Relationship between lime mortars and defects of the Great Wall built by the Ming Dynasty in China

Abstract A defining characteristic of the Ming Dynasty Great Wall is its use of dolomitic and calcium-rich lime mortars with minimal aggregates for bedding and pointing. These mortars exhibit remarkable strength and adhesion to both natural stone and kiln-fired bricks, owing to long-term mineralogical transformations. Analytical studies of unrestored sections in Beijing, Hebei, and Shanxi Provinces reveal the presence of calcite, aragonite, magnesium carbonates (predominantly magnesite, MgCO₃), and secondary minerals. Notably, the mortars possess higher capillary porosity than the bricks, a feature that historically enhanced the Wall’s durability in pre-industrial environments. However, structural degradation—manifested as cracks in mortar joints and bricks—is primarily attributed to chemical interactions between the mortars and air pollutants, with magnesium sulfates playing a key role in deterioration. Furthermore, post-restoration damage in certain sections has been linked to the use of modern cement for structural reinforcement. To address this issue, compatible natural hydraulic lime (NHL) and formulated lime (FL) mortars have been developed as sustainable alternatives to cement-based grouts. Experimental validation involved injecting enhanced grouts into 1 m 3 test cubes containing brick walls and stone fillings. After one year, the cubes were subjected to mechanical testing to assess flexural-tensile and compressive strength. This methodology was informed by prior studies evaluating the efficacy of modified NHL/FL mortars in filling cavities and cracks. Expanding on these findings, mortars based on formulated lime and NHL binders were systematically analysed, with particular emphasis on their pore structure and solid mortar properties. The results, detailed in the following sections, demonstrate their suitability for restoration applications, offering a scientifically validated approach to preserving this iconic structure.