钠钾盐梯度循环作用下高压实膨润土膨胀力衰变特性

高放废物深地质处置库近场环境中,高压实膨润土将长期遭受含盐地下水的循环化学作用,导致其膨胀性能不断衰变。针对干密度为1.7 g/cm3的高压实高庙子(GMZ)膨润土,分别开展了0.5 mol/L和1.0 mol/L两种盐梯度、Na Cl-水-KCl和Na Cl-水不同循环路径下的恒体积膨胀力试验,探讨盐梯度循环化学作用下其膨胀力的衰变特性。结果表明:盐梯度循环作用下,膨润土膨胀力的发展与离子种类、浓度和循环次数等因素有关。盐化阶段膨胀力不断降低,淡化阶段膨胀力有所提高;低盐度梯度循环下各阶段的稳定膨胀力均高于高盐度梯度循环时的膨胀力。随着循环次数的增加,稳定膨胀力逐渐下降,入渗溶液浓度越高,降幅越大,且衰减幅度随着循环次数的增加而减小。KCl溶液的入渗会引起膨润土发生矿物相变,膨胀力显著降低;当KCl溶液浓度达到一定值时,蒙脱石的充分溶解导致膨润土丧失膨胀能力。

Decay characteristics of swelling pressure of compacted bentonite under salinity gradient cycling

During the long-term operation of a deep geological repository of high-level radioactive waste (HLW), the bentonite barrier is likely to experience the chemical cycling paths of groundwater with various chemical components. In this case, the swelling capacity of bentonite can be reduced to a certain degree. In this work, GMZ bentonite powder is densely compacted to a dry density of 1.7 g/cm³ to carry out the swelling pressure tests under the constant-volume conditions. The salinization-desalinization effect is investigated by cyclical infiltrations of NaCl solution and distilled water. Furthermore, the cation exchange effect is emphasized by sequential infiltrations of NaCl solution, distilled water and KCl solution. For both infiltration paths, the salt concentrations (0.5, 1 mol/L) are taken into account. The results show that the swelling pressure characteristics of GMZ bentonite depend on the ion species and salt concentration of infiltration solution as well as the number of chemical cycles. The salinization effect of salt solution results in the decrease in swelling pressure, while the infiltration of distilled water improves the swelling behaviors. The swelling pressure decreases with the salt concentration of infiltration solutions. The stable swelling pressure and its reduced degree gradually decrease against the number of chemical cycles. Under the infiltration of KCl solution, some of the montmorillonite is dissolved, which results in the significant decrease in swelling pressure. No swelling pressure will be measured for GMZ bentonite if the concentration of KCl solution is higher enough.