Image processing of fabric evolution in granular salt subject to diffusive mass transfer

Because of its favorable creep properties and low gas permeability, salt rock is viewed as an attractive host medium for nuclear waste disposals and natural resources storage. Under high stress and temperature conditions, diffusive mass transfer in salt rock can result in crack rebonding and strength recovery. In order to track the evolution of voids between salt crystals with lower load levels but higher healing rates than what is practically encountered in underground storage, we carried out creep loading tests on table salt. We used different loading conditions and inclusion materials to study the potential recurrence of topological patterns at grain boundaries. We developed a dedicated multi-stage image processing procedure to enhance microscopic image quality, and presented a slicing method to track the evolution of microstructure in different sections of the sample. This allowed us to analyze not only the evolution of average void size and orientation, but also the evolution of the fabric. We found that creep deformation is due to pore shrinkage along a diagonal direction across the sample, without significant grain rearrangement. It was noted however that basalt and sand inclusions rotated during the first 136 days of the creep tests. The proposed image processing techniques presented herein are expected to provide a methodology to track the evolution of microstructure descriptors that can be used to define alternative fabric tensors in thermodynamic models.