Characterisation of concrete, mortar and calcium silicate hydrated phases (CSH) and thorium retention analyses by ion be
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Characterisation of concrete, mortar and calcium silicate hydrated phases (CSH) and thorium retention analyses by ion beam techniques. Ursula Alonso1, Tiziana Missana1, Miguel Garcia-Gutierrez1, Henar Rojo1, Alessandro Patelli2, Valentino Rigato3, Daniele Ceccato3 1 CIEMAT, Avda. Complutense 40, Madrid 28040, SPAIN 2 CIVEN, Via delle Industrie 5, Venezia-Marghera 30175, ITALY 3 LNL-INFN, Viale dell’ Università 2, Legnaro-Padova 35020, ITALY ABSTRACT Cement-based materials, like concrete and mortar, are widely used in radioactive waste repositories. A deep characterization of these heterogeneous materials, and of their main phases, is necessary to evaluate their capability of retaining critical radionuclides (RN). In this study, the ion beam technique micro- Particle Induced X- Ray Emission (PPIXE) is used to characterize the concrete and mortar used in the Spanish low level waste repository. Two calcium silicate hydrate (CSH) phases with different Ca/Si ratio are also studied, because they are known to be amongst the most relevant phases, formed upon cement hydration, that retain RN. The retention of thorium on the above mention materials, as relevant tetravalent actinide, is also analyzed. Results are compared with Scanning Electron Microscopy- Energy Dispersive X-Ray Spectroscopy (SEM-EDX) analyses. INTRODUCTION Cement-based materials, like concrete and mortar, are used as major components in the barriers of low-level and high-level radioactive waste repositories, because they are well suited to retain radionuclides (RN), both by physical and chemical processes [1-4]. To assess the long-term safety of cement barriers, the transport or retention of critical RN have to be studied. Cements are heterogeneous materials and RN migration would be conditioned by their heterogeneity and by the presence of trace minerals as well. Moreover, upon hydration, different cement phases are formed; calcium silicate hydrate phases (CSH) are known to be amongst the most relevant to retain RN by sorption [5 and references there in]. In this study, the ion beam technique micro - Particle Induced X- Ray Emission (PPIXE) is selected to characterize the mortar and concrete used in the Spanish low level repository. Two different CSH phases with different Ca/Si ratio (1.7 and 0.9) were also studied. In addition, the retention of thorium on these materials was analyzed, as representative of tetravalent actinides and as heavy element well detectable by ion beam techniques. PPIXE technique is selected because it allows obtaining quantitative elemental composition, because it includes a scanning system that provides mapping of studied areas (Pm2mm2), very useful for characterizing heterogeneous materials [6]. A similar methodology was preliminary tested to quantify Cs and I surface retention, at a mineral scale on solid concrete samples [7]. The high sensitivity of PPIXE allows cement phase deep characterization, detection of trace elements and it is expected to identify preferential areas where RN are retained.
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