Deciphering the Mortar Composition of Ancient Chinese Pagodas: A Comparative Analysis from the Tang to Ming Dynasties in Shaanxi Province

Abstract

The mortar materials from the internal and external walls of ancient Chinese pagodas in Shaanxi (Luoshan Temple Pagoda (847–858 AD), Daxiang Pagoda (960–1279 AD), and Qianjin Pagoda (1609 AD)) were investigated. Qualitative and semi-quantitative techniques including scanning electronic microscopy (SEM), energy-dispersive spectroscopy (EDS), X-ray diffraction (XRD), attenuated total reflection Fourier transform infrared spectroscopy (ATR-FTIR) and thermogravimetric analysis with differential scanning calorimetry (TGA-DSC) were employed. The physical and chemical characteristics including microstructural morphology, chemical composition, element distribution, and organic additives of the mortar samples were studied. SEM indicated that the mortar from the external wall exhibits a tightly bonded matrix, with the irregularly shaped particles, and the carbonation of the calcium carbonate in the mortars closely resembles that of aragonite crystals. The results of EDS, XRD, FTIR and TGA-DSC revealed that mortar from external wall of these ancient pagodas were air-hardened lime, and complete carbonization with no traces of calcium hydroxide, while mortars from internal wall primarily comprises local clay and lime. In addition, ATR-FTIR and iodine-starch discoloration tests confirmed that the presence of starch in the external wall of Daxiang Pagoda, whereas no organic additives were detected in the Qianjin and Luoshan Temple Pagoda.