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 Q(2) and Q(1) Si, while more Q(3) 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.