Advances on the knowledge of the buffer/backfill properties of heavily-compacted GMZ bentonite: Fid-Bau Portal

Advances on the knowledge of the buffer/backfill properties of heavily-compacted GMZ bentonite

Ye, Wei-Min / Chen, Yong-Gui / Chen, Bao / Wang, Qiong / Wang, Ju

AbstractGMZ bentonite has been selected as a potential material for the construction of engineered barrier in the Chinese program of geological nuclear waste disposal, for its high montmorillonite content, high cation exchange capacity (CEC) and large specific surface etc. Studies on mineralogy and chemical composition, mechanical properties, hydraulic behavior, swelling behavior, thermal conductivity, microstructure and volume change behavior of GMZ bentonite were performed from 1980s. Based on a review of the former studies, achievements on experimental and theoretic results obtained on compacted GMZ bentonite specimens including basic properties, thermal, hydraulic and mechanical behaviors are presented in this paper. Results show the thermal conductivity of GMZ bentonite and the bentonite-based mixtures influenced by its dry density, water content, mixture of other materials and degree of saturation etc. Water retention capacity of highly-compacted GMZ bentonite decreases as the temperature increases under confined and unconfined conditions. The hysteretic behavior in the water retention curves of the compacted GMZ bentonite is not so significant at 20 or 40°C. The unsaturated hydraulic conductivity of compacted GMZ bentonite under unconfined conditions is higher than that of under confined conditions. This is possibly induced by the difference in the mechanism of microstructural changes during hydration under different confining conditions. The compaction curves for GMZ bentonite with different dry densities are clearly step-phased. And the optimum water content for GMZ bentonite is about 15%. An exponential relationship between swelling pressure and dry density of highly-compacted GMZ bentonite was determined for the prediction of swelling pressure. Furthermore, the void ratio after swelling for unconfined sample also can be predicted using diffuse double layer (DDL) theory.

Research highlights► The thermal conductivity of compacted GMZ bentonite and the bentonite-based mixtures increase as the dry density and water content increase; for the same water content and dry density, the thermal conductivity of the mixture is higher than that of pure bentonite. When the saturation is higher than 20%, there is a linear relationship between the thermal conductivity and the saturation. ► The water retention capacity of compacted GMZ bentonite decreases as the temperature increases, regardless of confining conditions. At a certain temperature, the constraint conditions have little influence on the water retention capacity of the compacted bentonite at high suction, but the water retention capacity of the confined specimen is lower than that of the unconfined specimen at low suction. ► For the same suction, the unsaturated hydraulic conductivity of compacted GMZ bentonite under unconfined conditions is higher than that of under confined conditions. ► The dry density of GMZ bentonite increases when the compaction pressure increases, but the increase slows sharply when the compaction pressure is higher than 30 MPa. The optimum water content for GMZ bentonite is about 15%. ► The problems related to the influence of temperature on the behavior of the GMZ bentonite, the migration law of radionuclide in compacted bentonite and sealing properties of the GMZ bentonite-based materials under T-H-M-C coupled conditions, etc., should be the key issues to be explored in the future.