Compositional and Microstructural Characterization of Historical Architectural Mortars:a Comparative Study of Gray Backing Layers and Ridge Components from the Divine Kitchen Roof System at the Palace Museum
1 School of Civil Engineering and Architecture, University of Jinan, Jinan 250022, China 2 The Palace Museum, Beijing 100009, China 3 School of Materials Science and Engineering, Southeast University, Nanjing 211189, China
Abstract: The Forbidden City in Beijing is one of the greatest architectural projects in the history of human civilization. Its roofs comprise various kinds of lime-based mortar. Detecting the phase composition of the roofing lime mortar and the ratios between the different components using modern analytical approaches provides valuable insights and has practical significance for the repair of the ancient complex of the Forbidden City. Therefore, this work focuses on four kinds of lime mortar samples from the roofs of the Sacrificial Kitchen of the Forbidden City. The samples were analyzed using modern testing approaches, such as the polarizing microscope, acid corrosion, X-ray fluorescence (XRF) analyzer, X-ray powder diffraction (XRD) analyzer, thermogravimetry-differential scanning calorimetry (TG-DSC) analyzer, Fourier transform infrared spectrometer (FTIR), scanning electron microscopy-energy dispersive spectrometry (SEM-EDS), and mercury intrusion porosimetry (MIP), to characterize the petrography, phase composition, microstructure, and pore structure of the lime mortar. The results showed that hubanhui, or the first layer of mortar covering roofing boards, consisted of pure lime, whereas nihui and qinghui, or the second and third layers of mortar, as well as jialonghui, or the mortar used to fill the gaps on both sides of tiles, were mixed with peat soil, clay, and iron oxide red. After acid corrosion, the masses of the remaining impurities accounted for 5%, 70%, 9%, and 21% of the four mortar samples, respectively. Through analysis, we identified the presence of calcium silicate hydrate (C-S-H) in hubanhui, nihui, qinghui, and jialonghui. However, the quantitative analysis of XRD and TG-DSC shows that the content of C-S-H in the samples is very small. Carbonization gradually turned C-S-H into calcite, making it difficult to observe C-S-H through scanning electron microscopy. The process of traditional roofing bedding involves tapping, compaction, and plastering. The outer surface of the qinghui layer formed a smooth and dense protective layer after multiple compactions, and the jialonghui and nihui mortar became more solid after compaction or tapping.
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2025, Vol. 39 
