强碱性溶液扩散对高庙子膨润土的化学腐蚀

Corrosion of GMZ bentonite by diffusion of strong alkaline solution

高放废物地下处置库中,地下水腐蚀衬砌混凝土析出的强碱性溶液,会扩散进入膨润土缓冲屏障内部,引起屏障性能的退化。设计专门的扩散试验单元,模拟KOH强碱性溶液在压实高庙子膨润土试样内部的扩散过程,研究辐射升温条件下膨润土中蒙脱石矿物含量及微结构变化,为我国高庙子膨润土缓冲屏障长期性能评价提供依据。设计24组正交试验,对初始干密度为1.80 g/cm3的压实膨润土开展KOH扩散试验,工况组合条件为:pH值为12.6,13.0和13.5,环境温度T为30℃和60℃,反应时间t为7,14,21和28 d。扩散试验终结之后对试样开展X射线衍射(XRD)和扫描电镜(SEM)试验。试验结果证实,高庙子膨润土中蒙脱石矿物溶解损失,与溶液pH值和试验温度的升高成正相关关系,显著溶解发生在接触带2 mm以内,最严重的情形下蒙脱石含量从初始的44.4%减少到25.9%。蒙脱石矿物的溶失可能与蒙脱石水化产生的羽翼状胶体的溶解有关。碱溶液对膨润土结构的损伤程度随扩散深度减缓,表观孔隙率逐渐降低。接触界面上肉眼观察到腐蚀裂隙;接触带0~4 mm范围内,膨润土团聚体结构遭受一定的破坏;大于4mm的范围,微结构形态基本保持不变。关于高庙子膨润土缓冲屏障长期性能的化学演化,今后应关注溶液化学性质、水力条件及固液反应时间等因素的影响。

In the underground repository of high-level wastes,strong alkaline cementitious material dissolved by groundwater from the lining concrete will penetrate into bentonite buffer,resulting in a certain degradation of the performance of the buffer barrier. A diffusion cell was particularly designed to simulate the diffusion process of KOH solution into compacted GMZ bentonite specimens,so to investigate the possible changes in clay minerals and microstructure under elevated temperature for the purpose of long-term evaluation of GMZ bentonite buffer in China. Totally,24 sets of KOH diffusion tests were conducted on bentonite specimens with an initial dry density of 1.80 g/cm3,covering combinations of pH = 12.6,13.0 or 13.5,temperature of 30 ℃ or 60 ℃ and reaction time of t = 7,14,21 or 28 d. As soon as diffusion tests terminated,bentonite specimens were disassembled to carry out X-ray diffraction(XRD) and scanning electron microscopy(SEM) analysis. Research results verify that the dissolution loss of montmorillonite in bentonite has a positive correlation with pH and temperature and that the dissolution mainly concentrate within 0-2 mm from the contact surface with a max drop from original 44.4% to 25.9%. The alkaline dissolution of wing-like gels generated originally during bentonite hydration is responsible for the montmorillonite dissolution. Bentonite microstructure damage resulted from alkaline solution moderates with increasing the diffuse depth from the contact surface,accompanied by a decrease of the apparent porosity. Macro fissures on the contact surface are visible by naked eyes,aggregate destroy occurs within 0-4 mm from the contact surface but the microstructure almost keeps changeless while deeper than 4 mm. With respect to the long term chemical evolution of bentonite barrier,further attention should be paid to solution chemistry,hydraulic condition and time scale of mineral-solution reactions.