Abstract
Amongst all cement phases, hydrated calcium aluminate (AFm) plays a major role in the retention of anionic species. Molybdenum (Mo), whose 93Mo isotope is considered a major steel activation product, will be released mainly under the form of MoO42− in a radioactive waste repository. Understanding its fate is of primary importance in a safety analysis of such disposal. This necessitates models that can both predict quantitatively the sorption of Mo by AFm and determine the nature of the sorption process (i.e., reversible adsorption or incorporation). This study investigated the Cl−/MoO42− exchange processes occurring in an AFm initially containing interlayer Cl in alkaline conditions using flow-through experiments. The evolution of the solid phase was characterized using an electron probe microanalyzer and synchrotron high-energy X-ray scattering. All data, together with their quantitative modeling, coherently indicated that Mo replaced Cl in the AFm interlayer. The structure of the interlayer is described with unprecedented atomic-scale detail based on a combination of real- and reciprocal-space analyses of total X-ray scattering data. In addition, modeling of several independent chemical experiments elucidated that Cl−/OH− exchange processes occur together with Cl−/MoO42− exchange. This competitive effect must be considered when determining the Cl−/MoO42− selectivity constant.
Highlights
With more than seven billion cubic meters produced annually, cement is probably the most widely used material on Earth[1]
Amongst all cement phases, hydrated calcium aluminate (AFm) plays a major role in the retention of anions that enter into contact with cement-based materials
Understanding, and being able to model the mechanisms controlling these retention properties, requires a sound description of the sorption process from crystallographic to macroscopic scales. This methodological approach, which has been used successfully to study the mechanisms of ion adsorption by clay minerals[32,33,34,35,36], iron and manganeseoxides[37,38,39,40], and Mg-Al LDH41, has to date been seldom applied to cement phases
Summary
With more than seven billion cubic meters produced annually, cement is probably the most widely used material on Earth[1]. Understanding, and being able to model the mechanisms controlling these retention properties, requires a sound description of the sorption process from crystallographic to macroscopic (aqueous geochemistry) scales This methodological approach, which has been used successfully to study the mechanisms of ion adsorption by clay minerals[32,33,34,35,36], iron and manganese (hydr)oxides[37,38,39,40], and Mg-Al LDH41, has to date been seldom applied to cement phases. There have been no studies linking the determination of anion sorption sites, macroscopic geochemical selectivity constants, and the number and density of sorption sites involved in the retention mechanisms Such information is required to build robust predictive models for the binding and release of anions to AFm-Cl and, more generally, to cement-based materials.
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