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The impacts of sodium nitrate on hydration and microstructure of Portland cement and the leaching behavior of Sr2

Immobilization and solidification of low-level and intermediate-level radioactive waste liquid from nuclear power station involves the interaction between highly concentrated sodium nitrate solution and cement paste. This study aims to clarify the influence of sodium nitrate on the hydration kinetics of cement matrices, hydration products and microstructure. The hydration heat evolution in the first 28 days are tested by calorimetry. The hydration products are examined by XRD and DTG while the structure of C-S-H is characterized by NMR, FTIR and XPS. The results indicate that sodium nitrate reduced the degree of hydration and reduced the degree of the polymerization of C-S-H gel. The C-S-H of solidified cement matrices that contained the highest concentration of sodium nitrate is predominantly composed of Q² and Q¹ Si, while more Q³ units are found in the matrices without sodium nitrate. The decomposition temperature of Ca(OH)₂ decreased with the increasing concentration of sodium nitrate solution because of the geometrical transformation of Ca(OH)₂. Sodium nitrate in solidified cement matrices mainly existed in the form of granular crystalline phase. The cumulative leaching fraction of Sr²⁺ increases with the increasing concentration of NaNO₃ solution.

Adsorption of cesium on cement mortar from aqueous solutions

The adsorption of cesium on cement mortar from aqueous solutions was studied in series of bench-scale tests. The effects of cesium concentration, temperature and contact time on process kinetics and equilibrium were evaluated. Experiments were carried out in a range of initial cesium concentrations from 0.0103 to 10.88 mg L(-1) and temperatures from 278 to 313 K using coupons of cement mortar immersed in the solutions. Non-radioactive cesium chloride was used as a surrogate of the radioactive (137)Cs. Solution samples were taken after set periods of time and analyzed by inductively coupled plasma mass spectroscopy. Depending on the initial cesium concentration, its equilibrium concentration in solution ranged from 0.0069 to 8.837 mg L(-1) while the respective surface concentration on coupons varied from 0.0395 to 22.34 μg cm(-2). Equilibrium test results correlated well with the Freundlich isotherm model for the entire test duration. Test results revealed that an increase in temperature resulted in an increase in adsorption rate and a decrease in equilibrium cesium surface concentration. Among several kinetic models considered, the pseudo-second order reaction model was found to be the best to describe the kinetic test results in the studied range of concentrations. The adsorption activation energy determined from Arrhenius equation was found to be approximately 55.9 kJ mol(-1) suggesting that chemisorption was the prevalent mechanism of interaction between cesium ions and cement mortar.