Pore Surface Fractal Dimension Research Articles

Understanding the impact of synthesis parameters on the pore structure properties of fly ash-based geopolymers

This paper systematically investigated the effects of alkali activator modulus (AM), alkali dosage (AD), and water-fly ash ratio (W/FA) on the pore volume, pore size distribution, pore volume fractal dimension (Dv), and pore surface fractal dimension (Ds) of fly ash-based geopolymer (FABGs) by orthogonal test and mercury intrusion porosimetry test. Dv and Ds of FABG can be calculated by the Menger sponge model and Zhang and Li's models, respectively. The relationship between multiple synthesis parameters and pore characteristics of FABGs was detailedly studied through range analysis and correlation detection analysis, which provides a basic understanding of the influence of multiple parameters on the pore characteristics of FABG, as well as the relationship between pore characteristics and mechanical properties. The results show that AM has the greatest effect on the volume proportion of gel pores and transition pores, and AD and W/FA have the greatest effect on the volume proportion of capillary pores and large pores, respectively. W/FA has the greatest effect on the porosity, and the porosity and the most probable aperture increase with the increment of W/FA and decrease with a rise in AD. It is worth noting that AM, AD, and W/FA have little effect on the Dv of larger pores, but have a greater on the Dv of smaller pores, with the influence order of AD-W/FA-AM. Ds of the large pores and capillary pores range between 2.70–2.90, and the Ds of the transition pores and gel pores are mostly greater than 3.00, which suggests that there may be more ink-bottle pores in transition pores and gel pores. In addition, there was a strong linear relationship between the porosity and compressive strength of FABGs, and the compressive strength of FABGs showed a decreasing trend with the increase of porosity. Compared with Dv, there is a stronger correlation between Ds and compressive strength, indicating that the roughness of the pore surface and the pore size distribution have a greater influence on the compressive strength of FABGs.

Characteristics of Permeability Evolution and Pore Structure of Coal with High Gas

To study the influence of gas pressure on coal permeability evolution, we conducted experiments on coal samples from the No. 9 coal seam in Tangshan Coal Mine, Hebei Province, China. Different gas pressures (helium and nitrogen) were applied, and nitrogen-induced deformations were measured. We also analyzed the coal samples’ pore structure using mercury injection porosimetry, obtaining pore surface fractal dimensions. The increase in nitrogen pressure from 0.3 MPa to 3 MPa resulted in an elevation of adsorption strain from 0.168 × 10−3 to 1.076 × 10−3, with a gradual decrease observed in the extent of this increase. However, the permeability of coal samples initially decreased from 16.05 × 10−18 m2 to 4.91 × 10−18 m2 and subsequently rose to 5.69 × 10−18 m2. Helium showed similar trends to nitrogen, with average permeability 1.42–1.88 times higher under the same pressure. The lowest permeability occurred at 1.5 MPa for helium and 2.5 MPa for nitrogen. Gas absorptivity plays a crucial role in coal permeability evolution. Additionally, we observed coal’s compressibility to be 7.2 × 10−11 m2/N and corrected porosity to be 53.8%, considering matrix compression. Seepage pores larger than 100 nm accounted for 80.4% of the total pore volume, facilitating gas seepage. Surface fractal dimension Ds1 correlated positively with micropore volume, while Ds2 and Ds3 correlated negatively with pore volume and gas permeability.

New Insight into Organomineral Interactions in Soils. The Impact of Clay-Size Peat-Derived Organic Species on the Structure and the Strength of Soil Silt Aggregates

Knowledge of the effects of different organic species on soil structure and strength is gained mostly from experiments on natural soils amended with organic substances of various particle sizes, pH, ionic composition, and inorganic impurities. It greatly diversifies the experimental results and shadows individual effects of organic amendments. Therefore, to look for a clearer view, we examined the impact of HCl-washed clay-size organic species: peat, humic acids, residue after humic acid extraction, and two biochars, all derived from the same peat and having similar particles, on the structure and strength of artificial soil silt aggregates using mercury intrusion porosimetry, bulk density measurements, SEM, and uniaxial compression. Bulk density increased due to humic acid addition and decreased for the other amendments. The total pore volumes behaved oppositely. All organic substances except humic acid decreased the pore surface fractal dimension, indicating a smoothening of the pore surface. Humic acid appeared to occupy mostly the spaces between the silt grains skeleton, while the other species were also located upon silt grains. The latter effect was most evident for 600 °C heated biochar. Humic acid, peat, and the residue after humic acid extraction improved mechanical stability, whereas both biochars weakened the aggregates, which means that bulk density plays a smaller role in the mechanical stability of granular materials, as it is usually considered. A new equation relating maximum stress and the amount of the organic additives was proposed.

Real-time pore structure evolution difference between deep and shallow coal during gas adsorption: A study based on synchrotron radiation small-angle X-ray scattering

Coal seam CO2 sequestration is an important option to address global warming. A better knowledge on coal pore structure evolution during gas adsorption can provide guidance for coal seams CO2 sequestration. However, few investigations on the pore structure evolution differences between the deep and shallow coal were conducted during gas adsorption. In this study, based on the real-time synchrotron radiation small-angle X-ray scattering (SAXS) observation, the average pore diameter and pore surface fractal dimension evolution differences between deep and shallow coal were investigated from the aspects of coal compositions and stress history. Two types of coal deformation (inner-swelling and outer-swelling) coexist during gas adsorption. Coal compositions have significant impact on the dominance of deformation type. The dominance of inner-swelling in deep coal is induced by the higher ash contents, and there is the decrease of average pore diameter during gas adsorption. The impact of stress-history (burial depth) on adsorption-induced deformation is more prominent than that of gas adsorption capacity. In deep coal, the surface fractal dimension evolution presents a negative correlation with the evolution of pore diameters. In shallow coal, the surface fractal dimension evolution presents a Langmuir-type correlation with the adsorption time.

Characterization, factors, and fractal dimension of pore structure of fly ash-based geopolymers

Fly ash-based geopolymer is a new cementitious material with environmentally friendly characteristics. Generally, the mechanical properties of cementitious materials are influenced by their pore structure. The variation in the pore structural characteristics of the geopolymers with various influencing factors was investigated in this study. A smaller activator modulus and liquid-solid ratio lead to a finer pore structure, while higher activator concentrations also result in a finer pore structure. However, the densification of pore structure does not follow a monotonic relationship with the conserving temperature. Increasing the conserving temperature within a certain range decreases the most probable pore diameter and porosity. Nevertheless, excessive temperature increase has the opposite effect. Furthermore, the fractal dimension was introduced to characterize the pore structure comprehensively. The results show that a lower pore volume, a larger pore-specific surface area, and a smaller most probable pore diameter contribute to increasing pore surface fractal dimension. Further, compared to porosity, the microporous pore surface fractal dimension shows a better linear correlation with compressive strength.

Investigation on the rheology, self-shrinkage, pore structure, and fractal dimension of coral powder-cement slurry

There is a significant amount of coral powder in coral aggregates, which has a negative impact on the operational performance, mechanical characteristics, and durability of coral reef sand concrete. The present study investigates the effect of coral powder with varying fineness and content on rheological and self-shrinkage properties using an Anton Paar MCR102 rotational rheometer and YC-BWS bellows self-shrinkage apparatus, respectively. The dynamic yield stress, rheological index, and plastic viscosity are obtained according to the Bingham model, Modified Bingham model, and Herschel-Bulkley model. The pore structures of coral powder-cement slurry are studied using a combination of MIP and SEM tests. Fractal theory is used to characterize the pore size distribution. The results demonstrate that the coral powder-cement slurry exhibits a shear thinning phenomenon. The Herschel-Bulkley model is successful in simulating the rheological behavior of cement slurry. The dynamic yield stress and plastic viscosity increase gradually as the fine coral powder content increases. With the increase in the coarse coral powder content, the dynamic yield stress decreases gradually while the plastic viscosity increases. The self-shrinking value of the coral powder-cement slurry increases rapidly within 1d but gradually stabilize with age. The self-shrinkage value increases with the increase in the coral powder admixture. The self-shrinkage value of the fine coral powder-cement slurry is less than that of the coarse coral powder-cement slurry. It is found that increase in the coral powder reduces the compressive strength of the cement paste, however, cement replacement with 10% coral powder is acceptable. Pore surface fractal dimension and capillary pore volume is able to effectively characterize pore size distribution and pore volume, respectively. Furthermore, the strength has a positive and negative linear relationship with the pore surface fractal dimension and capillary pore volume, respectively. The ratio of pore surface fractal dimension to capillary pore volume can be used as a parameter of the strength model. A linear relationship between the strength and the ratio of pore surface fractal dimension to capillary pore volume is proposed, and the model exhibits high accuracy.

Influences of MgO and PVA Fiber on the Abrasion and Cracking Resistance, Pore Structure and Fractal Features of Hydraulic Concrete

Abrasion resistance and cracking resistance are two important properties determining the normal operation and reliability of hydropower projects that are subjected to erosion and abrasive action. In this study, polyvinyl alcohol (abbreviated as PVA) fiber and magnesium oxide expansive agents (abbreviated as MgO) were used together to solve the problems of cracking and abrasive damage. The effects of PVA fiber and MgO on the mechanical property, abrasion and cracking resistance, pore structures and fractal features of high-strength hydraulic concrete were investigated. The main results are: (1) The incorporation of 4–8% Type I MgO reduced the compressive strength, splitting tensile strength and the abrasion resistance by about 5–12% at 3, 28 and 180 days. Adding 1.2–2.4 kg/m3 PVA fibers raised the splitting tensile strength of concrete by about 8.5–15.7% and slightly enhanced the compressive strength and abrasion resistance of concrete. (2) The incorporation of 4–8% Type I MgO prolongs the initial cracking time of concrete rings under drying by about 6.5–11.4 h, increased the cracking tensile stress by about 6–11% and lowered the cracking temperature by 2.3–4.5 °C during the cooling down stage. Adding 1.2–2.4 kg/m3 PVA fibers was more efficient than adding 4–8% MgO in enhancing the cracking resistance to drying and temperature decline. (3) Although adding 4% MgO and 1.2–2.4 kg/m3 PVA fibers together could not enhance the compressive strength and abrasion resistance, it could clearly prolong the cracking time, noticeably increase the tensile stress and greatly lower the racking temperature; that is, it efficiently improved the cracking resistance to drying and thermal shrinkage compared with the addition of MgO or PVA fiber alone. The utilization of a high dosage of Type I MgO of less than 8% and PVA fiber of no more than 2.4 kg/m3 together is a practical technique to enhance the cracking resistance of hydraulic mass concretes, which are easy to crack. (4) The inclusion of MgO refined the pores, whereas the PVA fiber incorporation marginally coarsened the pores. The compressive strength and the abrasion resistance of hydraulic concretes incorporated with MgO and/or PVA fiber are not correlated with the pore structure parameters and the pore surface fractal dimensions.

Comparison between the Influence of Finely Ground Phosphorous Slag and Fly Ash on Frost Resistance, Pore Structures and Fractal Features of Hydraulic Concrete

Hydraulic concrete in cold regions is necessary for good frost resistance. The utilization of finely ground PS (FGPS) in the construction of hydropower projects could solve the pollution issue and the fly ash shortage problem. In this work, the influence of FGPS and fly ash on frost resistance, pore structure and fractal features of hydraulic concretes was investigated and compared. The main results are: (1) The inclusion of 15–45% FGPS reduced the compressive strength of plain cement concretes by about 21–52%, 7–23% and 0.4–8.2% at 3, 28 and 180 days, respectively. (2) The inclusion of FGPS less than 30% contributed to the enhancement of 180-day frost resistance. At the same dosage level, the FGPS concrete presented larger compressive strengths and better frost resistance than fly ash concrete at 28 and 180 days. (3) At 3 days, both the addition of FGPS and fly ash coarsened the pore structures. FGPS has a much stronger pore refinement effect than fly ash at 28 and 180 days. The correlation between frost resistance of hydraulic concrete and pore structure is weak. (4) At 28 days, the incorporation of FGPS and fly ash weakened the air void structure of hydraulic concrete. At 180 days, the presence of FGPS and fly ash was beneficial for refining the air void structure. The optimal dosage for FGPS and fly ash in terms of 180-day air void refinement was 30% and 15%, respectively. The frost resistance of hydraulic concretes is closely correlated with the air void structure. (5) The pore surface fractal dimension (Ds) could characterize and evaluate the pore structure of hydraulic concretes, but it was poorly correlated with the frost resistance.

Particle-size effect of recycled clay brick powder on the pore structure of blended cement paste

Herein, based on the selected particle sizes of 0–25 μm, 25–45 μm and 45–75 μm, the particle-size effect of recycled clay brick powder (RCBP) on the pore structure of blended cement pastes was investigated. Results show that RCBP particle size has a significant impact on the pore size and pore volume distributions and pore surface fractal dimension of blended paste throughout the observed age and on porosity and tortuosity only at later ages. Reducing RCBP particle size proved effective in improving the pore structure of blended paste. Notably, tortuosity is strongly correlated with compressive strength in an exponential correlation.

Surface-fractal-dimension characteristics of cementitious composites with multi-walled carbon nanotubes dispersed by silica fume

The effects of adding silica fume and multi-walled carbon nanotubes (MWCNTs) to cement mortar on its pore surface fractal dimension (SFD) are analyzed. The SFD is expressed through the Zhang and Li model based on mercury intrusion porosimetry (MIP) experiments. The silica fume serves as a filler and pore refiner for the paste and increases the SFD of the mesopores by forming a C–S–H gel through the pozzolanic reaction. Additionally, the well-dispersed MWCNTs increase the SFD by complicating the surfaces of the mesopores. The silica fume and MWCNTs considerably change the pore structure of the mesopores, but do not significantly affect the capillary pores. Although the correlation between the SFD and compressive strength of the cement mortar is rather poor, it has been shown to have an exponentially proportional relationship overall. Further, the SFD of the capillary pores has a greater effect than that of the mesopores on the compressive strength because of the load concentration effect.

Fractal Characteristics of the Pore Structure of Coral Powder–Cement Slurry under Different Fractal Models

In this study, coral powder with different contents and levels of fineness were incorporated into cement; then, the pore structure of a coral powder–cement slurry was measured using the MIP test at days 3 days and 28, respectively. Neimark’s model, Ji’s model, and Pfeifer and Avnir’s model were also used to analyze the fractal characteristics of the coral powder–cement slurry. The results show that the coral powder–cement slurry has multifractal characteristics when using Neimark’s model, and the entire pore size range of the cement slurry can be divided into three parts: Region I (1–200 μm), Region II (70 nm–4 μm), and Region III (5–500 nm). The pore surface fractal dimension of both Regions I and III is less than 3, while that of Region II is greater than 3. This indicates that Regions I and III have obvious fractal characteristics, which Region II does not. Meanwhile, the pore surface fractal dimension of Region I is positively correlated with hydration age, while the pore surface fractal dimension of Region III is less affected by hydration age and coral powder contents. Ji’s model reveals that coral powder–cement slurry also has multifractal characteristics, but the entire pore size range of the cement slurry can be divided into two parts: Region I (5.482 nm–500 m) and Region II (120 nm–370 μm). The pore volume fractal dimension of Region I is greater than 2 and less than 2.5, while that of Region II is greater than 2.9 and less than 3. Therefore, both Regions I and II have fractal characteristics. In addition, the coral powder admixture, fineness, and age have large effects on the pore volume fractal dimension and pore size range of Region II. Pfeifer and Avnir’s model reveals that the entire pore size range of cement slurry can also be divided into Region I (5.482–600 nm), Region II (120 nm–10 μm), and Region III (5–365 μm), and that the pore surface fractal dimension of both Regions I and III is less than 3, while that of Region II is greater than 3. This indicates that Regions I and III have fractal characteristics, while Region II does not have fractal characteristics.

Controlling Factor Analysis of Microstructural Property and Storage Capacity of Deep Longmaxi Formation Shale in Sichuan Basin

As the commercial development of shallow shale gas reservoirs has been successfully achieved in Sichuan Basin, the deep shale gas reservoirs become the key development target in the next step. The accurate quantitative characterization of the fluid storage and structural properties of deep shale is of great significance in resource evaluation and gas exploitation. In this study, fifteen deep Longmaxi Formation shale samples belonging to three sublayers are collected from the L208 well area, Luzhou Block, southern Sichuan Basin. Nuclear magnetic resonance and nitrogen adsorption are conducted on these samples to obtain the fluid storage capacity and microstructural information. Pore fractal dimension and surface fractal dimension are interpreted from these experimental data for quantitative evaluation of the pore distribution complexity and pore surface irregularity, respectively. The controlling factors of storage and microstructural properties of these deep shales are analyzed. From the perspective of stratigraphic division, the shales belonging to the lower sublayers possess a larger transverse relaxation time of the main peak, a weaker nitrogen adsorption amount, a lower pore volume, a lower specific surface area, and a higher surface fractal dimension as compared to those of the shales from the upper sublayer. From the perspective of shale composition, clay mineral content and total organic carbon content are the main and minor controlling factors of the storage capacity, pore fractal dimension, and surface fractal dimension of deep Longmaxi shale, respectively. Moreover, the pore surface irregularity is relevant to the average pore diameter and specific surface area.

Mechanical properties and fractal analysis of cement mortar incorporating styrene-butadiene rubber latex and carboxylated MWCNTs

Styrene-butadiene rubber (SBR) modified cement-based material owns many advantages, however, reduction in compressive strength within cement composite has limited its application. This paper evaluates the effect of carboxylated multiwall carbon nanotubes (MWCNT-COOH) on the mechanical properties and microstructure of SBR modified cement mortar. MWCNT-COOH was firstly dispersed within SBR by using ultrasonic vibration, and then the MWCNT-COOH/SBR suspension was incorporated into cement mortar. Four different kinds of mortars were compared in this study, i.e., plain cement mortar, pure SBR modified mortar, pure MWCNT-COOH modified mortar, and MWCNT-COOH/SBR hybrid modified mortar. The strength and fracture toughness of these composites were firstly analyzed, in which the experiment results showed that a significant improvement was achieved by incorporating MWCNT-COOH and SBR hybrid nano-composites. The pore structures and surface fractal dimensions (Ds) of cement mortars were determined by using mercury intrusion porosimetry (MIP) and a fractal model, the relationships between the mechanical properties and the porosity as well as the Ds values were then revealed and discussed in this study. It was found that the adding MWCNT-COOH/SBR hybrid and mono MWCNT-COOH greatly refined the pore size and reduced the porosity of cement mortar, resulting in higher mechanical properties. Fractal analysis indicated that there existed a positive relationship between Ds and compressive strength, and the addition of MWCNT-COOH and MWCNT-COOH/SBR hybrid greatly increased the Ds values of cement-matrix composite. Fourier transform infrared spectroscopy (FTIR) and field emission scanning electron microscopy (SEM) were used to characterize the phase composition, mineralogy and microstructure. Results indicated that the homogeneous dispersion of MWCNT-COOH/SBR hybrid nano-composites, a strong interfacial adhesion, and the formation of load-transfer networks are responsible for the considerable enhancement.

Repair of ordinary Portland cement concrete using alkali activated slag/fly ash: Freeze-thaw resistance and pore size evolution of adhesive interface

Adhesive interface is the weak part of old concrete after repair, which is an important factor affecting the functionality and durability of new-to-old concrete. In order to make full use of industrial solid waste, the old concrete was repaired with the mixture of slag and fly ash (SFA) as cementitious materials in this paper considering its strong cementitious effect under the condition of alkali activator. The modification of new-to-old concrete adhesive interface was studied using different mass ratio of slag to fly ash. The cohesiveness and durability of new-to-old concrete interface were investigated systematically. Pore surface fractal dimension (PSFD) was used to further explore the mechanism of improving durability adhesive interface durability. The results indicated that the splitting tensile strength and durability of adhesive interface can be improved by adjusting the mass ratio of slag to fly ash. With 50% slag addition, the defects and harmful pores in adhesive interface were the least. The contents of C-S-H, C-A-S-H, and N-A-S-H gels in adhesive interface repaired with AASFA were increased significantly, among which the OPC-G50 was most improved. The adhesive interface was the weakest part of the repaired structure, the utilization of AASFA promoted the formation of amorphous gels, which improved the microstructure density, interfacial pore volume content, mechanical behaviors and F-T resistance significantly.

PORE STRUCTURAL AND FRACTAL ANALYSIS OF THE INFLUENCE OF FLY ASH AND SILICA FUME ON THE MECHANICAL PROPERTY AND ABRASION RESISTANCE OF CONCRETE

Dam concrete suffers from serious abrasion damages in southwestern China, the abrasion resistance of concrete is therefore one of the most important factors determining the reliability even the safety of the dams. In the present work, the effects of fly and/or silica fume on the mechanical properties, drying shrinkage, as well the cracking and abrasion resistance of concrete were investigated, then the pore structures of concrete added with fly ash and silica fume and the pore surface fractal dimensions ([Formula: see text]) were determined by the mercury intrusion porosimetry (MIP) method and a fractal model, respectively. Finally, the relationships between the abrasion resistance of concrete and the porosity as well as the [Formula: see text] were revealed and discussed. The results indicate that silica fume significantly increases the drying shrinkage especially at early age, while the incorporation of fly ash and silica fume together can decrease the early plastic shrinkage-induced cracking risk and the final shrinkage to some degrees. Besides, the utilization of 5[Formula: see text]wt.% silica fume and 20[Formula: see text]wt.% fly ash together increases the abrasion resistance and mechanical property by about 4–9% at various ages. In addition, the compressive strength and the abrasion resistance of concrete are linearly correlated with the concrete porosity and the [Formula: see text]. Both the fly ash and silica fume could decrease the porosity of concrete and increase the [Formula: see text], therefore the concrete containing fly ash and silica fume have desired mechanical property and abrasion resistance. Moreover, [Formula: see text] has a more profound effect on the abrasion resistance of concrete than the concrete porosity. The addition of 5[Formula: see text]wt.% silica fume and 20[Formula: see text]wt.% fly ash together is suitable for the concretes used for wearing surfaces in terms of mechanical property, cracking and abrasion resistance.

Dynamic compressive behaviour of hybrid basalt-polypropylene fibre-reinforced concrete under confining pressure: Experimental characterisation and strength criterion

The dynamic compressive behaviour of hybrid basalt-polypropylene fibre-reinforced concrete (HBPRC) under confining pressure was investigated using a Hopkinson compression bar device with active confining pressure. The relationship between the pore structure and the dynamic compressive behaviour of HBPRC was discussed. The results indicate that the dynamic compressive behaviour of HBPRC under confining pressure shows evidently the strain rate dependence and the confining pressure dependence. The addition of basalt fibre (BF) and polypropylene fibre (PF) reduces the failure degree of concrete and improves the shear failure feature. The relationship between the pore surface fractal dimension and the strain rate effect of HBPRC indicates that the dynamic mechanical properties of HBPRC are affected not only by the fibre itself but also by the effect of fibre on the pore structure. In view of strengthening and toughening, BF is conducive to increasing the strength of HBPRC, whereas the PF is conducive to improving the toughness. Finally, based on the Hoek-Brown strength criterion, an empirical dynamic strength criterion for HBPRC accounting for the effect of fibre and pore structure is established and verified.

Microstructure and mechanical properties alterations in shale treated via CO2/CO2-water exposure

CO2 injections into shale reservoirs can enhance the recovery of oil and gas, while simultaneously sequestrating CO2. However, with the injection of CO2, the microstructure and mechanical properties alteration of shales caused by CO2/CO2-water exposure is should be noticed, which is of great importance to predicting the efficiency of CO2-enhanced shale oil/gas recovery and safety of CO2 sequestration in shale reservoirs. Therefore, this study clarifies the influence of sub/super-critical CO2-water exposure on the microstructure and mechanical properties of shale. Results show that after Sub/ScCO2 and Sub/ScCO2 water exposure, the contents of carbonate and clay in shale were decreased. Additionally, the proportion of micropores and mesopores were decreased, while the proportion of macropores was increased, leading to the increase of porosity and the improvement of pore connectivity, and the decreases of the pore surface fractal dimensions D1 and pore volume fractal dimensions D2 in shale. The uniaxial compressive strength (UCS) and elastic modulus (E) of shale were also decreased. Acoustic emission (AE) analysis results indicated that the cracks closure stage and stable crack propagation stage was prolonged and shorten, respectively, and AE signals show obvious multifractal characteristics. ScCO2 induced greater alteration in pore structure and mechanical properties than SubCO2, and the alterations were enhanced for CO2-water exposure. UCS and E of shale samples show a good negative correlation with porosity, and a positive correlation with D1 and D2. UCS in shale is positively related to the proportion of micropores and mesopores, and negatively related to the proportion of macropores, which indicated that microstructure alterations inevitably affected the macroscopic mechanical properties of shale.

Structures and fractal characteristics of pores in long-flame coal after cyclical supercritical CO2 treatment

Injecting CO2 in deep coal seams for enhanced coalbed methane recovery (ECBM) also benefits the environment through simultaneous CO2. Most studies have concentrated on the changes of pore structures that result from sustained supercritical CO2 (ScCO2) treatment. Conversely, the effects of cyclic ScCO2 treatment have been rarely investigated. In this study, a high-pressure reactor system was used to perform cyclic ScCO2 treatment of long-flame coal. Nuclear magnetic resonance techniques and low-pressure nitrogen gas adsorption were employed to determine the fractal dimensions of coal samples and quantitatively characterize their pore structures before and after cyclic treatment. The results demonstrated that the porosity and proportion of macropores increased significantly after treatment. Therefore, new pores were formed, and some small pores might have been converted into macropores. As the treatment duration increased, the daily average porosity rate exhibited an increasing trend that was presumably caused by the effects of coal matrix fatigue. Further, a decreasing tendency was captured in both the pore-surface fractal dimensions of adsorption pores and the pore-volume fractal dimensions of seepage pores, while increasing the cyclic treatment. This phenomenon after treatment indicated that pore roughness and complexity were decreased. A conceptual model was proposed to explain the mechanisms underlying the evolution of coal-pore structures during the treatments, and the effects of the cyclic injection on the CO2-ECBM field were also analyzed. Therefore, our findings have important guiding significance for selecting suitable CO2 injection methods for CO2-ECBM projects.

Fractal Dimension of Basalt Fiber Reinforced Concrete (BFRC) and Its Correlations to Pore Structure, Strength and Shrinkage.

In this study, we focused on exploring the correlations between the pore surface fractal dimensions and the pore structure parameters, strength and shrinkage properties of basalt fiber-reinforced concrete (BFRC). The pore structure of BFRCs with various fiber contents and fiber lengths was investigated using mercury intrusion porosimetry (MIP) measurements. Through Zhang’s model, the fractal characteristics of BFRCs in the whole pore size range and in different pore size ranges were calculated from the MIP test data. The results showed that the addition of BF increased the total porosity, total pore volume and pore area but decreased the average pore diameter, indicating that BFs refined the pore structure of the concrete. BFRC presented obvious fractal characteristics in the entire pore-size range and individual pore-size ranges; generally, the fractal dimension increased with increasing fiber content. Moreover, correlation analysis suggested that the fractal dimension of BFRC in the whole pore-size range (FD) was closely related to the fractal dimension in the macropore region (Dm) and average pore diameter (APD). The influence of pore structure factors on mechanical strength and shrinkage was studied by grey correlation theory, and the results showed that Dm showed positive correlations with strength and fracture energy, with increasing Dm tending to strengthen and toughen the concrete. An increase in fiber content and length was detrimental to reducing the drying shrinkage strain. In the transition pore region, the fractal dimension (Dt) at diameters ranging from 20 to 50 nm and shrinkage strain exhibited a highly linear relation. These results merit careful consideration in macro-property evaluation by using the pore surface fractal dimension in a specific region instead of the whole region. Finally, grey target theory was applied to evaluate the rank of the mechanical strength and shrinkage of concrete, and the results showed that the overall properties of concrete with a BF length of 18 mm and a BF content of 0.06% ranked the best.

Fractal characteristics of pore structure of hardened cement paste prepared by pressurized compact molding

The microstructure of hardened cement paste is directly related to its macroscopic properties. Pressurized compact molding can effectively improve the microstructure of hardened cement paste. In order to study the characteristics of pore structure of hardened cement paste and the mechanism of microstructure regulation and defect reduction after pressure compaction, Mercury Intrusion Porosimetry (MIP) and X-Ray Diffraction (XRD) was used to characterize the microstructure of hardened cement paste and the mass fraction of the main phase. Results indicate that the pressurized compact molding significantly reduced the total porosity and critical pore diameter of hardened cement paste, and the pore size was micro-nano. The pore surface fractal dimension (SFD) of the paste was multi-segmental fractal and scale dependent after compaction, and the irregular drift behavior in the transition region was not obvious. A new understanding of the pore volume distribution characteristics of hardened cement paste was proposed based on the change of main phase mass fraction.