Durability Analysis of Load-Bearing Recycled Concrete Blocks in Severe Cold Regions of Qinghai-Tibet

To realize resource utilization of solid waste and improve the performance of recycled concrete blocks in severe cold regions of the Qinghai-Tibet Plateau, this study investigates the effects of recycled fine aggregates and active supplementary cementitious materials on the impermeability, water resistance and frost resistance of loadbearing concrete blocks, based on 100% replacement of coarse aggregates with recycled aggregates. The microstructure of recycled concrete was analyzed by scanning electron microscopy (SEM) and nuclear magnetic resonance (NMR). A life cycle assessment (LCA) approach was employed to quantitatively evaluate material performance, costs and carbon emissions. The results show that the permeability and water absorption of load-bearing recycled concrete blocks increase with the increase of the proportion of recycled fine aggregates, while they decrease first and then increase with a reduction in the fly ash-to-slag blend ratio. Conversely, frost resistance and the softening coefficient exhibit the opposite trend. Under the condition of 100% recycled coarse aggregate replacement and a fly ashto-slag blend ratio of 3∶1, the compressive strength of the blocks reached 11.77 MPa, flexural strength was 3.89 MPa, softening coefficient was 0.99, water absorption was 0.7%, mass loss after 50 freeze-thaw cycles was 2.2%, and the loss rates of compressive and flexural strength were 10.2% and 13.9%, respectively. These properties meet the load-bearing and durability requirements for severe cold regions, making this mix ratio the recommended formulation. Microscopic analysis shows that alkali activation promotes secondary hydration in the composite cementitious materials, generating additional hydration products that fill internal pores, thereby enhancing the densification of the recycled concrete. However, with the increasing freeze-thaw cycles, the number of internal pores gradually increases, with micropores and mesopores envolving into macropores and cracks, leading to performance degradation.Based on the performance-cost-carbon emission analysis, using 100% recycled coarse aggregates alone is unfavorable for carbon reduction. In contrast, the recommended mix proportion incorporating both recycled aggregates and active SCMs demonstrates the optimal comprehensive benefits, achieving a carbon reduction rate of 31.03%.