted to pH value of ≥ 12 by using 5 mol/L NaOH solution and then transferred to a polytetrafluoroethylene (PTFE)-lined autoclave and crystallized isothermally

at (60 ± 1) ℃ for 14 d. The product was recovered by vacuum filtration using a Büchner funnel, washed for three times with deionized water, and dried in a vacuum oven at 50 ℃ for 72 h. The dried solid was then ground in a ball mill, passed through a 200-mesh sieve, and stored in a desiccator.

The total pore volume and pore-size distribution (PSD) of the C-S-H gels with varying Ca/Si ratios were determined by nitrogen sorption. The characteristic pore diameter and the gel pores below 10 nm, which were most relevant for transport and EDL overlap in C-S-H clusters, were measured. The Zeta potential (ζ) was measured in alkaline media (pH value of 12) over ionic strengths representative of pore solutions (e.g., 0.1-0.5 mol·L^-1 NaCl). The batch adsorption was conducted via equilibrating C-S-H powders in NaCl solutions at (20.0±0.1) ℃ for 14 d. Supernatants were filtered and analyzed by ion chromatography to determine equilibrium Cl^- concentrations, and adsorption capacities were calculated by mass balance and normalized to solid mass. The Gouy-Chapman-Stern (GCS) framework was implemented in cylindrical pores to couple measured ζ at different pore sizes, explicitly accounting for the Stern layer, diffuse layer, the Debye length, and the EDL overlap in nanoscale. The steady-state Poisson-Boltzmann/Nernst-Planck system was solved to obtain spatial electrostatic potentials and local Cl^- concentrations. The total adsorption was computed via integrating the EDL-related concentration profiles at different pore sizes. The model predictions were compared and validated by the experiments.

The C-S-H specimens with varying Ca/Si ratios normally exhibit unimodal gel-type porosity with dominant mesopores of 3-5 nm, accompanied by a tail extending into larger sizes. Increasing Ca/Si leads to densification, and the cumulative volume of gel pores below 10 nm decreases from approximately 0.11 cm³·g^-1 to 0.04 cm³·g^-1 when Ca/Si increases from 0.6 to 1.8. The zeta potential becomes less negative with increasing Ca/Si ratio, changing from -17 mV (at Ca/Si ratio of 0.6) to -10 mV (at Ca/Si ratio of 1.8) in the case of 0.1 mol·L^-1 NaCl. As electrolyte concentration increases from 0.1 mol·L^-1 to 0.5 mol·L^-1, and |ζ|decreases due to the classical EDL compression. The Debye lengths in the 0.3-1.0 nm imply a substantial EDL overlap within the characteristic gel pores (3-5 nm), eliminating the notion of an electroneutral pore core and promoting field-driven partitioning.

The role of Ca/Si ratio in chloride binding behavior reveals a composition-dependent balance. At a low Ca/Si ratio (i.e., 0.6), the large pore volume favors chloride accommodation, while the highly negative surface potential causes a great electrostatic exclusion of Cl^- near the C-S-H walls. Within confined gel pores, overlapping EDLs shift Cl^- towards the pore center and reduce near-surface concentrations, limiting net chloride uptake despite abundant void space. At an intermediate Ca/Si ratio (i.e., 1.0), the pore volume remains high, while |ζ| is moderated, yielding the most favorable balance between geometric access and electrostatic capture. The modeled profiles and measurements both indicate the maximum adsorption capacity at this intermediate Ca/Si composition, consistent with a synergy between transport accommodation and weakened repulsion. At a high Ca/Si ratio (i.e., 1.8), weaker repulsion can, in principle, increase the interfacial adsorption efficiency per unit area, but the gel densification causes a sharp loss of accessible pore volume (i.e., 0.04 cm³·g^-1), becoming a dominant limiting factor for the reduced overall Cl^- uptake.

A model-experiment agreement is established, demonstrating the non-linear Freundlich-like increase of chloride adsorption with solution concentration. The measured ζ and PSD provide a physically interpretable parameter set for quantitative prediction of Cl^- binding capacity of C-S-H. The densification (at a high Ca/Si ratio) or electrostatic exclusion (a low Ca/Si ratio) can dominate Cl^- adsorption, depending on the composition-controlled balance of the effects. The results identify an intermediate composition window that maximizes Cl^- binding via leveraging moderate surface charge and mesoporous access.

Chloride uptake by C-S-H could be controlled by a dual mechanism, i.e., with gel pore geometry dictating accommodation and transport, while EDL electrostatics governing near-surface adsorption. A low Ca/Si ratio (i.e., 0.6) provided large gel pore volumes but suffered great electrostatic exclusion. A high Ca/Si ratio (i.e., 1.8) weakened a repulsion but substantially densified the gel, resulting in a limited adsorption. An intermediate Ca/Si ratio (i.e., ≈1.0) achieved the optimum synergy between pore accessibility and moderated EDL effects, resulting in a peak Cl^- adsorption. The pore structure-EDL coupling framework proposed in this work could quantitatively link pore sizes and surface potential distribution to chloride adsorption capacity and provide actionable targets for binder design to enhance chloride resistance in service. Cementitious binders that preserved a mesopore network enriched in 3-5 nm pores and moderated surface charge exhibited a superior chloride binding capacity.