Life-Cycle Assessment of Underground Lining Concrete Incorporating Recycled Aggregates: Key Factors for Green Building Materials from Dispersed Waste

The incorporation of recycled coarse aggregates (RCAs) derived from dispersed waste concrete into lining concrete for underground engineering presents a potential strategy to reduce primary resource depletion. However, the actual environmental benefits of this approach are highly contingent upon logistical factors, such as transport distances, and the mix design adjustments necessary to compensate for RCA quality variations. In this study, a cradle-to-gate life-cycle assessment (LCA) was conducted for C30/37 underground lining concrete to evaluate the combined effects of the RCA replacement ratio, waste-concrete transport distance, and cement compensation assumptions. Four cement-compensation sensitivity scenarios (0%, 5%, 10%, and 15%) were evaluated at a fixed 30% RCA replacement rate. The results reveal that cement production accounts for over 80% of the total global warming potential (GWP). Under an equal cement content scenario, each 10% substitution of natural coarse aggregate with RCA reduces aggregate-related GWP by approximately 0.233 kg CO2 eq/m3. However, under the equal-cement, zero-compensation scenario (CC0), the total GWP is only 0.18% lower than that of conventional concrete. In contrast, cement-compensation levels of 5%, 10%, and 15% increase the GWP to 455.3, 474.7, and 494.1 kg CO2eq/m3, which are 4.26%, 8.72%, and 13.17% higher than conventional concrete, respectively. The previously identified deterministic break-even transport distance of 26.04 km is therefore specifically applicable only to the equal-cement CC0 aggregate substitution scenario. These findings demonstrate that the environmental feasibility of RCA-based lining concrete is jointly governed by the quality of the recycled material, the additional cement demand, and regional transport conditions. The outcomes provide a quantitative foundation for decision making in the production and environmental performance evaluation of underground lining concrete, particularly in the context of extreme environments, where material reliability and resource security are of paramount importance.