Volume and phase changes of swelling sulphates revealed through multitemporal micro-CT imaging

This study has the main purpose of investigating the micro-mechanisms involved in the expansion of anhydrite, a phenomenon that often occurs during excavations and may cause serious technical problems to civil tunnelling and other infrastructures (e.g., buildings, bridges, underground caverns). For this purpose, we developed an experimental investigation aimed at observing microscale changes occurring in anhydrite samples during water immersion by means of X-ray Computed Tomography (CT).  

We prepared six cylindrical specimens (diameter 10 mm and height 20 mm) of Triassic anhydrite from the Western Alps. Each of them was wrapped with an impervious cellophane sheet and partially submerged in calcium-sulphate saturated water. The upper surfaces were left in direct contact with air to force the water to cross the specimens. The specimens were put in water on the 15th of December 2023. Then, they were periodically scanned – over a total period of 1 year – through X-ray tomography using the CoreTOM scanner (TESCAN XRE) at Ghent University Centre for X-ray Tomography (UGCT), with a voxel size of 10 µm, as an EXCITE TNA project.

The elaboration of CT scans allowed to evaluate the volume and phase changes occurred during the test. All the specimens consistently showed a total expansion between 2% and 3% in volume. In addition, the multitemporal scans were examined using an algorithm of Digital Volume Correlation to deepen the mechanisms driving the expansion by visualizing the position where the expansion prevalently occurred.

The improved knowledge of the mechanism driving the process of expansion in anhydrite provides elements for a more accurate forecasting of the entity and the times of the phenomenon in real contexts (e.g., civil tunnelling, mining, slope stability, underground energy storage), driving to the possible refinement of existing constitutive models.