An assessment of weathering in sandstone masonry built with earthen mortars: the case of the ancient rope Bridge Ferry, China

The Ancient Rope Bridge Ferry, a culturally significant heritage site along the Silk Road, represents a key element of China’s historical legacy. However, prolonged neglect and inadequate maintenance have left the site increasingly vulnerable to natural weathering. Conservation efforts are further complicated by a limited understanding of the building materials and the unclear mechanisms of deterioration, which impede effective preservation strategies. To address this, this study assessed the conservation state of this monument through laboratory analysis and field tests. Several techniques, such as XRD, polarizing microscopy, and TESCAN Integrated Mineral Analyzer (TIMA) were used to investigate the weathering of sandstone, revealing that weathering accelerates feldspar alteration and carbonate mineral dissolution. Mortar was also found to contribute to salt weathering and the formation of gypsum crusts. Field non-destructive testing (NDT) and a CRITIC-based Integrated Weathering Index (IWI) were employed to quantify the degree of weathering. The results showed spatial variability in weathering: (1) the most pronounced deterioration occurred in the upper and lower wet areas of the wall due to precipitation and capillary water, respectively; (2) the northeast facades of walls were most heavily weathered, primarily influenced by wind-driven rains; (3) while weathering in the ferry and fort zones was similar, greater biological colonization was observed in the ferry zone. This study provides a reference for the future preservation and restoration of the monument.