Hydro-mechanical-chemical Properties of Na-bentonite Permeated with Alkaline Solutions

Bentonite is considered in many countries as a buffer material for the engineered barrier system (EBS) in deep geological high-level radioactive wastes (HLRWs) repositories. Understanding the thermal-hydro-mechanical-chemical (THMC) processes of the bentonite is essential for assessing the long-term stability of the EBS. In this study, the THMC properties of Wyoming sodium (Na) bentonite were experimentally determined. Column reaction tests on the compacted Na-bentonite specimens under different temperature conditions (25-75°C) were conducted using variably mixed KOH and Ca(OH)2 solutions as permeate solutions to simulate alkaline groundwater. Na-bentonite powder with a water content of 20% was placed in a metallic mold with a diameter of 5.9 cm and a thickness of 2.0 cm and then compacted to the dry density of 1.1 g/cm3 by using a hydraulic pump. Afterwards, the variably mixed KOH and Ca(OH)2 solutions were permeated into the compacted Na-bentonite specimens to determine hydraulic conductivity of the specimens. After permeation for more than 3 years, the column tests were terminated and the reacted Na-bentonite specimens were characterized. The hydro (water retention capacity), mechanical (swelling pressure and uniaxial compression strength), mineralogical (mineral components), and chemical (exchangeable cations, soluble salts, and cation exchange capacity) properties of the reacted bentonite specimens were determined. The results of this study suggest that the combined THC processes may affect HMC properties of compacted Na-bentonite at a given test period (approximately 3 years).