[M-IS05] Geomaterials in cultural heritage: weathering, investigation techniques, and conservation

[MIS05-P05] Microscopic weathering characteristics and deterioration mechanisms of sandstone heritage in the Nankan Grotto, northern Sichuan, China

*xuening Zhang¹, xiyong Wu¹, sixiang Ling¹ (1.Southwest Jiaotong University)

Keywords:Sandstone heritage, Deterioration mechanism, Deterioration pattern, Micro-petrography, Weathering

The mineralogy, micro-structure, geochemical mass balance, and porosity of the sandstone were analyzed to explore the weathering mechanism of the deterioration patterns and their influences on slope instability problems in the Nankan Grotto. The deterioration patterns consist of scaling, blistering, crust and disintegration, which had leaded to sliding, falling, bending, and toppling of the rock blocks. The results signify that the major elements Ca, Na, K, Al, and Si have been leached during the weathering of calcite, feldspar, and quartz. The cations (Ca²⁺, Na⁺) combined with CO₃²⁻ and SO₄²⁻ so gypsum, thenardite, and secondary calcite were precipitated. Fe and Mg exhibited different geochemical behaviors due to the dissolution of ankerite and biotite and the enrichment of secondary minerals. The porosity exhibited little variation from Zone-4 to Zone-2 and increased rapidly in Zone-1, which is attributed to the sharp increase in the abundance of the macropores in Zone-1. Based on the microscopic petrographic evidence, the stresses imposed by the biotite oxidation and clay mineral swelling/shrinking were the primary stimuli of the scaling. The disintegration was mainly attributed to the intense fragmentation of the calcite. The crystallization stress of the thenardite and calcite and the swelling/shrinking stress of the clay minerals led to blistering of the outer rock layer. Solution migration carried Ca²⁺ from the interior of the rock to the surface where it combined with CO₃^2- and SO₄^2- to produce gypsum and calcite on the surface during the drying process, resulting in the formation of the crust. Therefore, the mineralogical compositions (especially calcite and minor biotite) laid the foundation for the distinct development of scaling, blistering, crust, and disintegration. In addition, the micro-environments influenced the amounts and locations of salts crystallization, eventually leading to thenardite crystallizing within the rock at the blistering site, while the gypsum precipitated on the surface as a crust. Our research reveals the microscopic weathering mechanism of the sandstone deterioration in the Nankan Grotto, providing new insights for research on the deterioration mechanism of grottoes and the structural stability problem of the heritages, and paving the way for the subsequent conservation of the Nankan Grotto.