Optimal curing humidity for compacted bentonite-sand mixtures

Abstract

The buffer blocks used in deep geological repositories of high-level radioactive wastes (HLW) should be cured in an appropriate environment to prevent the deterioration of buffer blocks such as desiccation shrinkage and cracking. In this study, the bentonite-sand mixtures with different initial moisture contents (11.23-21.63%) were compressed in the laboratory to simulate the production of buffer blocks. The bentonite-sand mixtures were cured at different relative humidity (RH) of 33%, 75%, 85% and 100% in order to find the optimum curing humidity. During the curing process, the periodical mass change was weighed by a balance, the size change was measured by a vernier caliper, and the thermal conductivity was tested after the curing was balanced. The test results indicate that the moisture variation of bentonite-sand mixtures during curing was consistent with the soil-water characteristic curves (SWCCs) of the mixtures. When the compacted bentonite-sand mixtures were cured at the relative humidity of 33%, 75% and 85%, the samples were dehydrated and became dried, resulting in desiccation shrinkage and cracking. When the relative humidity was100%, the mixtures with a lower initial moisture content of 11.23-14.99% tended to absorb moisture from the environment and swelled. While the mixtures with a higher moisture content of 17.22-21.63% were desiccated to shrink, but no obvious cracks were observed on the surface of these cured specimens. Under the curing condition of RH =100%, the variation of water content and volume of the sample with an initial water content of 17.22% was the minimum, which was considered as the optimum curing humidity for buffer blocks. The optimal curing conditions (optimum RH) of industrial-scale buffer blocks used in the disposal repository can be estimated by the SWCCs of the small buffer blocks, and the development of drying shrinkage crack can be quantitatively evaluated by the thermal conductivity of buffer blocks.

Keywords

• high-level radioactive waste (hlw)
• bentonite-sand mixture
• buffer block
• curing
• relative humidity