膨润土水化裂缝演化过程的三维表征与量化

Three-dimensional characterization and quantification of hydration-induced cracking process of bentonite

关键词:  膨润土  水化裂缝  施工接缝  自封闭过程  微焦点计算机断层扫描

Key words:  bentonite  hydration cracking  technological void  self-sealing process  microfocus X-ray computed tomography

摘要:

为揭示高放废物深地质处置中膨润土在接缝自封闭阶段的水化开裂现象及其裂缝网络的时空演化规律,在径向接缝条件下针对压实膨润土开展了水化实验,并利用微焦点计算机断层扫描技术对不同水化时间下的膨润土进行了无损检测,基于三维重建图像对水化裂缝网络进行了定性和定量表征。结果表明:水化裂缝伴随着膨润土的膨胀变形经历了从起裂、扩展到闭合的完整演化过程；对水化裂缝的形态及几何参数进行统计后发现,其弗雷特直径和球形度分别集中分布在0.1~0.2 mm和0.6~0.8之间,可见水化裂缝网络由大量孤立的点状裂隙和少量连通的片状裂缝构成,虽然前者的数量超过95%,但后者的体积和表面积占比却达到了80%以上；在关于水化时间的对数坐标下,试样体积和接缝体积的线性变化表明膨润土的膨胀变形速率在水化过程中逐渐减缓,而膨胀速率、开裂速率和闭合速率在双对数坐标中随时间的线性递减以及膨胀速率始终高于开裂和闭合速率的特性则进一步揭示了膨胀变形与裂缝演化的内在联系,即膨胀变形既是水化开裂发生的前提,也是裂缝闭合的必要条件。通过对膨润土进行三维表征和定量分析,揭示了其水化裂缝的演化规律及发生机制,为缓冲/回填材料的设计和优化提供了参考依据。

Abstract:

To elucidate the hydration-induced cracking phenomenon and the spatiotemporal evolution of crack network in bentonite during the self-sealing process of technological voids in deep geological repository of high-level radioactive wastes, a hydration test was conducted on the compacted bentonite with a radial technological void. Microfocus X-ray computed tomography was utilized for detecting the bentonite non-destructively at different hydration time. Qualitative and quantitative characterizations of the crack network were performed after the three-dimensional reconstruction. The results demonstrate that the bentonite sample experienced an evolvement from crack initiation and propagation to crack closure.Statistical analysis of morphological and geometric parameters revealed that Feret diameter and sphericity were respectively distributed within the ranges of 0.1-0.2 mm and 0.6~0.8, indicating that the crack network was composed of a large number of dotted cracks in isolation (>95%) and a small number of flaky cracks of interconnection. Notably, the latter accountted for over 80% of the total volume and surface area.The linear relation between the logarithm of hydration time and both the sample and technological void volume suggested that the expansive deformation of bentonite gradually slowed down in the hydration process. Furthermore, the linear relations of swelling rate, cracking rate and closing rate versus time in double logarithmic coordinates, as well as the persistent dominance of swelling rate over cracking and closure rates, reflected an intrinsic connection between expansion deformation and crack evolution. Specifically, the expansive deformation served as both a prerequisite for cracking and a necessary condition for crack closure.Through three-dimensional characterization and quantitative analysis, this study revealed the evolutionary regularity and underlying mechanism of hydration-induced cracks, thereby providing critical references for the design and optimization of the buffer/backfill materials.