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The structure of C-S-H can change due to the hydration and polymerization reactions under the influence of temperature. Consequently, changes in the microstructure influences the ionic diffusivity in the hardened cement. Evaluation of ionic diffusivity is also a key issue for the design and assessment of a radioactive waste disposal system with a cement-based barrier. Especially, the influence of high temperature on the performance of a cement-based barrier should be thoroughly examined to build a disposal system with high reliability and safety. The temperature of the action depends on the actual concentration of exothermic radionuclides such as cobalt-60 in nuclear waste, assumed to be around 60°C below the design temperature, to prevent excessive influence of temperature on the disposal system. The duration of the exposure to the high temperature can extend over several decades or hundred years. However, until now, few studies have reported on the influence of long-term high temperature on the ionic diffusivity of hardened cement. Therefore, it is necessary to gain deeper understanding on the behavior of the hardened cement under the high temperature. The purpose of this study is to investigate the ionic diffusivity of hardened cement exposed to high temperatures over a long period. From the results of the investigation, we discuss the effectiveness of a prime candidate material for the disposal system.

Ionic Diffusivity and Pore Structure of Hardened Cement Paste Exposed to High Temperature Environment for Long Period