Microscopic Pore Research Articles

Study of mechanical characteristics and damage mechanism of sandstone under long-term immersion

The microscopic pore structure of sandstone gradually changes under long-term immersion, resulting in dynamic changes in the mechanical and damage properties of sandstone, which brings challenges to maintain the long-term stability of underground engineering. In this paper, the evolution of pore structure and mechanical behavior of sandstones at different immersion times (0d, 3d, 30d, 90d, 180d and 360d) is studied. A series of physico-chemical experiments and uniaxial compression tests (with simultaneous acquisition of acoustic emission (AE) signals) are conducted to explore the weakening mechanism and failure mode of sandstones during long-term immersion. The results show that long-term immersion has a significant effect on the pore structure and mechanical behavior of sandstones. Various physical parameters indicate that the change in the pore structure of a sandstone with increasing immersion time can be divided into two stages, namely the swelling stage of clay minerals (saturation process) and the corrosion stage of potassium feldspar (long-term immersion process). The variation of pH value and K+ concentration in soaking solution can provide a basis for the division of pore structure stages. From the perspective of mechanical behavior, the uniaxial compressive strength (UCS) and cumulative AE counts of a sandstone decrease by 83% and 96.8% respectively after 360 d of immersion, with enhanced plastic characteristics. The macroscopic failure mode of a sandstone changes from combination of shear and tensile crack surfaces to a single shear crack surface. Based on the results of physico-chemical experiments and mechanical tests, it can be assumed that microcracks in long-term immersion sandstones are more prone to develop along weakened cementation surfaces. Under the effect of stress, corrosion pores formed by potassium feldspar provide space for the rotation and movement of quartz grains, making it more prone to shear slip. The study results enhance our understanding of weakening mechanisms at long-term immersion and provide a basis for assessing the long-term stability of underground engineering.

Advances in Microscopic Pore Structure Characterization of Fine-Grained Mudrocks

Advances in Microscopic Pore Structure Characterization of Fine-Grained Mudrocks

Preparation of high performance porous carbon by microwave synergistic nitrogen/phosphorus doping for efficient removal of Cu2+ via capacitive deionization

Sargassum biochar has potential advantages as an electrode material due to its natural microscopic pore channels. However, conventional pyrolysis method is prone to thermal damage to the biochar, and incapable to form a complete pore structure resulting in poor biochar electrode performance. In this study, a strategy of microwave pyrolysis coupled with KOH activation was used to prepare nitrogen/phosphorus double-doped graded porous biochar (STC) using ammonium dihydrogen phosphate as dopant. The carbon material STC-1.24-800 prepared by the optimal parameters had a high specific surface area (SSA) of 1367.6 m2 g −1 and a total pore volume of 1.499 cm3 g−1. The precise inside-out heating characteristics of microwave facilitated the generation of suitable meso-micropore distribution ratios in carbon, and the graded porous structure provided abundant active sites for charge accumulation and ion diffusion. The doped nitrogen/phosphorus atoms responding to the microwave field, generated spin to promote microwave absorption, introducing surface structural defects to produce electron density differences. The change in the nature of the electron donor and its electron density enhanced the electrical conductivity and chemical stability of STC. Nitrogen/phosphorus polar surface functional groups improved hydrophilicity and wettability. STC-1.24-800 had a higher specific capacitance of 531 F g−1 and exhibits great cycle performance in capacitive deionization (CDI) applications (1.0 V, 50 mg L−1 Cu2+) as well as adsorption performance (56.16 mg g −1). The present work can provide a novel feasible idea for preparing diatomically doped graded porous biochar for CDI electrode application by microwave irradiation.

Effect of alkaline hydrogen peroxide assisted with two modification methods on the physicochemical, structural and functional properties of bagasse insoluble dietary fiber.

Bagasse is one of major by-product of sugar mills, but its utilization is limited by the high concentration of lignin. In this study, the optimal alkaline hydrogen peroxide (AHP) treatment conditions were determined by the response surface optimization method. The results showed that the lignin removal rate was 62.23% and the solid recovery rate was 53.76% when bagasse was prepared under optimal conditions (1.2% H2O2, 0.9% NaOH, and 46°C for 12.3 h), while higher purity of bagasse insoluble dietary fiber (BIDF) was obtained. To further investigate the modification effect, AHP assisted with high-temperature-pressure cooking (A-H) and enzymatic hydrolysis (A-E) were used to modify bagasse, respectively. The results showed that the water holding capacity (WHC), oil holding capacity (OHC), bile salt adsorption capacity (BSAC), and nitrite ion adsorption capacity (NIAC) were significantly improved after A-H treatment. With the A-E treatment, cation exchange capacity (CEC) and BSAC were significantly increased, while WHC, OHC, and glucose adsorption capacity (GAC) were decreased. Especially, the highest WHC, OHC, BSAC and NIAC were gained by A-H treatment compared to the A-E treatment. These changes in the physicochemical and functional properties of bagasse fiber were in agreement with the microscopic surface wrinkles and pore structure, crystallinity and functional groups. In summary, the A-H modification can effectively improve the functional properties of bagasse fiber, which potentially can be applied further in the food industry.

Pore structure and fractal characteristics of Wufeng–Longmaxi formation shale in northern Yunnan–Guizhou, China

In this study, the microscopic pore characteristics of shale in marine strata are evaluated. Based on field emission scanning electron microscopy (FE-SEM), low-temperature N2 adsorption (LT-N2GA), low-pressure CO2 adsorption (LP-CO2GA) and high-pressure methane adsorption (HPMA) experiments, the pore characteristics of 12 shales from the Wufeng–Longmaxi Formations in northern Yunnan and Guizhou are characterized qualitatively and quantitatively. Fractal Frenkel–Halsey–Hill (FHH) theory is used to analyse the fractal characteristics, and the adsorption pore characteristics of shale are discussed. The correlation between the fractal dimension and pore structure and adsorption performance is determined. The results show that the total organic carbon (TOC) contents of the 12 shales are in the middle–low level, ranging from 0.43% to 5.42%, and the shales are generally in the highly mature to overmaturity stage (vitrinite reflectance (Ro) values between 1.80% and 2.51%). The mineral composition is mainly quartz and clay minerals. The average clay mineral content is 40.98% (ranging from 24.7% to 63.3%), and the average quartz content is 29.03% (ranging from 16.8% to 39.6%), which are consistent with those of marine shale in the Sichuan Basin. FE-SEM and LT-N2GA isotherms reveal a complex shale pore structure and open pore style, mainly ink bottle-shaped and parallel plate-like pores. The total pore volumes (PVs) range from 0.012–0.052 cm3/g, and the specific surface area (SSA) values range from 18.112–38.466 m2/g. All shale samples have abundant micropores and mesopores, accounting for >90% of the total SSA. The fractal dimensions, D1 and D2, were obtained from N2 adsorption data, with different adsorption characteristics at 0–0.5 and 0.5–1.0 relative pressures. The fractal dimensions increase with increasing BJH PV and BET SSA and decrease with decreasing average pore diameter (APD). The fractal dimensions are positively correlated with the TOC and clay mineral contents and negatively correlated with the quartz content. The fractal dimension can be used to evaluate the methane adsorption capacity; the larger the fractal dimension is, the larger the methane adsorption capacity is. Fractal analysis is helpful to better understand the pore structure and adsorption capacity of shale gas reservoirs.

Experimental study on effect of freeze-thaw cycles on dynamic mode-Ι fracture properties and microscopic damage evolution of sandstone

The investigation of dynamic fracture characteristics of sandstone after freeze-thaw (F-T) treatments is of great significance for the optimization of blasting parameters and stability analysis of rock masses engineering in cold or high-elevation regions. In the present study, the P-wave velocity, porosity, T2 spectrum curve, and micromorphology of sandstone specimen after different F-T cycles were measured using nuclear magnetic resonance (NMR) and scanning electron microscopy (SEM) techniques. The F-T damage factor was defined according to the reduction in wave impedance. Moreover, the dynamic mode-Ι fracture test was carried out using notched semicircular bend (NSCB) sandstone specimen. Meanwhile, the fracture process of NSCB specimen was captured by adopting a high-speed photography system. Test results show that F-T cycles can irreversibly deteriorate the microscopic pore structure and the cementation between mineral particles, which is manifested by an increase in the proportion of macropores and mesopores. The dynamic fracture pattern of sandstone specimen is affected by the coupling of F-T cycle number and loading pressure. Furthermore, the dynamic fracture toughness of sandstone constantly increases with the increase of loading rate at a specified F-T cycle number, while the F-T cycle treatment has a significant weakening effect. There has a negative exponential correlation between dynamic fracture toughness and F-T damage factor.

Study on Microscopic Pore Structure Classification for EOR of Low Permeability Conglomerate Reservoirs in Mahu Sag

The microscopic pore structure controls the fluid seepage characteristics, which in turn affect the final recovery of the reservoir. The pore structures of different reservoirs vary greatly; therefore, the scientific classification of microscopic pore structures is the prerequisite for enhancing the overall oil recovery. For the low permeability conglomerate reservoir in Mahu Sag, due to the differences in the sedimentary environment and late diagenesis, various reservoir types have developed in different regions, so it is very difficult to develop the reservoir using an integrated method. To effectively solve the problem of microscopic pore structure classification, the low permeability conglomerate of the Baikouquan Formation in Well Block Ma18, Well Block Ma131, and Well Block Aihu2 are selected as the research objects. The CTS, HPMI, CMI, NMR, and digital cores are used to systematically analyze the reservoir micro pore structure characteristics, identify the differences between different reservoir types, and optimize the corresponding micro pore structure characteristic parameters for reservoir classification. The results show that the pore types of the low permeability conglomerate reservoir in the Baikouquan Formation of the Mahu Sag are mainly intragranular dissolved pores and residual intergranular pores, accounting for 93.54%, microfractures and shrinkage pores that are locally developed, accounting for 5.63%, and other pore types that are less developed, accounting for only 0.83%. On the basis of clear pore types, the conglomerate reservoir of the Baikouquan Formation is divided into four types based on the physical properties and microscopic pore structure parameters. Different reservoir types have good matching relationships with lithologies. Sandy-grain-supported conglomerate, gravelly coarse sandstone, sandy-gravelly matrix-supported conglomerate, and argillaceous-supported conglomerate correspond to type I, II, III, and IV reservoirs, respectively. From type I to type IV, the corresponding microscopic pore structure parameters show regular change characteristics, among which, porosity and permeability gradually decrease, displacement pressure and median pressure increase, maximum pore throat radius, median radius, and average capillary radius decrease, and pore structure becomes worse overall. Apparently, determining the reservoir type, clarifying its fluid migration rule, and formulating a reasonable development plan can substantially enhance the oil recovery rate of low permeability conglomerate reservoirs.

Study on powder particle behavior in powder spreading with discrete element method and its critical implications for binder jetting additive manufacturing processes

ABSTRACT Understanding powder bed system behaviour in powder spreading is a fundamental issue in binder jetting additive manufacturing (BJAM). This work established a discrete element model incorporating a parallel bond model to compatibly depict local cross-links between powder particles. BJAM parameters including layer thickness, gap compensation, recoat speed, rotation speed, and layer number were studied quantitatively for their effects on recoated powder's packing density and microscopic pore size and bonded layer's breakage and layer shift. Evolutions and influence mechanisms on both layer shift and bond breakage were further elucidated. Some practical implications include: gap compensation corresponding to an ideal recoated powder structure is ∼75 μm; rotation speed should be controlled at 40–120 rad/s to avoid low-rotation-speed layer shift surge and high-rotation-speed breakage; layer shift occurring at a certain stage is irreversible and must deserve well-maintained. This research can provide theoretical guidance for developing BJAM and even support-free powder bed – based additive manufacturing.

Accumulation Conditions and Pattern of Tight Oil in the Lower Submember of the Fourth Member of the Shahejie Formation in the Damintun Sag, Bohai Bay Basin

To determine the accumulation conditions and pattern of tight oil in oil shales in the Damintun Sag, Bohai Bay Basin, this study investigated the basic geological conditions of the source rocks and reservoirs in the sag using methods such as organic carbon analysis, whole-rock XRD analysis, and field emission scanning electron microscopy. The results show that: (1) The high-quality source rocks in the lower submember of the fourth member of the Shahejie Formation (E2S42) in the Damintun Sag have high organic matter abundance, favorable organic matter types, high hydrocarbon expulsion efficiency, and high fluidity. Therefore, they provide sufficient oil sources for tight oil accumulation.; (2) During the burial of organic-rich shales, the thermal degradation of organic matter produces large amounts of organic acids, which can dissolve carbonate minerals. In this way, secondary pores are formed.; (3) The special microscopic pore structure that connects fractures to pores is the key to the enrichment of tight oil is a key factor for the high oil saturation of pores in oil shales; (4) The breakthrough pressure (up to 100 MPa) and specific surface area of dolomitic mudstones in the E2S42 submember are significantly higher than those in other horizons. As a result, the dolomitic mudstones can effectively seal the underlying tight reservoirs; (5) Compared with the tight oil in tight sandstones, the tight oil in the oil shales in the study area has significantly superior geological conditions for reservoir formation, such as the favorable arrangement of hydrocarbon expulsion channels, low filling resistance, and the presence of reservoir spaces.

Core String Parallel Experiment Study for Volcanic Rock in Xushen Gas Field

Volcanic gas reservoir in Xushen gas field has complex microscopic pore throat structure and dense physical property, which is quite different from that of conventional high-permeability sandstone gas reservoir. It is of great significance to know its seepage mechanism for scientific and efficient development technology countermeasures. Through the parallel and series experiments of volcanic fracture cores and matrix cores, the gas flow changes of fractures and matrix cores with different permeability levels under different pressure gradients were measured, and the percolation mechanism of volcanic gas reservoirs with different pressure gradients was revealed when fractures and matrix cores were in parallel or series. The experimental results show that the fracture is the dominant channel relative to the matrix, and the supply and exhaust capacity of the fracture is much larger than that of the matrix, and the permeability of the matrix determines the supply and exhaust capacity of the series system composed of fracture and matrix

MULTIPHASE TRANSPORT OF FOAM FLUID IN POROUS STRUCTURES FOR ENHANCING OIL RECOVERY

Foams in pores affect the sweep range and oil displacement efficiency for low-permeability reservoir. In this paper, regular porous media with different coordination numbers are constructed to study how the parameters of microscopic pore structure affect foam flooding, and to identify the relationship between foam flooding efficiency and pore size. A two-dimensional microscopic pore structure model is established by computed tomography (CT) and the Imagej software. The sweep range and oil displacement efficiency of foam flooding are compared with that of water flooding. The results show that porous media with low coordination numbers can lead to higher foam flooding efficiency. When the pore size of porous media with a coordination number of 3 is 1 μm, the oil displacement effect is the best. Compared with water, foam can significantly improve oil displacement efficiency. Foam can block the dominant channel and drive out the residual oil in the narrow pore throat with higher efficiency. When the gas-liquid ratio of the foam is 3:1, the residual oil saturation is the lowest, and the flooding effect is the best. The higher contact angle of the wetting wall can produce better efficiency of foam flooding. When the wetting angle is 45°, the oil displacement effect is the best. When the injection rate is 0.5 m/s, the oil displacement effect is the best, but when the injection rate reaches 5 m/s, the residual oil saturation is the smallest. When the coordination number is 4, the oil displacement effect is the best.

The Microscopic Pore Structure Change and Its Correction with the Macroscopic Physicomechanical Properties of Sandstones after Freeze–Thaw Cycles

The Microscopic Pore Structure Change and Its Correction with the Macroscopic Physicomechanical Properties of Sandstones after Freeze–Thaw Cycles

STUDY ON THE EVOLUTION LAW OF PORE STRUCTURE AND MECHANICAL PROPERTIES OF MUDSTONE UNDER THE EFFECT OF WATER-ROCK CHEMISTRY

The influence of different pH and concentration of water chemical solutions on the mechanical properties of mudstone is studied. The triaxial compression test and nuclear magnetic resonance (NMR) test of mudstone are carried out, and the root cause of the deterioration of mechanical properties of mudstone during the hydration damage process is explored, combining macroscopic and microscopic. The results show that hydrolysis and chemical ion exchange are responsible for the deterioration of mudstone shear strength parameters. The essence of mudstone hydration damage is that water chemistry changes the original pore space structure and pore size distribution inside. Part of the minerals dissolve and fall off under the influence of the water chemical solution, which expands the pore size and increases the porosity. The connection force between mineral particles decreases, macroscopically, the mudstone undergoes a softening evolution from brittleness to ductility, and the shear strength parameters deteriorate. The results show that mudstone is more sensitive to acidic solution than neutral and alkaline solution. The higher the pH of the alkaline solution or the higher the concentration of the neutral solution, the more significant the deterioration of mudstone. Under the influence of hydrochemical solution, the fractal dimension of mudstone gradually tends to 2 from 3, which indicates that the process of hydration damage reduces the complexity of mudstone pore structure. On this basis, the damage variable D<sub>n</sub> defined based on the change of porosity before and after hydration damage of mudstone, to a certain extent, quantitatively reflects the law of mudstone shear strength parameters accompanied by microscopic pore structure evolution. The shear strength parameters of mudstone decrease significantly with the increase of damage variable D<sub>n</sub>. The test analyzes the root cause of the deterioration of the macroscopic mechanical properties of mudstone during the hydration damage process from a microscopic point of view, and the conclusions obtained provide a good reference for the quantitative study of the effect of water chemistry on the mechanical properties of mudstone.

Multi-scale Pore Structure Characterization of Lacustrine Fine-Grained Mixed Sedimentary Rocks and Its Controlling Factors: A Case Study of Lucaogou Formation in Jimusar Sag

The microscopic pore structure of fine-grained mixed sedimentary rocks (FMSRs) is of great significance for the occurrence mechanism and sweet spot optimization of mixed shale oil. At present, there is no clear understanding of the microscopic pore system of FMSRs and their coupling relationship with rock composition, which restricts the fine understanding of the reservoir forming mechanism of this type of reservoir. Taking the FMSR samples of the Permian Lucaogou Formation in the Jimusar Sag, the southeastern margin of the Junggar Basin, China, as an example, this paper quantitatively characterizes the full-scale pore structure of lacustrine FMSRs by casting thin sections, scanning electron microscopy, low-temperature nitrogen adsorption (LTNA), high-pressure mercury injection, and nuclear magnetic resonance (NMR); analyzes the coupling relationship between the full-scale pore structure and rock composition; and then summarizes the controlling factors. The results showed that the full-scale pore size distribution (PSD) can be revealed by combining the LTNA and NMR methods. The PSDs of FMSRs are evaluated, mostly corresponding to small pores (30–200 nm) and mesopores (200–1500 nm). The sediment grain size, mineral composition, sedimentary environment, and diagenesis jointly controlled the development of FMSR reservoirs. The development of coarser grain components (such as fine sand and arenaceous dolomite), high content of feldspar, low content of clay and calcite, and appropriate content of dolomite (less than 20%) were conducive to high-quality reservoirs. A near-provenance sedimentary pore throat is better. Siltstones, dolomitic siltstones, and dolarenites were mainly developed in shoreland sandy bar, distal sand bar, and dolomitic mud flat microfacies; type I pore throats with residual intergranular pores and dissolved pores are the most favorable for the Lucaogou Formation.

Reconstruction of LWD-NMR T2 water spectrum and fluid recognition based on microscopic pore structure constraints

The existing fluid property identification methods are not applicable to low resistivity reservoirs. In order to accurately identify low-resistivity oil reservoirs, the effects of fluid properties on the NMR T2 spectrum under different pore structures were analyzed. The results show that the T2 spectrum of the low-resistivity oil section shows a broad and gentle hilly shape, and the T2 spectrum of the water section shows a steep and mountainous shape. Based on the difference characteristics of T2 spectrum of reservoirs with different fluid properties, a new NMR water spectrum reconstruction and fluid property identification method based on microscopic pore structure constraint is innovatively proposed. Based on the drilling NMR logging data, three parameters characterizing the microscopic pore structure are firstly extracted from the NMR T2 spectrum of standard water reservoirs, which reflect the overall shape of the T2 spectrum in terms of transverse relaxation time, longitudinal pore fraction and These three parameters reflect the pore structure, water and oil and gas information of the reservoir in terms of the transverse relaxation time, the longitudinal pore fraction and the overall morphology of the T2 spectrum. Secondly, the microscopic pore structure parameters are used as the coordinate axes to construct a 3D water spectrum sample library, and the standard water formation T2 spectrum samples of the target block are stored in the 3D water spectrum library through the constraints of the three parameters. Finally, the NMR T2 spectrum parameters are extracted from the whole well section and the samples in the water spectrum library are called to realize the water spectrum reconstruction. The parameters that can distinguish oil and water formations are extracted from the measured T2 spectrum and the reconstructed water spectrum and fluid identification is performed. The results show that the reconstructed water spectrum is highly similar to the actual measured T2 spectrum in the production-proven standard water layer, which verifies the accuracy of the water spectrum reconstruction technique; in Caofeidian A oilfield, the fluid identification was performed on 20 production-proven test sections, of which 18 reservoirs were correctly identified, and the identification compliance rate reached 90%. The method circumvents the influence of pore structure in NMR T2 spectrum and highlights the information of its fluid properties, which improves the fluid property identification capability of logging with drilling (LWD) NMR logging.

Real-Time Monitoring of Pore-Fracture Structure Evolution during Coal Creep Based on NMR

Understanding the structural change of coals at micro- and mesoscales during the creep process is essential for exploring the time-dependent properties of deep coal and the long-term safe and effective operation of deep underground engineering. The conventional triaxial compression and creep tests on coal are carried out using a nuclear magnetic resonance (NMR) apparatus with a complete mechanical loading system at different confining pressures. The variations of parameters such as peak areas of various pore distributions, pore percentage, and porosity increment are quantitatively analyzed by real-time monitoring of T2 spectra during loading. Combined with nuclear magnetic resonance imaging (NMRI), accurate characterization and visualization of microscopic pore structures in coal are realized. The experimental results show that the adsorption pore proportion in the pore structure of the deep coal sample from Pingdingshan, Henan Province, is more than 80%. The changes in seepage pores and microfractures are most obvious in the triaxial compression and creep test. The creep deformation leads to the development of pore structure, and the creep porosity growth rate M increases gradually with increasing stress levels. M can accurately describe the rapid development of the pore structure and the damage accumulation due to time effects during creep. During creep, the most obvious changes in pore structure are the first and last levels of creep. The fractal dimension shows an “inverted V” trend during creep and is more sensitive to the inhibition of pore-fracture structure evolution due to increasing confining pressure. The creep strain increment and porosity increment during graded creep appear to be a similar behavior, indicating that the change of pore structure in the creep process based on the NMR system test can reveal the creep damage mechanism and indirectly characterize the deformation behavior of coal creep.

Effects of Clay Mineral Content and Types on Pore-Throat Structure and Interface Properties of the Conglomerate Reservoir: A Case Study of Baikouquan Formation in the Junggar Basin

Many factors need to be considered in the evaluation of tight conglomerate reservoirs, including the microscopic pore-throat structure, pore connectivity, lithology, porosity, permeability, and clay mineral content. The contents and types of clay minerals reflect the mineral evolution process during the deposition of the reservoir and can reflect the reservoir’s physical properties to a certain extent. In this study, cores from the Baikouquan Formation in Mahu were used to comprehensively analyze the effects of the clay mineral content on the physical properties of a tight conglomerate reservoir, including field emission scanning electron microscopy (FE-SEM), casting thin section observations, X-ray diffraction (XRD), interface property testing, high-pressure mercury injection, low temperature N2 adsorption, and nuclear magnetic resonance (NMR)-movable fluid saturation testing. The results revealed that differences in different lithologies lead to differences in clay mineral content and pore structure, which in turn lead to differences in porosity and permeability. The interface electrification, adsorption, and specific surface area of the reservoir are positively correlated with the clay mineral content, which is mainly affected by the smectite content. As the clay mineral content increases, the proportion of nanoscale pore throats increases, and the core becomes denser. The saturation of the movable fluid controlled by the >50 nm pore throats in the tight conglomerate ranges from 8.7% to 33.72%, with an average of 20.24%. The clay mineral content, especially the I/S (mixed layer of Illite and montmorillonite) content, is negatively correlated with the movable fluid. In general, the research results clarified the relationship between the lithology and physical properties of clay minerals and the microscopic pore structure of the tight conglomerate reservoirs in the Baikouquan Formation in the Mahu area.

Study on the Low Temperature Cracking Mechanism of Steel Slag Asphalt Mixture by Macroscale and Microscale Tests

Steel slag’s physical and mechanical properties are close to natural gravel, which can replace natural aggregate with paving asphalt mixture pavement. Based on macro- and microscales, it is necessary to study the low-temperature cracking characteristics of steel slag asphalt mixture (SAM). This paper aims to study the low-temperature cracking characteristics and influence mechanism of steel slag asphalt mixture by macro- and microscale tests. This article analyzes the strain field distribution and cracks propagation law of SAM at low temperatures by the three-point bending test combined with digital image correlation (DIC). The microscopic pore characteristics of SAM were studied by nuclear magnetic resonance (NMR). Based on fracture mechanics and weight function theory, the variation law of the stress intensity factor at the crack tip of the asphalt mixture with crack propagation is derived. The correlation between microscopic pores and low-temperature cracking characteristics of asphalt mixture was analyzed based on the grey correlation theory. The results show that SAM has more small and less large pores, and its total porosity is lesser than that of the basalt asphalt mixture (BAM). The initial elastic stage of SAM resisting bending deformation is longer than that of BAM, the bending strain energy density (dW/dV) is larger, and the crack penetration is slower. The horizontal strain energy density (DE) of SAM is greater than BAM, and the crack tip stress is greater. The porosity correlates well with the low-temperature crack resistance of the asphalt mixture.

Strength Characteristics of Biochar-Amended Clay Covered Soil Mixed with Methane-Oxidizing Bacteria

Adding biochar to soil can improve the soil’s physical–chemical properties, microscopic pore structure, and bacterial habitat. This affects the soil’s strength characteristics and the oxidization of methane. Using a Humboldt pneumatic direct shear instrument, this study investigated the effect of the amount of biochar in the soil, the soil’s methane-oxidizing bacteria, aeration time, and carbon content on the strength characteristics of a biochar-amended clay. The results show that when the biochar content is low, the soil’s stress–strain curve shows a strain hardening state as the strain increases. When the biochar content is greater than 10%, the methane-oxidizing bacteria increase as the shear strain increases. The stress–strain curves of the biochar–clay mixture all showed a softened state. Under the same biochar content, the soil’s stress–strain curves show strain softening as the methane filling time increases. However, with an increase in the amount of biochar, cohesion gradually increased and the internal friction angle did not change significantly. A scanning electron microscope (SEM) image of the biochar–clay mixture with methane oxidizing bacteria revealed the influence of the evolution law of the samples’ micropore structure on the soil’s stress–strain curve and strength properties.

Correlation between macroscopic properties and microscopic pore structure in steel-basalt hybrid fibers reinforced cementitious composites subjected to elevated temperatures

Steel-basalt hybrid fibers reinforced cementitious composites (SBFRCC) have excellent mechanical properties, and current studies mainly focused on the macroscopic or microscopic properties of SBFRCC at room temperature separately. However, the macroscopic and microscopic properties of SBFRCC subjected to elevated temperatures, especially their correlation relationships are still unclear. Hence, this study aims to investigate the macroscopic and microscopic properties and their correlation relationships of SBFRCC subjected to elevated temperatures. Firest, the mass loss rate, three-point bending, uniaxial compressive, mercury intrusion porosimetry (MIP) and scanning electron microscopy (SEM) tests were conducted subjected to elevated temperatures. Then, the correlations between mass loss rate, fracture, uniaxial compressive behavior and pore structure were analyzed based on a correlation analysis approach. The results show that steel fiber and basalt fiber can effectively restrict crack initiation and propagation subjected to elevated temperatures. Uniaxial compressive strength and elastic modulus decrease with temperature while peak strain increases below 600 °C and drops after 600 °C. With the increase of temperature, the gel pore and transitional pore decrease, the large pores increase, the porosity increases, and the pore structure is “coarsened” significantly. The correlation coefficient between uniaxial compression strength and porosity is the highest, and it is reasonable to use porosity to characterize the changes of uniaxial compressive strength subjected to elevated temperatures. Gel pore has the highest correlation with initiation fracture toughness while large pore has the highest correlation with mass loss rate, unstable fracture toughness, fracture energy, and elastic modulus. Gel pore directly affects the generation of cracks while large pore directly affects the propagation of cracks.