Insights into the effect of NaOH on the hydration products of solidified cement-NaNO3 matrices and leaching behavior of Sr2.

Abstract The low-level and intermediate-level radioactive waste is generally an alkaline solution with high concentration of sodium salts. Moreover, extra NaOH will be added into the matrices when the high content of mineral admixtures is used in the preparation of solidified cement matrices. This study mainly researches the influence of NaOH on the structure of C-S-H gels and the leaching rate of Sr2+ in the solidified cement matrices by Isothermal Conduction Calorimetry, X-Ray Diffractometry, Thermogravimetry, 29Si Nuclear Magnetic Resonance, Mercury Intrusion Porosimetry and Inductively Coupled Plasma. The results show that the addition of NaOH accelerated the hydration of cement in the early age but decreased the normalized hydration heat until 28 d. Also, the fraction of total Ca(OH)2 decreased with the increasing content of NaOH. Moreover, NaOH promoted the formation of C-S-H gels along the main chain by increasing the mean chain length but decreased the Q3 sites. However, the addition of NaOH has optimized the pore structure through reducing the porosity and median pore diameter. At last, the cumulative leaching fraction of Sr2+ at the NaOH content of 10% decreased by 17.5% compared to the NaOH content of 0.82%. Based on the results, it can be concluded that NaOH enhanced the adsorption capacity of C-S-H gels to Sr2+ and improved the immobilization performance of solidified cement matrices.