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

Abstract

Ion diffusion through cement-based barrier is key to safe radionuclide transfer in intermediate radioactive waste disposal facilities. The purpose of this research is to elicit the behavior of ionic diffusivity and pore structure of hardened cement paste specimens for the barrier system when exposed to high temperatures (up to 80°C) for long periods (up to one year). The cement paste is made of an ordinary Portland cement base- and a low-heat Portland cement / fly ash / limestone filler (LF)system. The results of the adsorption isotherms of N₂ and water vapor of the specimens exposed to limesaturated water of different temperatures, 20, 40, 50, 60, and 80°C, are presented, and their pore size distributions are analyzed by the Barrett-Joyner-Halenda method. The change of specific surface areas is measured and the influence of high temperature on the pore structure is discussed. Additionally, results of chloride ionic diffusivity test for the specimens after the high-temperature exposure are contrasted with the tendency of pore structure change. It is found that the LF system has much higher resistance to high-temperature exposure than ordinary Portland cement; however, temperatures greater than or equal to 60°C deteriorate the ionic diffusivity. These experimental results may indicate the ionic diffusivity of hardened cement paste can be influenced by the spatial electrical charge in micropores depending on the electrical charge of pore wall and the ion composition of pore solution.

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