Changes In Pore Structure Research Articles

Analysis of mechanical properties and deterioration mechanisms of mudstone under wetting-drying cycles

During seasonal rainfall periods, natural rocks typically undergo repeated wetting-drying cycles, which directly affects the micro-structure of the rock samples and consequently leads to the deterioration of their mechanical properties, especially in mudstone. Therefore, this paper investigated the degradation pattern of strength parameters and failure pattern of mudstone samples under wetting-drying cycles through a series of laboratory tests and quantitatively characterized the micro-structural changes in the samples. Additionally, numerical simulations were utilized to delve deeper into the degradation mechanism of the samples. The results indicate that with an increasing number of wetting-drying cycles, the degradation of peak strength and elastic modulus becomes more pronounced. Moreover, there is a continuous increase in micro-porosity within the samples, accompanied by changes in pore structure and morphology. According to the simulation results, the wetting-drying cycles change how the samples fail, transitioning from mixed tensile-shear failure to predominantly shear failure. This study not only deepens the theoretical understanding of the deterioration mechanisms in mudstone but also provides important guidance for practical applications.

Microscopic Mechanism of CO2 Displacement Oil in Different Pore Throat Radius Segments of Tight Sandstone Reservoirs.

The micropore structure of tight sandstone affects the efficiency of CO2 displacement of crude oil. As the pressure changes, the oil displacement efficiency (E d) in segments with different pore radii changes, and the asphaltene precipitation in the pores causes alterations in the pore structure and wettability, which constrain E d. Ten samples of tight sandstone from the Yanchang Formation in the Ordos Basin were selected for this study. A variety of methods, including X-ray diffraction (XRD), casting thin sections (CTS), scanning electron microscopy (SEM), high-pressure mercury intrusion (HPMI), CT scanning, and nuclear magnetic resonance (NMR) combined with CO2 displacement, were used to study the efficiency of crude oil utilization and the amount of asphaltene deposited at different pore-throat radii, and then the impacts of pressure, pore structure, and wettability changes on E d were discussed. The findings indicate that samples have three types: macropore-fine throats (MF), medium pore-tiny throats (MT), and small pore-microthroats (SM). The MT exhibits a favorable configuration. The pore-throat radius of each sample can be divided into two segments, namely, large pore segments (PL) and small pore segments (PS), and the PL has a significant E d. The E d of the MF-type PS is constrained by pressure. The E d of PL is significantly affected by the pressure sensitivity for the MT, while the E d of PL for the SM structure is more affected by pressure. Changes in wettability and the precipitation of asphaltene are the results of the reaction between crude oil and CO2. In the MF, asphaltene precipitates from the PL, while in the MT and SM, asphaltene precipitates both from the PL and PS. The amount of asphaltene precipitation strongly affects the E d in PS. The oil wettability increases more obviously with better pore-throat configurations. This study offers a reference and foundational understanding for evaluating CO2 displacement in tight sandstone reservoirs.

Molecular structure and evolution mechanism of shale kerogen: Insights from thermal simulation and spectroscopic analysis

During thermal evolution, the kerogen in shale formation undergoes significant chemical, structural and compositional changes, continuously influencing the shale storage capacity, hydrocarbon generation potential, and gas occurrence state. This study systematically examines the chemical structural changes during the thermal evolution of Longmaxi shale kerogen using hydrothermal laboratory experiments from 420 to 850 °C. Additionally, a range of characterization techniques, including Fourier Transform Infrared (FTIR) spectroscopy, Raman spectroscopy, High-Resolution Transmission Electron Microscopy (HRTEM), X-ray Diffraction (XRD), and Solid-state 13C Nuclear Magnetic Resonance (13C NMR), were employed to systematically investigate the kerogen structure and its evolution patterns. Results indicate that Longmaxi Formation kerogen comprises a large molecular carbon framework with aromatic structures, long-chain aliphatic components, and oxygen-containing functional groups. As thermal maturity reaches 2.6 %, the alignment of aromatic fringes gradually increased, with over 60 % aligning in the main direction. Throughout thermal evolution (1.9–3.2 %), fringes in the 5–10 Å range within aromatic structures peak at over 20.45 %, with fringes over 50 Å rapidly increasing from 3 % to over 15 %. Early in the thermal evolution, the long-chain aliphatic component of the kerogen decreases rapidly and part of the structure forms small aromatic rings through aromatisation. The mature to over-mature stage can be divided into three phases: 1) 1.7–2.4 % (loss of oxygen-containing functional groups), 2) 2.4–3.5 % (formation of larger aromatic structures via condensation reactions), 3) over 3.5 % (continued aggregation of aromatic rings, resulting in an increase in the length of aromatic fringes). At a macroscopic level, this induces changes in kerogen pore structure and carbonization features. This study thus provides new insights into the thermal evolution of shale kerogen, which in turn improve understanding of shale reservoirs for hydrocarbon exploitation.

The Relation between Soil Moisture Phase Transitions and Soil Pore Structure under Freeze–Thaw Cycling

The process of soil moisture phase transitions (SMPT) under freeze–thaw cycling is considered a key factor driving changes in soil pore structure. However, there is still no consensus on which indicators related to SMPT affect the soil pore structure. The objectives of this study were to compare SMPT and soil pore characteristics under freeze–thaw cycling, and to analyze the inherent relationship between them as affected by different bulk densities. Hence, we employed thermal pulse time-domain reflection technology (T-TDR) and X-ray CT scanning technology (X-CT) to quantitatively study the process of SMPT and pore characteristics of soil core samples (60 mm diameter, 100 mm height) repacked with three different bulk density levels: 1.10 g·cm−3 (NC), 1.30 g·cm−3 (LC) and their combination (1.10 g·cm−3 for upper half, 1.30 g·cm−3 for lower half, SC) under freeze–thaw cycling. Our results showed that compared with NC, the porosity of LC’s 0–5 cm soil column decreased by 0.070 cm3·cm−3, the imaged porosity (ϕ>60μm) decreased by 0.034 cm3·cm−3, and the maximum soil ice content (MIC) decreased by 0.030 cm3·cm−3. The pores within the range of 200−300 mm (ϕ2) and 300–400 mm (ϕ3) contribute the most significantly to ϕ>60μm (50–60%). Soil initial moisture content (IMC) and MIC explained 50.1% of the change in ϕ2, and the bulk density explained 49.3% of the change in ϕ3. During the melting process, higher moisture content promotes the thaw collapse of soil particles, resulting in a decrease in ϕ>60μm. The mean pore radius of the limiting layer (MRLL) and the hydraulic radius (HR) show that changes in bulk density from 1.10 g·cm−3 to 1.30 g·cm−3 do not have significant differences. Our results show the relationship between SMPT and pore structure change during freeze–thaw cycles as affected by initial soil bulk density and moisture condition.

Sea/coral sand in marine engineered geopolymer composites: Engineering, mechanical, and microstructure properties

AbstractUtilizing marine waste and resources for eco‐friendly building materials is pivotal in promoting sustainable development in island and coastal construction industries. Among the potential alternatives, coral sand as well as sea sand stands out as a promising material. This research seeks to explore the potential of coral/sea sand as a fine aggregate in the creation of environmentally sustainable marine engineered geopolymer composites. An assessment was conducted on the influence of varying proportions of coral sand, meant as a substitute for sea sand, on the flowability, drying shrinkage, mechanical properties, and microstructure of the marine engineered geopolymer composites. The findings indicate that as coral sand replaces sea sand, flowability and drying shrinkage decrease, while compressive strength experiences an initial rise followed by a decline. Encouragingly, a combination of coral and sea sands enhances tensile ductility. Overall, a 20 wt.% coral sand mixture yields optimal results, with the compressive strength is 54.4 Mpa and the tensile strain capacity is 2.397% after 28 days. Moreover, microscopic tests reveal changes in hydration products and pore structure. Our research delves into the potential of coral/sea sand as a fine aggregate in the creation of environmentally sustainable marine engineered geopolymer composites.

The effect of deposit morphology on soot oxidation in non-catalytic and catalytic processes

Investigating the oxidation behavior of soot particles with varying deposit morphologies is crucial for understanding the mechanisms behind uneven regeneration in diesel particulate filters and enhancing regeneration strategies. This study explores the impact of two distinct deposit morphologies on soot oxidation within non-catalytic and catalytic processes, with a particular focus on the associated changes in pore structure during oxidation. Findings indicate a minimal difference in oxidation activity between loose and compacted soot particles in non-catalytic oxidation, while a significant enhancement in oxidation activity is observed for compacted soot particles during catalytic oxidation. Specifically, compacted soot particles exhibit a pronounced reduction in total pore volume, particularly in macropores, and a shift in pore size distribution towards smaller diameters during non-catalytic oxidation. In the catalytic oxidation process, compacted soot particles predominantly form 2–10 nm mesopores, with limited formation of pores larger than 10 nm. Oxidation activity in the non-catalytic process positively correlates with pore volume, especially for 100–200 nm macropores. In contrast, during catalytic oxidation, oxidation activity inversely relates to pore volume, emphasizing the critical role of catalyst-soot contact in catalytic oxidation efficiency.

Capillary sealing capability alteration of shale caprock induced by CO2-brine-rock interaction: Implication for CO2 geological storage

The capillary sealing mechanism plays a pivotal role in the sealing capability of caprock in CO2 geological storage. The interactions between CO2, brine, and rock induced by CO2 injection may potentially impact the capillary sealing capability of the caprock. To characterize the capillary sealing capability of Yanchang shale, breakthrough pressure (Pb) tests were conducted before and after CO2-brine-rock interaction while analyzing the contributions of pore structure and wettability to Pb alteration. The results revealed that Pb of Yanchang shale samples decreased by 7.14%–22.22% after CO2-brine-rock interaction, indicating a deterioration in their capillary sealing capability. This degradation can be attributed to changes in pore structure and wettability, as they collectively influence the capillary sealing capability of shale caprock. The increase of small-pore (<200 nm) porosity and contact angle promoted the reduction of Pb. The alteration of shale caprock's capillary sealing capability due to CO2-brine-rock interaction significantly impacts CO2 storage potential and long-term safety, which should be considered prior to large-scale CO2 injection.

Study on mechanical properties and meso-damage mechanism of carbon-polyvinyl alcohol hybrid fiber reinforced recycled coarse aggregate concrete under the coupling action of sodium sulfate and dry-wet cycles

In order to investigate the influence of the coupling effect of sodium sulfate dry-wet (DW) cycles on the mechanical properties of carbon-polyvinyl alcohol hybrid fiber reinforced recycled coarse aggregate concrete (HFRAC), HFRAC specimens were subjected to uniaxial compression tests. The total volume fraction of fibers was kept constant at 0.4%, and various volume fractions of carbon fibers (CF) were added to the recycled coarse aggregate concrete (RAC), namely 0%, 0.1%, 0.2%, 0.3%, and 0.4%, with polyvinyl alcohol (PVA) fibers replacing a portion of CF in equivalent volume. Uniaxial compressive stress–strain curves were obtained for HFRAC specimens subjected to different numbers of DW cycles (0, 30, 60, and 120 cycles). Microscopic testing methods including acoustic emission (AE), scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDS), X-ray diffraction (XRD), and nuclear magnetic resonance (NMR) were employed to analyze the microstructure, erosion product composition, and pore structure changes of the specimens. The results revealed that, for RAC specimens, the peak stress (σp) and elastic modulus (E) initially increased and then decreased, whereas the peak strain (εp) initially decreased and then increased with an increase in the number of DW cycles. After adding hybrid fibers, the σp values of CF, Hybrid 3:1, Hybrid 1:1, Hybrid 1:3, and PVA specimens increased (or decreased) by 12.0%, 11.43%, 0.9%, −9.5%, and −8.5%, respectively, compared to RAC. The Hybrid 1:3 specimen exhibited a negative hybrid effect. With an increase in the erosion age, the Hybrid 3:1 specimen demonstrated enhanced resistance to sulfate attack and achieved the highest σp value after 120 cycles. The influence of sulfate DW cycles coupling on the microscopic damage mechanism of HFRAC was analyzed using a statistical damage constitutive model. The microscopic damage evolution process was characterized by five characteristic parameters: E0, εa, εh, εb, and H. The analysis results indicated a clear correlation between the variation of these characteristic parameters and the number of DW cycles. An effective connection was established between the microscopic damage mechanism and the macroscopic nonlinear constitutive behavior. The phenomenon of relative lagging of acoustic emission signals during the uniaxial compression process was explained from the perspective of effective stress. This study provides a theoretical basis for the promotion and application of RAC under sulfate DW cycle environments.

Experimental study on mechanical property and pore structure of nano-modified concrete in the large temperature difference environment of cold-region tunnels

To reveal the mechanism of nanomaterials on concrete in the large temperature difference environment, three different types of nano-modified concrete were explored in this study. The mechanical properties and pore structure characteristics of concrete under these different nano-modified measures were investigated. Furthermore, mathematical models of the strength and the pore structure were established. The experimental results demonstrate that: Untimely entering the large temperature difference environment is unfavourable for the strength of concrete; The incorporation of nanomaterials is conducive to the strength and pore structure of concrete, of which 2 % Nano-SiO2 shows the best effect; When the pre-curing time ranges from 1.5 days to 3 days, the strength of 2NS group develops slowly, which is consistent to the change of pore structure; The multi-factor strength mathematical model established can express the quantitative relationship between concrete’s strength and pore structure.

Effects of supercritical CO2 fluids on pore structure and fractal characteristics of bituminous coal

Enhanced coalbed methane recovery with CO2 coal seam storage (CO2-ECBM) technology is an important way to achieve China's strategic goals of carbon peak and carbon neutrality. Presently, to date there has been rarely research conducted on the effect of coal sample scale on pore structure under supercritical CO2 (ScCO2) fluids. In this study, a high-pressure geological environment simulation system was adopted to analyze coal samples of different scales for ScCO2 saturation. Subsequently, low-pressure nitrogen gas adsorption (LP-N2GA), mercury intrusion porosimetry (MIP), and low-field nuclear magnetic resonance (LF-NMR) were used to analyze the pore structure and fractal dimension changes in saturated coal samples at different scales. The experimental results show that the mesopore ratios of cylindrical and granular coal decrease by an average of 1.68% and 2.30%, respectively, after the saturation of ScCO2. The proportion of macropores in cylindrical coal increased by an average of 5.50% after ScCO2 saturation, while the proportion of macropores in granular coal changed by 176.86% compared to cylindrical coal. The fractal dimension of the ScCO2 saturated coal samples obtained with LP-N2GA, MIP, and LF-NMR all show a decreasing trend, again confirming the modification of the coal pore surface by ScCO2. Finally, a conceptual model is presented to analyze the mechanism of the effect of coal sample scale on the pore structure under ScCO2. The difference in the transport paths of ScCO2 molecules at different coal scales is the main reason for the difference in the evolution of the pore structure. In addition, the impact of the amount of adsorption obtained in the laboratory using coal samples of different scales on the assessment of the CO2 storage capacity was discussed. Therefore, the results of this study are expected to provide a reference for the CO2 storage capacity assessment of the CO2-ECBM project.

The structural evolution of undisturbed loess due to water infiltration

Loess structure is the physical key factor that determines its stability and consists of macro-pores, loose texture, and water sensitivity. The structural change characteristics and effects of the undisturbed loess before and after water infiltration are studied using mechanical CT and simulation tests in order to study the structural change process within the undisturbed loess caused by water infiltration. The change in particle state is as follows: the peak frequency point of the equivalent diameter of the loess particles after infiltration ranged from 16.75 to 23.76 μm, and the eroded fine particles consisted primarily of fine particles. The smaller loess particles are removed by water infiltration resulting in coarsening of soil particles. The sphericity of the loess particles gradually changes from spherical pores to angular and dendritic pores. The particle inclination angle transitions to a range greater than 70°, and its proportion is approximately 61%. The change in pore structure is as follows: The loess porosity after infiltration increased by approximately 20%, and the increase in the pore area ratio of the mesopores and the macropores was higher than that of the micropores. Additionally, the small pores increased by more than 5 times the original state of the undisturbed loess. The connected pores expanded less than 60% of the initial state to more than 90% after infiltration, thus, increasing the dominant seepage channel of the undisturbed loess. These changes in particle and porosity further increase the water filtration intensity and promote the migration of fine particles (mainly silt particles), linking loess catastrophes and are the leading cause of loess settlement and slope instability. The process of water infiltration into the loess, the mechanism of loess collapsibility, and the influence of salinity on the loess structure and strength are discussed in this study.

Salt Dissolution and Pore Structure Changes Induced by Fracturing Fluid in Shale Reservoirs

Salt Dissolution and Pore Structure Changes Induced by Fracturing Fluid in Shale Reservoirs

Investigation on the stability and durability of PVDF membranes under prolonged exposure to household bleach cleaning agents

This study delves into the impact of aging on PVDF membranes due to exposure to various NaOCl-based household bleach products, namely Bayclin, Proclin, and Soclin, over different durations (0, 1, 3, and 5 days). The primary objective is to assess the influence of household bleaches as cleaning agents on the stability and durability of PVDF membranes. Our findings underscore that prolonged exposure to NaOCl-based cleaning agents induces changes in pore structure, morphology, mechanical strength, and flux performance of the membranes owing to oxidation by hypochlorite. However, these alterations are mitigated when using household cleaning agents, particularly Soclin and Proclin. For instance, after 5 days, membranes aged with NaOCl exhibited an enlargement of pore size up to 0.2 mm, whereas those aged with Soclin (MS) and Proclin (MP) only experienced an enlargement to 0.12 mm from the original pore size of 0.05 mm. This increase in pore size is undesirable as it adversely affects the selectivity performance of the membranes. Additionally, the hydrophilicity of membranes was found to be better maintained when aged with household products compared to pure NaOCl, as evidenced by the higher water contact angle observed in MP and MS membranes. Furthermore, the flux recovery ratio (FRR) exhibited an upward trend with prolonged aging duration, with Bayclin showing the highest FRR of 180 %, closely to that of pure NaOCl at nearly 190 %, among the other two products. This significant increase in FRR is undesirable as it results from the enlargement of pore morphology caused by damage from exposure to hypochlorites which is stronger in the chemical-grade NaOCl compared to the household bleach products. In summary, this study highlights that household bleaches, especially Soclin and Proclin, present a promising option in terms of stability for prolonged use. These results are crucial for identifying chemicals that are not only more efficient in the cleaning process but also less damaging to the membranes with good stability in long-term use.

Impact of NH4OH treatment on the ion exchange and pore characteristics of a metakaolin-based geopolymer.

We investigated the viability and influence of NH4OH post-synthetic treatment on the pore characteristics of geopolymers. Geopolymers are a class of materials with amorphous aluminosilicate three-dimensional frameworks, regarded as amorphous analogues of zeolites. Similar to zeolites, when geopolymers are used in catalysis or adsorption applications, post-synthetic treatments such as ion exchange with NH4 + salts (e.g., NH4Cl and NH4NO3) and desilication (using strong bases such as NaOH) are necessary to introduce active sites and modify their pore structure, respectively. Recently, it has been shown that treatment with NH4OH combines these two steps, in which acidic sites are introduced and the pore structures of zeolites are modified simultaneously. Considering the increasing interest in geopolymers in catalysis and adsorption applications, understanding the impact of such treatment on the structure of geopolymers is needed. Our diffuse reflectance infrared Fourier-transform spectra show that NH4 + exchanges Na+ in the geopolymer, and laser diffraction with scanning electron microscopy images show that the particle size of the powdered geopolymer decreases after NH4OH treatment. N2 sorption isotherms and 129Xe and 1H NMR measurements revealed information about the changes in pore structures: micropores were larger than mesopores and inborn mesopores increased in diameter, thereby reducing the surface area to volume ratio. However, pore accessibility and pore connectivity were not altered by NH4OH treatment. Since solid-state NMR and X-ray fluorescence revealed desilication, these changes in particle size and pore characteristics are considered to be due to desilication caused by NH4OH treatment.

Cement-based external wall insulation material with thermal performance improvement by partial substitution of calcium silicate

External wall composite insulation materials have shown great potential for energy saving and emission reduction in the construction industry, but cement-based material as one of the external wall composite insulation materials has the problem of high thermal conductivity. Herein, a composite insulation material was prepared using the press method by expanded polystyrene (EPS) and cement of different calcium silicate content. The addition of porous calcium silicate provides nucleation sites for cement hydration to promote hydration reaction and increase the porosity of the inorganic cementitious phase from 75 % to 85 % and refine the average pore size from 2826.09 nm to 421.31 nm. The cement-based composite insulation material which the calcium silicate content is 20 % has 0.0423 W/(m·K) in the thermal conductivity, which is 23.1 % lower than the non-calcium silicate content of cement-based composite insulation material (0.055 W/(m·K)). The simulation analysis proves that the improved insulating properties are attributed to the increased interface between solid-phase and gas phase, thinner heat transfer walls, and longer heat transfer paths due to the pore structure changes in the inorganic cementitious phase. This proposed composite insulation material provides a new choice for external wall insulation.

Study on strength deterioration mechanism of foamed concrete under freeze-thaw cycles: Experiment and numerical simulation

In cold regions, freeze-thaw (F-T) cycles pose a severe challenge to the long-term durability and structural reliability of foamed concrete (FC). In response, this study proposes a comprehensive multi-level evaluation system integrating macro and micro scales, aiming to thoroughly investigate the decline mechanisms of compressive strength of FC under F-T cycles. Through macroscopic experiments, the extent of mass loss and strength degradation has been quantified. The evolution of internal damage, progression of surface displacements, and changes in pore structures have been effectively tracked and captured by using non-contact techniques. The findings reveal that after F-T cycles, the mass loss rate and strength loss rate of FC can be up to 6.62 % and 37.61 %, respectively. There is a marked acceleration in the expansion rate of non-stable internal cracks; peak displacements show significant growth, and the failure mode changed from brittle to brittle-ductile. Furthermore, the porosity and the proportion of irregular pore shapes have increased by 25.84 % and 18.39 %, respectively. Additionally, a discrete element model incorporating irregular pore characteristics has been established, elucidating the profound impact of F-T cycles from a microscopic physical perspective. Specifically, the weakening and the increased number of fracture events of the contact forces between cement particles lead to the deterioration of FC's strength.

Influence of biomass fly ash on durability of self-consolidating cement-tailings grout: Resistance to freeze-thaw cycles and sulfate attack

The durability of backfills is a crucial factor in maintaining the long-term stability of the mine backfill area. Partial replacement of cement by biomass fly ash (BFA) is a cost-effective and sustainable way for viscosity control of self-consolidating cement-tailings grouts (CTG), however, the unfavorable changes of pore structure may affect the durability during the service life. This study investigates the resistance of self-consolidating CTGs to 90 freeze-thaw cycles and 180 days sulfate attack, with and without BFA addition. The mechanical properties and relative dynamic modulus of CTGs were measured and analyzed during the exposure to freeze-thaw cycles and sulfate attack. The changes in pore structure, solid phase, and microstructure were also studied. The results demonstrate that the role of 30 % BFA addition on resistance to freeze-thaw cycles and sulfate highly depends on the water-to-binder ratios of hardened CTGs. At a low water-to-binder ratio of 0.6, the addition of BFA can enhance the performance of concrete in terms of its resistance to freeze-thaw cycles and sulfate attacks. At the water-to-binder ratios of 0.8 and 1.0, BFA can increase the resistance to sulfate attack but degrades the performance after freeze-thaw cycles. At a high water-to-binder ratio of 1.2, the addition of BFA reduces resistance to freeze-thaw cycles and sulfate attack. The varying effects of BFA with different water-to-binder ratios are attributed to the pore structures. When the water-to-binder ratio is 0.6, the CTG containing BFA exhibits the best resistance to freeze-thaw cycles and sulfate attack. This provides insights into the design and application of novel self-consolidating CTGs in engineering sites.

Micromechanical analysis of contact erosion under cyclic loads using the coupled CFD‒DEM method

Different layers of soil often have distinct particle sizes. When exposed to the natural environment, soil is easily affected by natural rainfall, rising groundwater levels, and human activities, leading to particle contact erosion, which reduces the safety and service performance of the soil structure. In this paper, a coupled computational fluid dynamics–discrete element method (CFD–DEM) model was employed to investigate the particle migration phenomena, mechanical response of contact interfaces, variations in flow fields, and macroscopic deformation during the contact erosion process under cyclic loads at different frequencies and amplitudes. The conclusions are presented as follows: (1) Within one cycle of cyclic loading, both compression during loading and stress relaxation during unloading are the main factors triggering the migration of fine particles. (2) The migration and loss of fine particles mainly occur in the early stages of cyclic loading, where strong contact force chains are formed within the fine particle layer, leading to significant plastic deformation of the soil at the macroscopic level. (3) Under cyclic loading, changes in the soil pore structure cause an upwards hydraulic gradient in the initial quiescent water flow field. This hydraulic gradient can rupture weak contact force chains and cause particle pumping. (4) Increasing the frequency and amplitude of cyclic loading intensifies the erosion of fine particles, causing greater axial deformation of the soil. Compared to cyclic loading frequency, the amplitude of cyclic loading has a greater impact on contact erosion.

Enhanced Hoek-Brown (H-B) criterion for rocks exposed to chemical corrosion

Underground constructions often encounter water environments, where water–rock interaction can increase porosity, thereby weakening engineering rocks. Correspondingly, the failure criterion for chemically corroded rocks becomes essential in the stability analysis and design of such structures. This study enhances the applicability of the Hoek-Brown (H-B) criterion for engineering structures operating in chemically corrosive conditions by introducing a kinetic porosity-dependent instantaneous mi (KPIM). A multiscale experimental investigation, including nuclear magnetic resonance (NMR), X-ray diffraction (XRD), scanning electron microscopy (SEM), pH and ion chromatography analysis, and triaxial compression tests, is employed to quantify pore structural changes and their linkage with the strength responses of limestone under coupled chemical-mechanical (C-M) conditions. By employing ion chromatography and NMR analysis, along with incorporating the principles of free-face dissolution theory accounting for both congruent and incongruent dissolution, a kinetic chemical corrosion model is developed. This model aims to calculate the kinetic porosity alterations within rocks exposed to varying H+ concentrations and durations. Subsequently, utilizing the generalized mixture rule (GMR), the kinetic porosity-dependent mi is formulated. Evaluation of the KPIM-enhanced H-B criterion using compression test data from 5 types of rocks demonstrated a high level of consistency between the criterion and the experimental results, with a coefficient of determination greater than 0.96, a mean absolute percentage error less than 4.84%, and a root-mean-square deviation less than 5.95 MPa. Finally, the physical significance of the porosity-dependent instantaneous mi is clarified: it serves as an indicator of a rock’s capacity to leverage the confining pressure effect.

Porosity and pore structure evolution during the weathering of black shale

Pore type and pore structure evolves systematically across continuous black shale weathering profile. However, the extend and process of pore structure change is still an enigma. In this study, we try to unveil the pore structure evolution during weathering process through studying Cambrian Hetang shales in southern China. Fourteen shale samples, from protolith zone (PZ), fractured and weathered shale zone (FWZ), and saprolite zone (SZ), were collected to elucidate how porosity and pore structure develop during black shale weathering under subtropical condition. Through low pressure argon (Ar) gas adsorption (LP-ArGA), high pressure mercury intrusion (HPMI), nuclear magnetic resonance(NMR) and field emission scanning electron microscope (FESEM) observation, the results reveal significant differences in physical properties and pore structures among the PZ, FWZ, and SZ samples. Specifically, compared to PZ, FWZ and SZ samples are characterized by higher clay mineral content, lower organic matter (OM), and the absence of carbonates and pyrite. Total porosity, determined through HPMI and NMR, exhibits a gradual increase from PZ (6.70 % and 6.41 %) to FWZ (20.47 % and 13.45 %) and SZ (23.22 % and 12.48 %). Ar adsorption isotherms indicate a change in pore type from predominantly ink-bottle and slit-shaped in the PZ to mainly slit-shaped in FWZ and SZ. Integrated analysis of LP-ArGA, HPMI, NMR and SEM observation suggests a substantial decrease in the contribution of micropores to total pore volume (PV) and a concurrent increase in larger pores (meso-macropores) with the increase of weathering intensity. This results in smoother surfaces of micro-transition pores but rougher surfaces of macropores. Changes in mineralogy composition during weathering play a crucial role in influencing pore structure of shales and further accelerating the release and migration of toxic elements in black shale. Our study provides the essential theoretical foundation for the remediation of soil and water environmental pollution caused by black shale weathering.