Molecular Dynamics Modeling of Chloride Binding to the Surfaces of Calcium Hydroxide, Hydrated Calcium Aluminate, and Calcium Silicate Phases

Molecular dynamics computer simulations are performed to study the structure and dynamical behavior of chloride and associated cations at the interfaces between aqueous solutions and portlandite (Ca(OH)<sub>2</sub>), Friedel's salt ([Ca<sub>2</sub>Al(OH)<sub>6</sub>]Cl·2H<sub>2</sub>O), tobermorite (Ca<sub>5</sub>Si<sub>6</sub>O<sub>16</sub>(OH)<sub>2</sub>), and ettringite (Ca<sub>6</sub>[Al(OH)<sub>6</sub>]<sub>2</sub>[SO<sub>4</sub>]<sub>3</sub>·26H<sub>2</sub>O). These phases are important in calcium silicate and calcium aluminate cements and are models of important poorly crystalline cement phases. They are also representative of many hydrous hydroxide, aluminate, and silicate materials stable near room temperature and pressure. The MD simulations use a recently developed semiempirical force field and take into account the flexibility of surface OH groups and allow for energy and momentum transfer between the solid and solution to effectively simulate the sorption. The principal focus is on the structure at and near the solution/solid interfaces and on the molecular mechanisms of adsorption of aqueous Cl<sup>-</sup>, Na<sup>+</sup>, and Cs<sup>+</sup> ions on a neutral portlandite surface and comparison to the Cl<sup>-</sup> sorption behavior on the positively charged surface of Friedel's salt. Power spectra of molecular motions for bulk and surface species, diffusion coefficients for Cl<sup>-</sup>, Na<sup>+</sup>, and Cs<sup>+</sup> ions in different surface-related environments, and mean residence times on surface sites are calculated. Relative to the diffusion coefficients in bulk solution, those of Cl<sup>-</sup> in an inner-sphere surface complex are reduced about an order of magnitude, those in outer-sphere complexes are reduced less, and for both types the coeffcients are reduced more for Friedel's salt than for portlandite. No Cl<sup>-</sup> adsorption was observed on tobermotite, and little, on ettringite. The simulation results are in good qualitative agreement with experimental sorption and <sup>35</sup>Cl NMR studies. The MD results provide further confirmation that chloride binding on C−S−H, which is the most abundant phase in many cements, can be thought of as due to sorption on surface sites similar to those on portlandite.