Chloride Binding of Cement Estimated by Binding Isotherms of Hydrates

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Chloride binding behaviors of cement, which is important for estimating the chloride penetration into concrete, are investigated. Firstly chloride binding isotherms of major cement hydrates are examined and then a chloride binding iso-therm of cements is estimated from the binding isotherms of these hydrates. The results show that the monosulfate hydrate and C-S-H phases have significant chloride binding capacities, whereas ettringite and portlandite have no capacity to bind chlorides. However, there is a possibility that ettringite in cement paste dissolves at high chloride concentration to produce Friedel's salt. The chloride binding by monosulfate hydrate is attributed to the formation of Friedel's salt and the binding isotherm shows a good fit to a Freundlich-type adsorption. The chloride-binding isotherm of C-S-H shows a good fit to a Langmuir-type adsorption. The binding capacity of C-S-H is saturated at about 0.6 mmol/g at high chloride concentration over than 2 M. Based on the chloride binding isotherm of monosulfate hydrate and C-S-H, the chloride binding isotherms of cements are shown to be realistically predicted.
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