Effects of phase change materials on the freeze–thaw performance of backfill materials for geothermal energy piles in cold regions

This study evaluates the performance of phase change materials (PCM)-enhanced backfill materials under repeated freeze–thaw cycles (FTCs) for geothermal energy piles (GEPs) in cold region applications. Cementitious backfill materials containing PCM with a melting point of around 6 °C at three different contents (0%, 3%, and 5 wt%) were prepared and cured at 6 °C to replicate subsurface conditions in cold region climates such as Edmonton. The cured backfill material samples were subsequently subjected to 0, 14, and 28 FTCs to evaluate the influence of FTCs on thermal and mechanical performance. The findings exhibited that although FTCs adversely affect the performance of backfill materials—such as decreasing thermal conductivity (TC) and unconfined compressive strength (UCS)—the incorporation of PCM substantially mitigated these adverse effects. After 28 FTCs, the oven-dry TC decreased by 33.3% in PCM-free samples but by only 14.1% in the sample with 5% PCM. This mitigating effect enabled the 5% PCM sample to retain a higher TC than the PCM-free sample (1.76 vs. 1.54 W/m·K) after 28 FTCs. Additionally, after 28 FTCs, the UCS of 5% PCM sample decreased by 21.3%, compared with 35.8% for the PCM-free sample. Furthermore, differential scanning calorimetry (DSC) results showed that PCM-enhanced samples retained over 78% of their specific latent heat after FTCs, indicating that the PCM remained thermally effective. Overall, PCM-enhanced backfill materials can effectively mitigate freeze–thaw-induced degradation and provide superior performance for GEPs in cold regions, addressing the critical challenge of maintaining thermal and mechanical stability under repeated FTCs while providing a comprehensive evaluation of PCM-enhanced GEP backfill materials under such conditions.