Performance evaluation and application of nano-metakaolin modified natural hydraulic lime for stone cultural heritage restoration

Stone cultural heritage suffers from significant degradation due to long-term environmental exposure, necessitating repair materials that exhibit high workability, compatibility, and durability. In this study, a natural hydraulic lime (NHL5)-based composite mortar modified with nano-metakaolin (NMK) was developed. The workability, compatibility, and durability of the composite mortar were systematically evaluated through multiple characterization techniques, with the enhancement mechanism elucidated from a microscale perspective. Results show that the proposed material exhibits shortened setting times, high early strength, adjustable final strength, low shrinkage, and reduced soluble salt content. Moreover, it demonstrates excellent resistance to erosion, maintaining its structural integrity and mechanical performance even after dry-wet or freeze-thaw cycles. Microstructural analysis revealed that the incorporation of NMK enhances pozzolanic reactivity, leading to the formation of additional hydration products and a denser matrix. Notably, a successful application at Yungang Grottoes confirms its potential as an effective strategy for the conservation of stone heritage.