Deterioration mechanisms under combined low-amplitude thermal-fatigue and sulphate attack of concrete lining materials for underground thermal energy storage

Underground thermal energy storage (UTES) is an emerging decarbonisation technology for storing heat. However, the complex working environment with fluctuating temperature and potentially varying water chemistry may pose challenges to the lining materials, threatening the stability and durability of UTES infrastructure. This study establishes a thermo-chemical testing system integrating an in-situ monitoring platform to investigate the real-time deterioration process of UTES lining materials. An accelerated deterioration (i.e. strength decreased by 90% within 50 cycles/25 days) of lining mortar under low-amplitude thermal-fatigue and sulphate attack is observed for the first time to the best knowledge of the authors. Results show that specimens under the coupled action develop radial cracks and circumferential cracks with a delamination phenomenon at the circular plane surface and polygonal cracks at the curved surface, which are mainly developed during the cooling period. The deterioration mechanism is further explored by micro-CT, SEM-EDS, and ion concentration tests. Leaching and ettringite filling are considered as two main causes for the accelerated deterioration process. In one aspect, calcium leaching intensified by sulphate ions and heating process generates new defects and channels, promoting ion diffusion and weakening bond strength. In the other aspect, sulphate ions penetrate the specimen through pre-existing and leaching-induced defects, particularly within the outer 5 mm, and react with hydration products to form ettringite, thereby reducing porosity, increasing stiffness, and potentially mitigating shrinkage deformation. Theoretical and numerical analysis further demonstrate that the increased stiffness at the outer region of the specimen results in a higher hoop tensile stress at the edge during the cooling process, promoting the radial crack initiation. The difference of thermal expansion coefficients (i.e. shrinkage deformation) between inner and outer components further generates a radial tensile stress at the interface, leading to the formation of circumferential cracks and delamination. This new deterioration phenomenon and clarified mechanisms provide vital reference for assessing the stability and durability of UTES infrastructure.