With the gradual depletion of natural sand due to over-exploitation, alternative building materials, such as manufactured sand aggregate (MSA), have attracted much attention. In order to interpret the evolution of pore structure and fractal characteristics in MSA mortar over long-term water saturation, the 1H low-field nuclear magnetic resonance (LF-NMR) relaxation method was used to investigate the temporal evolution of the pore structure in five single-graded MSA mortars and synthetic-graded mortars with small amplitudes in particle size. MSA presents a fresh rock interface characterized by a scarcity of pores, which significantly reduces the porosity of the mortar. The surface-to-volume ratio (SVR) is employed for characterizing the MSA gradation. Through an analysis of parameters, such as total porosity, pore gradation, pore connectivity, and pore fractal dimension of mortar, a correlation model between pore structure parameters and aggregate SVR is constructed. The fractal characteristics of pores and their variations are discussed under three kinds of pore gradations, and the correlation model between fractal dimension and porosity is established. These results demonstrate the high impermeability and outstanding corrosion resistance of synthetic-graded mortar. The fractal model of the pore structure evolution of MSA mortar has a guiding effect on the pore distribution evolution and engineering permeability evaluation of MSA mortar in engineering.

The evolution of the rock pore structure is an important factor influencing rock mechanical properties in cold regions. To study the mesoscopic evolution law of the rock pore structure under freeze-thaw weathering cycles, a freeze-thaw weathering cycle experiment was performed on red sandstone from the cold region of western China with temperatures ranging from -20°C to +20°C. The porosity, T2 spectral distribution, and magnetic resonance imaging (MRI) characteristics of the red sandstone after 0, 20, 40, 60, 80, 100, and 120 freeze-thaw weathering cycles were measured by the nondestructive detection technique nuclear magnetic resonance (NMR). The results show that the porosity of sandstone decreases first and then increases with the increase of the freeze-thaw weathering cycles and reaches the minimum at 60 of freeze-thaw weathering cycles. The evolution characteristics of porosity can be divided into three stages, namely, the abrupt decrease in porosity, the slow decrease in porosity, and the steady increase in porosity. The evolution characteristics of the T2 spectrum distribution, movable fluid porosity (MFP), and MRI images in response to the freeze-thaw weathering process are positively correlated with the porosity. Analysis of the experimental data reveals that the decrease in the porosity of the red sandstone is mainly governed by mesopores, which is related to the water swelling phenomenon of montmorillonite. Hence, the pore connectivity decreases. As the number of freeze-thaw cycles increases, the effect of the hydrophysical reaction on the porosity gradually disappears, and the frost heaving effect caused by the water-ice phase transition gradually dominates the pore evolution law of red sandstone.

Mineral resources are increasingly being developed in cold and permafrost regions. However, the mechanical and physical properties of cemented tailings backfill (CTB) cured at normal temperature are no longer applicable. To clarify the reasons for this variability, a series of tests were performed. The mechanical properties of CTB with different cement–tailings ratios (CTR, 1:4, 1:8, 1:12, 1:16, and 1:20) were tested at different curing ages (3, 7 and 28 days) and curing temperatures (20 °C, 5 °C, −5 °C, and −20 °C). The differences of CTB in mechanical and physical properties under positive- and negative-temperature curing conditions were analyzed, and the microscopic failure process of CTB under negative-temperature curing conditions was discussed. The results revealed that the mechanical properties and deformation behavior of CTB under positive- and negative-temperature curing conditions were different. The frozen CTB had higher early strength than the standard-temperature curing condition (20 °C), and the lower the temperature, the higher the early strength. The low-temperature curing condition, on the other hand, was not beneficial to CTB’s long-term strength. The low-temperature curing condition was not conducive to the long-term strength of CTB. After yielding, strain hardening and strain softening appeared in the deformation behavior of frozen backfill, indicating ductility. In contrast to the typical-temperature curing condition, the frozen CTB showed a new failure pattern that has little relation to curing time or CTR. Furthermore, the failure process of frozen backfill was reviewed and studied, which was separated into four stages, and altered as the curing time increased. The results of this study can act as a guide for filling mines in permafrost and cold climates.

Infrared radiation thermal imaging technology is proposed for rock mechanics tests. Through the infrared radiation temperature and stress–strain curves of sandstone with different porosity during the process of uniaxial compression loading obtained by an infrared camera and uniaxial compression testing machine, the characteristics of infrared radiation on sandstone are systematically analyzed during uniaxial compression loading. The results show that: (1) The strength of sandstone decreases with the increase of porosity, the infrared thermography of sandstone has obvious characteristics of stage evolution and is closely related to the deformation and failure stage of rock in the process of uniaxial compression; (2) During the rock failure process, infrared thermography is differentiated, and the high-temperature radiation zone can reflect the location, size and shape of rock fractures; (3) During the uniaxial compression loading process, the evolution law of the average infrared radiation temperature (AIRT) is closely related to the porosity and uniaxial compressive of the rock, and it is possible to invert all processes of rock failure. The research results demonstrate that the technique of infrared thermal imaging can be applied to rock mechanics tests, and the evolution law of the infrared radiation characteristics can provide a reference for stability analysis of the underground rock engineering.

To study the dynamic response of granite to different levels of microwave power, an intelligent microwave rock-breaking instrument is used to irradiate different power from three directions. The servo universal testing machine is used to carry out a uniaxial compression test on the granite after microwave damage to analyze the strength damage characteristics and the degree of pore damage. Pore fractal characteristics are analyzed based on nuclear magnetic resonance to establish the microwave damage degradation model. In parallel, the energy evolution process of granite under the influence of various power levels is analyzed using the theory of energy dissipation. Simultaneously, based on the energy dissipation theory, we analyze the energy evolution process of granite under the action of different powers. The results show that with higher microwave power, the peak strength and modulus of elasticity show a linear decreasing law. The degree of fragmentation is more obvious, showing the damage characteristics with two big ends and little in the middle. The higher the power, the greater the porosity and the more sensitive the micropore becomes to microwaves. Additionally, the damage degradation model established to evaluate the microwave damage of the rock showed that it was feasible. The higher the power, the lower the total energy, elastic energy, and dissipation energy, and the granite is gradually transformed from elastic deformation to plastic deformation. The elastic energy ratio decreases, the dissipation energy ratio increases, and the degree of damage becomes more and more serious. This study provides theoretical support for exploring the mechanical behavior and mechanism of microwave-assisted rock breaking and is of great practical significance.