The products transformation and leaching behavior in solidified cement matrices under the coupling actions of “water-heat-chemistry”

Abstract

Solidified cement matrices for immobilization of intermedium- and low-level radioactive waste stream will be subjected to the coupling actions of water, heat and chemistry at the near-surface disposal. This study mainly researches the products transformation and leaching behavior of Ca2+ and Sr2+ in solidified cement matrices under the coupling actions. The products are characterized through 29Si Nuclear Magnetic Resonance and X-ray Photoelectron Spectroscopy while the ion concentrations are examined by Inductively Coupled Plasma. The results show that ettringite and AFm change from crystal to amorphous phase while the coordination number of Al decreases from 6 to 4 under the coupling actions of high temperatures and NH4Cl concentrations. Meanwhile, the decalcification and polymerization of solidified cement matrices occurred in deionized water with high temperature (60 °C and 80 °C). However, the maximum degree of decalcification and polymerization was reached at 60 °C instead of 80 °C because of the precipitation of large NH4Cl crystals in the pores on the surface of solidified cement matrices. Furthermore, the diffusion of Sr2+ in solidified cement matrices was accelerated by the decalcification, and the apparent diffusion coefficients of Ca2+ and Sr2+ under coupling actions showed linear relationship. The corresponding results obtained in this study can provide the basic data to the further building of reaction-transport model in the immobilization of intermedium- and low-level radioactive waste stream by cement-based materials.