Chunbo Zhou

The effects of MATE-ADV densifier dosage on CT-resolvable pore characteristics and compressive strength in fly ash–slag concrete remain insufficiently quantified. An L9 orthogonal array was used to evaluate cement content, sand ratio, fly ash (FA) content, and densifier dosage. X-ray computed tomography (X-CT) characterized the specimens at 28 d, and compressive strength was measured at 7, 14, and 28 d. Across the nine specimen groups, mean slice porosity ranged from 0.75% to 2.50%, while 28 d compressive strength ranged from 59.49 to 68.37 MPa. Exploratory pooled-error ANOVA suggested main effects of cement content (F_(2,4) = 8.05, p = 0.040) and sand ratio (F_(2,4) = 7.71, p = 0.042) on mean slice porosity. Cement content and sand ratio showed the largest main effects on 28 d strength; FA content was also significant, whereas densifier dosage was not (F_(2,18) = 2.79, p = 0.088). X-CT analysis showed variation in pore-section area and circularity among the groups. A second-order logarithmic model summarized the observed porosity–strength relationship (R² = 0.948; leave-one-group-out RMSE = 1.422 MPa). Finally, a candidate for the optimal mix proportion was identified by jointly considering the evaluated 2D pore characteristics and 28 d compressive strength.

Given that shale reservoir characteristics necessitate hydraulic fracturing for extraction, while traditional hydraulic fracturing faces significant challenges related to water consumption and pollution, the development of new waterless/low water fracturing technologies has become an urgent industry need. Low temperature fracturing, particularly methods utilizing liquid nitrogen to cool reservoirs, holds great promise. This study systematically reveals, through numerical simulation, the influence of initial temperature, confining pressure, and material heterogeneity on the stress evolution patterns in shale during liquid nitrogen cooling. An increase in initial temperature intensifies the stress intensity and response complexity within shale during cooling. Confining pressure dominates stress redistribution by suppressing tensile stresses, amplifying compressive stresses, and limiting deformation, thereby magnifying stress fluctuations. Furthermore, the heterogeneous distribution of thermal expansion coefficients and thermal conductivity increases the stress difference between interior and exterior regions, while the heterogeneity of thermal expansion coefficients and Young’s modulus exacerbates the heterogeneous stress distribution. This study provides a theoretical basis for low temperature fracturing.

As coal extraction advances to greater depths, a refined understanding of the coupled evolution of involved physical effects and mechanisms in gas-bearing coal under excavation-induced disturbances becomes indispensable. In this context, “chain evolution” characterizes the progressive and interdependent interplay among stress redistribution, damage propagation, and seepage adjustment. Building upon a seepage–stress–damage coupling model for gas-bearing coal, and supported by triaxial compression tests for validation, this study explores multifield evolution during roadway excavation across lateral pressure coefficients ξ of 0.5, 0.8, 1.0, 1.2, and 1.5. The results reveal that the lateral pressure coefficient fundamentally regulates both the orientation and intensity of this coupled process by reshaping the initial stress regime and associated unloading constraints. At relatively low values ξ < 0.8 , damage and high-permeability zones preferentially propagate along the sidewalls, facilitating rapid gas pressure dissipation. Under near-equilibrium conditions 0.8 ≤ ξ ≤ 1.2 , the stress, damage, seepage, and energy fields evolve in a coordinated, annular configuration surrounding the roadway. By contrast, at higher coefficients ξ > 1.2 , damage localization and permeability enhancement are concentrated in the roof and floor, accompanied by pronounced vertical energy accumulation. These findings underscore that the lateral pressure coefficient not only governs the spatial distribution of individual physical fields but also orchestrates the pathways of their coupled evolution. The study thus provides a robust, mechanism-oriented basis for optimizing support design and implementing targeted hazard mitigation strategies in deep gas-bearing coal seams.

Geothermal energy can be obtained from hot dry rock (HDR). The target temperatures for heat extraction from HDR range from 100 to 400 °C. Artificial fracturing is employed to stimulate HDR and create a network of fractures for geothermal resource extraction. Liquid nitrogen (LN₂) is environmentally friendly and shows better performance in reservoir stimulation than does conventional fracturing. In this study, triaxial compression experiments and acoustic emission location techniques were used to evaluate the impacts of temperatures and confining pressures on the mechanical property deterioration caused by LN₂ cooling. The numerical simulation of LN₂ fracturing was performed, and the results were compared with those for water and nitrogen fracturing. The results demonstrate that the confining pressure mitigated the deterioration effect of LN₂ on the crack initiation stress, crack damage stress, and peak stress. From 20 to 60 MPa, LN₂-induced reductions in these three stress parameters ranged between 7.73–18.51%, 3.46–12.15%, and 2.51–8.50%, respectively. Cryogenic LN₂ increased the number and complexity of cracks generated during rock failure, further enhancing the fracture performance. Compared with those for water and nitrogen fracturing, the initiation pressures of LN₂ fracturing decreased by 61.54% and 68.75%, and the instability pressures of LN₂ fracturing decreased by 20.00% and 29.41%, respectively. These results contribute to the theoretical foundation for LN₂ fracturing in HDR.

Microwave and liquid nitrogen have shown great potential applications in the mining, geological, petroleum, and particularly metal mining fields. In order to further promote their application in rock tunneling engineering, this paper conducted a series of I-model fracture tests on hard rock (granite) to explore the impact of liquid nitrogen, microwave, and liquid nitrogen and microwave pretreatment on the fracture characteristics of hard rock. The results showed that compared with the control group, all pretreatment groups had varying degrees of reduction in wave speed and fracture toughness, especially the microwave and liquid nitrogen group.