INTERACTION OF ALKALIS (CS-) WITH CALCIUM SILICATES HYDRATES P. FAUCON*, T. CHARPENTIER**, P. HENOCQ*, J.C. PETIT*, J. VIRLET*, F. ADENOT***, *Service de Chimie Mol~culaire, CEA Saclay, 91191 Gifsur Yvette, France. "**Service de Physique de l'Etat Condense, CEA Saclay, 91191 Gifsur Yvette, France. ***Service d'Entreposage et de Stockage des D~chets, CEA Saclay, 91191 Gif sur Yvette, France ABSTRACT C-S-H of different Ca/Si ratios were synthesized in suspension. Cesium chloride (0.5 M) was put in contact with these C-S-H in reactors for 30 days at 25 'C with solution/solid ratios of 50. The quantities of cesium fixed by the C-S-H was determined by microanalyses and the mechanisms of retention in relation with the C-S-H structure was investigated by ' 33 Cs Nuclear Magnetic Resonance (NMR). The influence of the humidity yield in which the C-S-H were stored was also studied. At 100% of humidity, some cesium ions are trapped in the pore network of the C-S-H. However, most cesium ions are incorporated in the hydrated interlayers of the C-S-H between the silica chains of the structure. They are mobile in this interlayer space. At high Ca/Si ratio, the charge of incorporated Cs' ions in the structure should be compensated by the associated Cl- incorporation. At low Ca/Si, the presence of silanol groups in the C-S-H structure, which becomes closer to that of tobermorite (Ca/Si about 0.8), makes possible the exchange or substitution H'<->Cs'. The resulting retention of cesium in the C-S-H then becomes higher. Keywords: Cement paste, Cesium, C-S-H, retention, NMR, Chloride INTRODUCTION Cement-based materials may be used for various purposes (waste matrices, technological barriers, civil engineering structures, etc) both in interim storage surface sites and in deep-underground repositories for final disposal. The understanding of the retention mechanisms of radionuclides in the cement hydrates may thus be a key issue for any relevant modeling and performance assessment, in particular where cements are considered as waste forms. The poor retention of alkalis (e.g. Cs- ) by Portland cements constitutes a well known, major limitation for the use of this material. Therefore, we have studied the retention of Cs' on/in Calcium Silicate Hydrates (C-S-H), which are the main component of the ordinary Portland cement (these phases have a variable stoichiometry 0.66 < Ca/Si < 1.7). Recently, we reported the results of a preliminary study [1 ] where we used an initial 0.4 M CsCl solution. The maximum exchange capacity of the C-S-H was obtained at low Ca/Si molar ratio (Figure 1). Moreover, the use of a radioactive tracer (' 37 Cs) allowed to follow the kinetics of Cs fixation without collecting samples for analysis in the chemical reactor. This experiment has demonstrated that only 5 days were 551 Mat. Res. Soc. Symp. Proc. Vol. 506 0 1998 Materials Research Society