Hydraulic and physicochemical properties of dense thin bentonite layers permeated with variably mixed alkaline solutions of KOH and CaCl₂ at various temperatures (25–75 °C) for 3 years

Hydraulic conductivity tests were performed using mixed alkaline solutions of KOH and CaCl₂ (pH ∼12) on thin Na-bentonite layers under various temperature conditions (25–75 °C) for 3 years. For dense thin Na-bentonite (dry density of 1.12 Mg/m³) in a mixed alkaline solution of 0.03 M KOH and 0.03 M CaCl₂, the hydraulic conductivities at 50 °C and 75 °C were approximately 10 times higher than that at 25 °C. The bentonite samples permeated with the mixed solution at 50 °C and 75 °C achieved almost complete cation exchange of Na ions by Ca and K ions. However, only slight cation exchange occurred in the bentonite specimens permeated at 25 °C, regardless of the type of permeant. The free swell index of the reacted bentonite permeated with a mixed solution of 0.03 M KOH and 0.03 M CaCl₂ significantly decreased at 50 °C and 75 °C compared with that at 25 °C. X-ray diffraction analysis revealed that the decreases in the relative intensities of the peaks of accessory minerals, such as opal-cristobalite/tridymite, quartz, and feldspar, were enhanced at 50 °C and 75 °C.