Developing sustainable heat-storage backfill materials for geothermal energy piles in cold regions using limestone calcined clay cement (LC 3 )

This study develops phase change material (PCM)-enhanced backfill materials using limestone calcined clay cement (LC3) in geothermal energy pile (GEP) systems. Backfill materials incorporating PCM (1–5 wt.%) and flake graphite (6–12 wt.%) were prepared using LC3. The PCM melting point and curing/testing temperature were set around ∼6 °C to represent typical underground thermal conditions in cold regions like Edmonton, Canada. The results showed that PCM significantly increased the specific latent heat of the backfill materials, from 2.07 J/g to 11.25 J/g as PCM content increased from 1 to 5 wt.%. Although MPCM reduced the thermal conductivity (TC), the addition of flake graphite effectively compensated for this reduction. The mixture containing 5 wt.% PCM and 12 wt.% graphite achieved TC values of 2.12 W·(m·K)−1 in oven-dry and 2.83 W·(m·K)−1 in saturated states. Moreover, life cycle assessment (LCA) showed that LC3 significantly reduced environmental impacts, lowering global warming potential from 625.55 to 418.05 kg-CO2-eq per m3 compared with OPC (33% reduction). Overall, the developed PCM-enhanced backfill materials prepared using LC3 show strong potential for sustainable cold-region GEP applications.