Effect of water-rock interaction on the weathering and erosion of Pi-sandstone, Southern inner Mongolia, China

Pi-sandstone, a set of fluvial clastic rock, is not a scientific term, but a common name that the local people call a sedimentary layer exposed on Ordos Plateau of China, integrally wearing red, developed in the Triassic, Jurassic and Cretaceous period. Its lithology gives priority to white, gray, and aubergine sandstones and siltstones, in which muddy sandstone, sandy mudstone, mudstone, few gravel and shale are immingled. Pi-sandstone area, one of the main sources of coarse sediment of the Yellow River, is an intense erosion center in the middle reaches of the Yellow River. The paper analyzes the impact of water on rock weathering and rock erosion through field investigations, sampling, indoor simulated experiment and so on. The testing results of rock samples show that the mineral composition of rocks, in which clay minerals content is between 10% and 40% and the sum of average contents of unstable chemical composition accounts for 11.52 percent of the total chemical composition, is dominated in clay minerals which are unstable, easily weathered and have strong hydrophilicity. At the same time, micro-structure characteristics put up particle sizes with great disparity, disorder arrangement and developed porosity, and the cementation is clay minerals. The testing results of water samples collected from indoor simulated experiment of water-rock interaction and the rain water and spring water are simulated using inverse modeling based on PHREEQC software. The simulated results indicate that there are three main influences on Pi-sandstone caused by the water: 1) Weaken the cohesion within the rock mass, and expand the porosity , for example, the dissolution of quartz, kaolinite, calcite, feldspar, dolomite, gypsum and halite makes rock porosity enlarge; (2) Chemical corrosion takes place, and that feldspar and other minerals are easily weathered into kaolinite and illite expands the fissures of the rock, (3) the precipitation amount of Ca-montmorillonite is much, and its characteristic of expanding when meeting water can increase the internal pressure. Water-rock interaction influences on the physical and mechanical properties of Pi-sandstone and intense the weathering and erosion of Pi-sandstone.