Calcium silicate hydrate (CSH) studies

This project aims at determining adsorption onto and co-precipitation processes with CSH phases as possible immobilisation mechanisms for radionuclides in the near field and in the alkaline disturbed zone (pH plume). The studies are performed in a pH range between 11.5 and 13.3, simulating the chemical conditions prevailing during the first and second stage of the cement degradation.

The project is carried out within the framework of a co-operation with and partially funded by CRIEPI and contributes to the ECOCLAY II project, partially funded through the 5^th EU framework programme. The project comprises experimental studies with Sr(II) and Eu(III) for the ECOCLAY II project, as well as investigations with Th(IV) and U(VI) for the joint CRIEPI-PSI project.

5.4.1 CSH synthesis and characterisation

In the last year CSH phases (C:S mol ratios of 1.07, 1.29, 1.5 and 1.82) synthesized in MilliQ water were characterised by chemical analysis of the bulk composition and the equilibrium solutions as well as by quantitative XRD. The XRD measurements were carried out in collaboration with the Laboratory of Environmental Geotechnics and Clay Mineralogy (Swiss Federal Institute of Technology, Zürich). The well-characterized materials were then used in the sorption and co-precipitation studies.

The synthesis of CSH phases was conducted in water because it was demonstrated that, under ACW conditions, CSH phases with target C:S mol ratios above 1.29 cannot be prepared in a pure form. The CSH phases are always contaminated with Ca(OH)2, and thus, have effective C:S ratios significantly below the target C:S ratios. The results from the characterisation studies show that the Ca solubility of the CSH phases synthesized in MilliQ water is a factor of 10 higher than the Ca solubility of the same phases synthesized in ACW. By contrast, the Si solubility is a factor of 5 to 10 lower. The XRD measurements confirm that in all cases a CSH phase is formed. Furthermore, all samples are free of Ca(OH)2 except the sample with the highest C:S ratio, which contains 7.7 wt% of Ca(OH)2. Based on this finding, the corresponding effective C:S mol ratio of this CSH phase was estimated to be 1.6.

Table 5.1: Characterisation of CSH phases synthesized in MilliQ water (pH =

5.4.2 Studies of the sorption and co-precipitation of Sr(II) and Eu(III)

Investigations on the Sr(II) sorption kinetics and measurements of the sorption isotherms as well as desorption tests have been brought to a close. These experiments were conducted on CSH phases with different C:S ratios under ACW conditions. In addition, a series of sorption and co-precipitation experiments were performed using CSH phases synthesized in MilliQ water (pH = 12 - 12.5).

The results corroborated previous findings that, under ACW conditions, the sorption and co-precipitation processes are fast, and give rise to constant Rd values after equilibration for approximately three days. Sorption and co-precipitation isotherms are linear over the entire concentration range, and the Rd values deduced are similar.

Slightly increasing Rd values with increasing number of ACW replacements were obtained from sequential desorption tests. Nevertheless, uncertainty on the data also increases with decreasing Sr inventories in these systems, and thus, the significance of the increase in the Rd value might be in question. In the case of the sorption and co-precipitation experiments at lower pH value, the isotherms are also linear, and the Rd

values obtained are similar. In this case, however, sequential desorption tests result in Rd values similar in value to those determined from the sorption or co-precipitation experiments, indicating reversibility of the uptake process.

Furthermore, it was observed that the Rd values of Sr decrease with increasing C:S ratio in both sorption and co-precipitation experiments (Figure 5.2a). This decrease in the Rd

value further coincides with an increase in the Ca concentration in the equilibrium solution (Figure 5.2b). This finding implies a competition between Ca and Sr for

sorption sites on the CSH phases. An attempt will be made to further assess the effect of the Ca concentration in connection with the high Na and K concentrations under ACW conditions on Sr uptake by modelling the data with the GEMS code.

Part of the above work was presented at the MRS meeting in Kalmar, Sweden, and will be published in the MRS proceedings (TITS et al. 2003b).

Sorption studies of Eu indicate strong uptake by CSH phases (Rd = 5·10⁵ L kg^-1 to 5·10⁶ L kg^-1). In the last year the sorption studies were complemented by some desorption tests. The Rd values obtained from these tests were found to be similar in value to those determined in the sorption experiments. Nevertheless, any detailed interpretation is limited since the measured activities are extremely low after desorption.

Fig. 5.2a: Dependence of the Rd values of Sr on the C:S ratio of CSH phases from sorption and co-precipitation experiments in ACW. The solid line is added to guide the eye.

Fig. 5.2b: Dependence of the aqueous Ca equilibrium concentration on the C:S ratio of CSH phases. The solid line is added to guide the eye.

5.4.3 Studies of the sorption and co-precipitation of Th(IV) and U(VI)

Sorption and co-precipitation studies of Th(IV) and U(VI) on CSH phases prepared using the CRIEPI method for CSH synthesis have been conducted. Synthesis of the CSH phases is described in FUJITA et al. (2003). The CSH phases were prepared and the experiments started during a stay of T. Fujita at PSI in July and August 2002. PSI staff carried out the follow-up of the experiments (sampling, activity measurements, data compilation) and finished the experimental work.

The experimental studies consisted of a series of sorption kinetic tests and co-precipitation tests of Th and U on CSH phases having different C:S ratios. The CSH phases were synthesized in ACW and in water. Fig. 5.3 shows selected results from U uptake studies carried out at pH ~12 (CSH phases synthesized in water).

Fast kinetics was observed for both processes, i.e., co-precipitation and adsorption, and equilibrium conditions were established within a few days. It is worth mentioning that both processes lead to similar Rd values, indicating that the same process controls U immobilisation. It is to be noted that these conclusions also hold for the Th/CSH system. The same type of experiment is currently being performed on CSH phases which were prepared using the PSI method for CSH synthesis. A direct comparison of the two data sets will enable us to assess the influence of CSH preparation and differences in the C:S ratios of CSH phases on sorption and co-precipitation processes.

Furthermore, XAFS measurements on the U/CSH systems have been carried out at the Rossendorf Beamline (ROBL) at the European Synchrotron Radiation Facility (ESRF), Grenoble, France, complementing the above wet chemistry studies. The CSH phases were prepared using the CRIEPI method for CSH synthesis and loaded with U. Data analysis is ongoing.