ABSTRACT
In this study we examine the microstructures of reconsolidated rock salt after rapid pre-compaction to a porosity of 5%, followed by stress relaxation testing. Synthetic NaCl powder was used for the deformation experiments. Nominally dry material was exposed to laboratory air during stress relaxation at a nominal porosity of about 5%. The relaxation behaviour of this material could be described by a power law with an (apparent) n value of ∼5. The microstructure of polished and lightly etched material was characterized using scanning electron microscopy (SEM) and electron back scattered diffraction (EBSD). A large number of grains were strain free, although some extensively deformed grains were also present. Intra- and intergranular porosity, grain indentations, and channel-island grain boundary microstructures, were also observed. These microstructural features confirm that, in addition to crystal plastic flow, several other mechanisms were operating during reconsolidation. In particular, extensive recrystallization by fluid activated grain boundary migration had occurred. When recrystallization occurs, the strained material is reset to an undeformed state. In view of this, and the drastic work hardening flow that must have occurred during pre-compaction, stress relaxation did not involve steady state deformation. Other mechanisms such as pressure solution may also have operated.
