Nano-Micro Synergistic Design of Limestone Calcined Clay Cement for Sustainable Geothermal Tunnels: Mechanical Resilience, Phase Stabilization, and Life-Cycle Carbon Reduction

The construction of deep transportation tunnels faces increasing challenges from extreme geothermal conditions ( > 100 ° C , ∼ 10 % relative humidity), which compromise concrete durability and inflate life-cycle burdens. This study develops a solution via a nano-micro engineered limestone calcined clay cement ( LLC 3 ). The system incorporates nano-alumina and calcium carbonate whiskers, which operate synergistically across scales. Nano-alumina provides reactive Al sites that stabilize ettringite, while the microscale whiskers bridge microcracks, enhancing toughness. This multiscale reinforcement, chemically compatible with the LLC 3 matrix, significantly refines the pore structure and suppresses crack propagation. The modified composite demonstrates exceptional thermal resilience, retaining over 91% of its compressive strength after exposure to 105°C. A cradle-to-gate assessment reveals a ∼ 31 % reduction in embodied CO 2 compared to ordinary portland cement (OPC). When expressed as the binder-level global warming potential/thermal resistance coefficient (GWP/TC) based on measured high-temperature strength retention, the value is reduced by ∼ 70 % relative to OPC, indicating the potential of this nano-micro modification strategy for performance-oriented low-carbon binder design under severe hot-dry exposure.