Spectroscopic Investigation of Cement Hydrate Phases and Their Chloride Binding Properties
The chloride binding capacity depends on the structure and surface properties of the individual hydrates. Portlandite and AFm surfaces have high affinity for chloride due to their positive zeta-potentials. For C-S-H samples, the chloride binding capacity increases with increasing C/S ratio due to increased numbers of surface Ca-OH sites and decreased polymerization of silicate chains. Aluminum substitution in C-S-H is unfavorable for chloride binding due to reduced layer charge and increased chain polymerization. For all hydrate phases the mechanism of chloride binding is not simply electrostatic attraction. Formation of metal-chloride clusters in the solution and sorption in the Stem layer may contribute substantially. The bound chlorides near the cement hydrate surfaces are in water solvated environments similar to those in the bulk solution and are in rapid exchange (>2kHz) with free chloride in the bulk solution. The chloride concentration near the surfaces is much higher than in the equilibrium bulk solution, and the reorientational frequency of the water molecules solvating the bound chloride is slower than those in the bulk solution. Chloride has a well-defined structural site in Friedel's salt. In solid solutions between Friedel's salt and hydroxyl-AFm, chloride occurs predominantly in Cl-rich domains except at high OH-contents.
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