Study on material degradation mechanism of brick masonry defensive wall heritages under different environmental erosion

The brick masonry defensive walls are important cultural heritages. However, under the influence of environmental erosion and human factors over hundreds of years, the constituent materials have suffered severe degradation. In order to better conserve and restore these precious heritages, it is necessary to promptly investigate and analyze the main factors causing the degradation of their original structure and underlying mechanisms. This study, through on-field investigations, experimental tests, and theoretical analysis, has conducted research on the types and mechanisms of the degradation of the Great Wall and the Ming City Wall in Nanjing. The results show that the degradation mechanisms of brick masonry defensive walls, under varying climatic and environmental conditions, are primarily attributed to multiple factors: wet-dry cycling, freeze-thaw cycling, salt crystallization, temperature fluctuation, acid rain corrosion, hydrolysis, microbial activity, and human activities. These identified drivers further result in specific degradation outcomes, including wall expansion, wall cracking, brick weathering, crenelation damage, brick pavement degradation, biological erosion, lime mortar loss, and anthropogenic damage. These interconnected degradation processes not only damage the material fabric but also continuously degrade the heritage feature of the walls by altering texture, structural features, and overall form. The results of this study can provide scientific basis for the conservation and restoration of the brick masonry defensive wall heritages.