Zhenya Zhang

To elucidate the evolution of coal pore structure under coupled temperature–time effects, this study conducted programmed temperature oil-bath experiments to treat coal samples under varying thermal conditions. The pore characteristics, including pore volume, specific surface area, and pore size distribution, were systematically analyzed through low-temperature nitrogen adsorption (LTNA) measurements and BET theory. Furthermore, the FHH fractal theory was employed to quantitatively characterize the pore structure heterogeneity. The results indicate that the shape of the coal adsorption–desorption curves remained largely unchanged with increasing heating temperature and duration. In terms of pore volume distribution, the mesopore fraction exhibited a consistent decreasing trend, the micropore fraction initially declined before increasing, while the macropore fraction exhibited an initial increase followed by a subsequent reduction. Regarding the surface area distribution, the proportion of mesopores and macropores exhibited an initial decrease followed by an increase, whereas the micropore fraction demonstrated a converse trend, increasing first and subsequently decreasing. The fractal characteristics of pores are significant, with the fractal dimension D₁ > D₂ indicating that the surface pores exhibit rougher structures compared to the internal pore network. Under fixed heating duration, the elevation of temperature lead to concurrent decreases in both D₁ and D₂ values. When maintaining constant temperature conditions, the value of D₁ demonstrated an initial decline followed by recovery, while that of D₂ showed a trend of initial increase and subsequent reduction. This reveals the dynamic evolutionary process of pore structure.

The coal spontaneous combustion (CSC) in the goaf of a coal mine poses a significant safety hazard. This study investigates the gas production characteristics, apparent activation energy, and limiting parameters of coal samples with varying particle sizes during the low-temperature oxidation stage under both primary and secondary oxidation conditions through a temperature-programmed experiment. The results indicate that smaller particle size leads to higher rates of O₂ consumption, CO generation, and heat release intensity in coal samples; The O₂ consumption rate, CO generation rate, and heat release intensity during the secondary oxidation of coal samples are relatively high in the initial stage of the experiment. However, in the later stage of the experiment, there is a reversal in these parameters; the selection of the most appropriate model from the 22 commonly observed reaction kinetics mechanism functions revealed that the apparent activation energy of the coal sample decreases during secondary oxidation compared to primary oxidation within the temperature range of 80℃ to 130℃. However, a reversal occurs between temperatures of 140℃ and 170℃, indicating that secondary oxidation initially enhances the low-temperature oxidation characteristics but weakens them in later stages; The various particle sizes under both primary and secondary oxidation conditions significantly influence the limit parameters of CSC, with secondary oxidation being more prone to inducing SC of coal in goaf compared to primary oxidation.

Mineral admixtures (MA), like fly ash (FA), silica fume (SF), and slag (S), are usually added to cement-based materials to improve their compactness and further enhance their mechanical properties, permeability resistance, and durability. In this study, low-field nuclear magnetic resonance (LF-NMR) is adopted to explore the evolution of the early hydration characterization of cement-based materials with MA by testing the transverse relaxation time T2. Meanwhile, the effect of MA on mechanical properties is analyzed by measuring compressive and flexural strength. The results show that, in the early hydration (0–7 days), the T2 distribution shows a trend of gradually moving to a short relaxation time and changes from a double peak to a single main peak. The decrease in T2i (main peak vertex) means that the evaporated water is gradually distributed in smaller pores with more motion constraints. However, the type and content of MA have little effect on T2i. Porosity gradually decreases in the period of early hydration. The addition of MA causes the porosity to decrease, and the order influence is FA > S > SF, i.e., the porosities of cement paste with 0%MA, 10%FA + 10%SF, 10%FA + 10%S, and 20%FA at 7 days are 48%, 44.5%, 40.7%, and 40.2%, respectively. Additionally, the addition of MA to cement-based materials also decreases the early strength, and the influence order is FA > S > SF, i.e., the compression strength values of cement paste with 0%MA, 10%FA + 10%SF, 10%FA + 10%S, and 20%FA at 7 days are 47.8 MPa, 40.1 MPa, 38.6 MPa, and 37 MPa, respectively.

A five-story moment frame structural model with a base isolation system was tested on a shaking table. The isolation system comprised both linear natural rubber bearing (LNR) and nonlinear viscous dampers (NLVDs). Seven ground motions were employed: including three far-fault (FF) and four near-fault (NF) earthquake ground motions. The performance of the isolation system was evaluated by measuring the displacement and base shear of the isolation bearings. Furthermore, the axial force and displacement of the NLVDs were measured. The evolution of the fundamental dynamic frequency of the frame during the test was also determined. During strong earthquakes, NF ground motions caused larger story drifts and floor accelerations of the superstructure than FF ground motions. The displacement and base shear of the isolation base was very large when the isolated structure was subjected to Kobe_TAK000 and ChiChi_TCU102/278 pulse-like NF ground motions. Furthermore, the LNR s experienced tension and uplift when the PGA of input earthquake ground motions was larger than 0.80 g. Although the NLVDs performed very well in combination with the LNRs, the severe responses of the isolation bearings were caused by NF ground motion with a pulse period T_p neighboring the fundamental period of the isolated structure.