西北地区砂岩石窟盐害特征及影响因素分析

Characteristics of salt damage to sandstone grottoes in Northwest China and contributing factors

西北地区砂岩石窟数量众多,由于岩性条件及赋存环境等原因遭受着严重的盐害。现场调查发现,盐害特征具有一定的区域性规律,采样后通过离子色谱法测试可溶盐的含量及成分,结合岩性、环境等因素分析盐害特征及成因。结果表明:陕北、陇东地区盐害发育严重,陇中、河西地区盐害相对较为轻微,仅在局部发育;石窟风化层含盐总量分布于0.11%-5.73%,各离子含量差异较大;含盐量影响盐害的宏观表现形态,含盐种类影响盐分的微观结晶形式,其中NaCl、CaSO_(4)结晶形式为片状盐壳,Na_(2)SO_(4)为酥粉状,KNO_(3)表现为絮状结晶;石窟岩体的孔隙率影响盐害发育程度,水分的迁移活动影响盐害的出露位置。研究结果可为砂岩石窟的盐害防治工作提供依据。

Background,aim,and scope Many sandstone grottoes in Northwest China,due to their lithological properties and occurrence environment,suffer severe salt damage.Field investigations revealed that the salt damage of sandstone grottoes showed regional differences.The characteristics and causes of salt damage in the grottoes were analyzed by disease investigations from such perspectives of salt distribution,stratigraphic lithology,environment condition and other factors.Materials and methods Samples were collected from 18 grottoes,which were divided by location into four areas.The content and types of salt were analyzed by ion chromatography.Data about the occurrence conditions and manifestation of salt damage in grottoes in each area were collected through field surveys and local chronicles of Gansu and Shaanxi provinces.Results Severe salt damage was observed in Northern Shaanxi and the Longdong area,while relatively milder damage was observed in the Longzhong and Hexi Corridor areas.The total salt content in the weathered layer of the grottoes was between 0.11%and 5.73%,but the salt content showed substantial regional differences.Discussion At a low content of salt,the salt precipitated only on the surface,and as the salt content increased,the form of damage evolved into flaking and powdering.The crystallization form of NaCl and CaSO_(4) was salt crust,Na_(2)SO_(4) mainly effloresced on the surface.Serious salt damage was observed in grottoes with a high rock porosity,because the formation in those grottoes,especially grottoes in Cretaceous strata,had strong permeability and frequent pore water activities.The salt content in grottoes was found to change with the movement of capillary water.Conclusions The salt content determines manifestations of damage,and the types of salt affect the crystallization form.The migration of water affects the position of exposure to salt damage in the grottoes.Recommendations and perspectives The regional characteristics and causes of salt damage in sandstone grottoes were summarized and discussed in depth.The research findings are expected to provide a basis for protection of sandstone grottoes.