Analysis of the composition, structure, and deterioration mechanisms of brick carvings in the Beijing Tianning Temple Pagoda

The Tianning Temple Pagoda in Beijing, a representative Liao Dynasty structure, features extensive brick carvings that exemplify the craftsmanship and aesthetic traditions of early Chinese masonry. After centuries of exposure to harsh climatic conditions and atmospheric pollution, the carvings have suffered severe deterioration, including salt efflorescence, delamination, and surface disintegration. To elucidate their material characteristics and deterioration mechanisms, this study employed multiple analytical methods—XRF, XRD, FTIR, TG-DTG, SEM-EDS, PLM, and IC—to investigate the composition, microstructure, and salt contamination of both the brick carvings and their lime-based binding materials. Results show that the bricks were made from high-silica–alumina clay (SiO₂ + Al₂O₃ + Fe₂O₃ > 70%) fired above 1000 °C, with quartz, feldspar, and clay minerals as the main phases. The lime binder has fully carbonated into calcite (CaCO₃), providing structural adhesion between brick units. However, the brick matrix exhibits a loose and porous microstructure, which facilitates the migration and crystallization of soluble salts such as sulfates, nitrates, and chlorides. Ion chromatography indicates that the surface layers contain significantly higher concentrations of SO₄²⁻, NO₃⁻, and Cl⁻ compared with the interior, exceeding the critical thresholds defined in the WTA standard. These salts mainly originate from the long-term deposition of SO₂ and NOₓ emitted by nearby industrial sources, which, through acid-rain processes, react with silicate and feldspar minerals to form soluble sulfate and nitrate salts. Repeated wet–dry and temperature cycles promote salt crystallization and generate crystallization pressure, leading to flaking, granular disintegration, and loss of carved details. The study clarifies the physicochemical mechanisms responsible for the deterioration of Liao Dynasty brick carvings and provides a scientific basis for conservation strategies targeting salt weathering under polluted urban environments.