Simulating Weathering Research Of The Effects Of Air Pollutant （SO₂、 NO_x） On The Deterioration Of Yungang Grottoes

Yungang Grottoes are situated at the southern foot of WuZhou mountain, Datong city, Shanxi province, having had over 1500-year history since they were first constructed at North Wei dynasty. These grottoes are vitally important materials and references for the studying of Chinese Buddhist art development, as well as significant historical and cultural heritage. Chinese researchers who work for cultural relics conservation began to introduce modern science and technology into the preservation and restoration of Yungang Grottoes in order to provide them with better protection since the 60s.Shanxi province is one of the coal-rich provinces in China and its coal reserves ranked second in the country, accounting for a quarter of identified reserves. Yungang Grottoes are close to one of coalfields called Yungang coalfield, therefore, there was an explosive development in the coal mining industry from the early 80s to the late 90s near Yungang Grottoes. The traffic volume of 109 State Road which was 350 meters away from Yungang Grottoes increased dramatically, resulting to an increase of the atmospheric pollutant emissions of dust, carbon dioxide and nitrogen oxides, plus the city dwellers’ behaviour of burning coal for heating in winter, which leaded to a severe problem of air pollution in the region of Yungang Grottoes. Dust carrying sulfides, nitrides, cyanide and other air pollutants accumulated on the surface of Yungang Grottoes’sculptures and walls, which have caused serious decay on Yungang sandstones. Former researches carried out many studies about the effects of water including the impact of acid rain in terms of the weathering problem of historic stones, but there lack the study about the influence of air pollutants without the participation of water on the deterioration of Yungang sandstones. We carried out a series of simulating weathering experiments, using sulfur dioxide, nitrogen oxide as erosion gas, in order to research how air pollutants effect the deterioration of Yungang sandstone without the participant of liquid water such as acid rain. Two rounds of experiments were conducted and each round lasted three months. The decay process of Yungang sandstone and the damaging mechanism of air pollutants were explained in this dissertation through the analysis of phase transformation and pore structure changes as well as the discussion of gas adsorption and capillary condensation phenomenon happening in the pores of Yungang sandstones during the simulating experiment. These equipment included a polarizing microscope, scanning electron microscope (SEM), wavelength dispersive X-ray fluorescence spectrometer (XRF), sample level high-power X-ray powder diffraction (XRD) and automatic surface area and porosity analyzer. Capillary condensation phenomenon was a very important stage in Yungang sandstones’deterioration in the simulating weathering experiment, which directly determined the degree of the deterioration of Yungang sandstones. The results show that, after the experiments, the damaging types of Yungang sandstone samples in our study coincided the major damaging types found in natural weathering Yungang sandtones, including salt crystallization, surface discoloration, crisp powder, massive spalling and fissures. Particle size and natural moisture content of sandstones have a certain impact on sandstones’deterioration. Sulfur dioxide have greater impact than nitrogen oxides on sandstones’deterioration. Microscopic changes of Yungang sandstone samples are mainly enhancements of the process that feldspar gradually transformed into clay, and the development of intergranular pore, dissolution pores and micro-cracks. New salt crystals were mainly gypsum (CaSO4·H2O) and epsomite (MgSO4 or MgSO4·7H2O) filling in micro pores and cracks. There were mesopores (pore diameter is between 2 nm to 50 nm) and macropores (pore diameter is larger than 50 nm) in Yungang sandstone samples.