Xiuyuan Yang’s research while affiliated with China University of Mining and Technology and other places

In the process of geothermal energy development, there is groundwater vapor condensation, and the rock is in an alternating dry and wet environment, so it is of great significance to study the influence of high temperature dry and wet cycle on argillaceous sandstone. In this study, the argillaceous sandstone samples were heated at 200 °C after saturation with water and subjected to 120 drying-wetting cycles to study the damage characteristics of rock under high-temperature drying-wetting cycles. The changes in the damage characteristics of rock samples were monitored by acoustic emission (AE). The results highlighted that during the heating stage, there was a decrease in the continuity of thermal AE signal and an increase in the signal strength with an increase in the number of cycles. During the constant temperature stage, the intensity of the thermal AE signal was weak through 0–40 cycles and subsequently increased significantly with the increase in the number of cycles. There was a gradual increase in the number, length, and width of fractures on the surface of argillaceous sandstone with the increase in the number of cycles. The surface fracture rate increased linearly, reaching 2.8% after 120 cycles, forming a fracture network. According to the changes observed in the saturated water absorption rate, the process of fracture development can be divided into the initiation stage (0–40 cycles), expansion stage (41–80 cycles), and penetration stage (81–120 cycles). Moreover, there was an increase in the saturated water absorption rate and loss of mass in each stage with the increase in the number of cycles. This study can provide a theoretical basis for the development of geothermal resources.

Due to precipitation and groundwater level changes, rocks often cycle between dry and wet states. This can cause weathering, which may be exacerbated by the presence of salt. Sandstone is a widely used material in ancient and modern buildings, so the effects of salt during wet-dry cycling are, therefore, of great interest. It is also important in geotechnical engineering to understand its effects on the physical properties of rock. In this paper, sandstone from Wanzhou District, Chongqing, China, was soaked in 0%, 2%, 4%, 6%, or 8% NaCl solutions for 200 dry–wet cycles. The quality, chromaticity, and roughness of the sandstone were measured, and changes in the internal pore structure were studied using nuclear magnetic resonance. The sandstone exhibited three stages of mass loss: a decrease (0–10 cycles), a slow increase (10–140 cycles), and a sharp increase (140–200 cycles). In the third stage, the chromaticity, roughness, porosity, and pore size change significantly. Higher NaCl concentrations cause more serious rock weathering. Salt crystallization increases the pore size, which indirectly accelerates the weathering rate.

The study on the change of rock pore structure during the weathering of purple mudstone is of guiding significance to the stability of the bank slope of the three gorges reservoir. In this paper, the pore changes in the wet and dry circulation of purple mudstone in the three gorges reservoir area are studied by means of nuclear magnetic resonance (NMR). The results show that the simulated weathering of wet and dry circulation has a great influence on the purple mudstone. With an increase in the number of dry-wet cycles, the purple mudstone pore volume ratio significantly changed. Originally, it consisted of a small pore structure with a single pore diameter of 0.01–0.1 µm and changed to a variety of pore structures with various pore diameters of 0.001–100 µm. With the increase in the number of dry-wet cycles, the micropores (0.001–0.1 µm) were transformed into macropores (0.1–1 µm). The area of the second peak of the three samples (large pores 0.1–1 µm) increased from 0.9413, 0.9974, and 0.6779 to 0.9871, 1.1498, and 0.9901, respectively.

To quantitatively study the influence of temperature and strain rate on the brittleness of sandstone, the mechanical parameters of sandstone under different temperatures and strain rates are collected from the previous literature, and two empirical equations for calculating rock brittleness are used to quantitatively calculate and evaluate the brittleness of sandstone. The results show that both BI1 and BI2 can characterize the brittleness of sandstone, but the applicable conditions are different. The BI1 method is more accurate in calculating the variation in the sandstone brittleness with a strain rate, while the BI2 method is more accurate in calculating its variation with temperature. The brittleness of sandstone increases with the increase in the strain rate, especially when the strain rate exceeds 100 s⁻¹. Under low-temperature conditions, the strength and brittleness of rocks increase due to the strengthening of ice. Under the condition of high temperature, the thermal damage to sandstone is intensified after 400°C, and the quartz phase changes after 600°C, which leads to the increase in microcrack density and the decrease in brittleness of sandstone. The conditions of low temperature and high strain rate are beneficial to the enhancement of sandstone brittleness.

Due to the complex natural geological conditions, many slope-related geological hazards occur in the Three Gorges Reservoir area in China. This study focuses on the characteristics of landslide development and their underlying mechanisms in this area. A statistical analysis is conducted to determine the characteristics of landslide development in the Wushan area, including the landslide distribution as a function of the elevation, slope, landslide material composition, scale, lithology, boundary conditions, instability mechanism, stratigraphic age, attitude, and sliding direction. The mechanisms of slope instability and the effect on the occurrence of landslides are analyzed. This study provides important reference material for landslide research in the Three Gorges Reservoir area and similar stratigraphic areas.

Fly ash (FA) is a powdery substance discharged by power plants after burning coal or other fossil raw materials. The storage and reuse of FA have been under debate since the release of FA into the atmosphere could be detrimental to human health. Fortunately, FA has been mostly collected by plants, and these large amounts of collected FA have led to its reuse, which not only reduces pollution, but FA can be also used in lieu of other materials, such as cement, for environmentally friendly building materials. However, the properties of building materials will undergo deterioration at high temperatures. Therefore, in this study, the influence of heat treatment at high temperatures on FA cement mortar is examined in relation to the color and thermal conductivity changes. Color changes are an apparent indicator, for example, of the variation in moisture content. Thermal conductivity (λ) is an important parameter for evaluating the heat transport properties of materials. Mass loss and density also affect thermal conductivity. In this study, 15 FA cement mortar samples are made from mixtures that contain 3 different FA replacement percentages: 10%, 20% and 40%, for a total of 45 samples. The samples undergo heat treatment at temperatures that range from room temperature to 800 °C before testing is carried out. A color meter and a thermal conductivity tester are used to obtain the color parameters L∗, a∗, b∗ and thermal conductivity, respectively. The samples are also sprayed with a phenolphthalein reagent after treatment to qualitatively observe the changes in pH. It is found that a temperature range of 400–600 °C is critical in which the pH of the mortar decreases due to carbonization and the decomposition of calcium hydroxide, which lead reduced lifespan of the FA cement mortar. The thermal conductivity also decreases at higher temperatures, and is linearly correlated with the density ratio. The experimental results show that the FA percentage has a considerable influence and therefore, there could be an optimum FA percentage.

In this study, the rate of emission of arsenic during the burning process of different kinds of coal is examined in order to study the volatile characteristics of arsenic during coal combustion which have negative effects on the ecological environment and human health. The results show that the emission rate of arsenic gradually increases with increased burning temperature, with a threshold of approximately 700°C to 800°C in the process of temperature increase. Then, the relationships among the arsenic emission rate and combustion environment, original arsenic content, combustion time, burning temperature, air flow and amount of arsenic fixing agent are discussed, and it is found that except for the original arsenic content, the rest of the factors have a nonlinear relationship with the emission rate of arsenic. That is, up to a certain level, they all contribute to the release of arsenic, and then their impact is minimal. The original arsenic content in coal is proportional to the arsenic emission rate. Therefore, taking into consideration the nonlinear relationships between factors that affect the arsenic emission rate can reduce contamination from arsenic.