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Molecular Dynamics Modeling of Chloride Binding to the Surfaces of Calcium Hydroxide, Hydrated Calcium Aluminate, and Calcium Silicate Phases
journal contribution
posted on 2002-07-23, 00:00 authored by Andrey G. Kalinichev, R. James KirkpatrickMolecular 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)2), Friedel's salt ([Ca2Al(OH)6]Cl·2H2O), tobermorite (Ca5Si6O16(OH)2), and ettringite (Ca6[Al(OH)6]2[SO4]3·26H2O). 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-, Na+, and Cs+ ions on a neutral portlandite surface and comparison to the Cl-
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-, Na+, and Cs+
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- 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- adsorption was observed on tobermotite, and little, on
ettringite. The simulation results are in good qualitative agreement with experimental
sorption and 35Cl 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.