Fine Pore Structure Research Articles

Variation Characteristics of Mass-Loss Rate in Dynamic Seepage System of the Broken Rocks

When the collapse column and its adjacent rocks in complex geological structures are disturbed by mining, concomitant fine particle migration, mass loss, and porous structure variation during the water seepage process in broken rocks are the inherent causes for collapse column activation and water inrush. Studying the time-varying characteristics of the mass-loss rate in the dynamic seepage system of the broken rocks is of theoretical importance for the prevention of water inrush from the collapse columns. In this study, the seepage tests of the broken mudstone were carried out using the patented pumping station seepage method, and the time-varying function of the mass-loss rate was generalized. Then, the optimal coefficients in the function of mass-loss rate were computed using the genetic algorithm. At last, the mass-loss rate in the dynamic seepage system of the broken rocks with consideration of the acceleration factor was calculated using Lagrange discrete element method. The results showed that (1) the mass-loss rate was expressed as a time-dependent, exponential function with its coefficient related to the porosity, and its time-varying characteristics were affected by gradation; (2) the time-varying curves with Talbol power exponents less than 0.6 had a rapid change stage and a slow change stage, while the time-varying curves with Talbol power exponents greater than 0.6 had an initial gradual change stage, a rapid change stage and a slow change stage; (3) at the early seepage stage, the mass-loss rate decreased with Talbol power exponent increasing; and (4) after long time seepage, the mass-loss rate was close to zero and unrelated to Talbol power exponent, and the porous structure in broken rocks remained stable with its porosity close to a certain stable value.

Influence of nano metakaolin on thermo-physical, mechanical and microstructural properties of high-volume ferrochrome slag mortar

This study investigates the effect of nano metakaolin (NMK) on the thermo-physical, mechanical and microstructural properties of hardened cement mortar containing high amount of ferrochrome (FeCr) slag. Sand was replaced with FeCr-slag aggregate at the ratio of 50 mass, %. Cement was partially replaced with different amounts of NMK at the ratios of 2, 4, 6, 8, 10, 12 and 14 mass, %. Compressive strength, flexural strength, drying shrinkage, capillary water absorption and thermal conductivity of the hardened nano-modified mortars were determined in accordance with ASTM standards at 28 days of water curing. The microstructure characteristics of the hardened samples were investigated by scanning electron microscope (SEM) equipped with energy dispersive analytical X-ray unit (EDAX). The results showed considerable improvements in the mechanical and thermal performance of cement mortar, due to the incorporation of FeCr-slag; in addition, the replacement of cement by NMK was found to be effective in providing an additional enhancement in the mechanical strength and thermal conductivity. Furthermore, NMK containing mortar has considerably reduced drying shrinkage and capillary water absorption. The reduction in drying shrinkage for the mortar containing 50 mass, % FeCr-slag and modified with 10 mass, % NMK reached 87% relative to the reference mortar. The microstructure of the hardened mortar containing NMK appeared quite dense and compact with finer pore structure.

Hypervelocity impact testing on stochastic and structured open porosity cast Al-Si cellular structures for space applications

A hypervelocity (6.7–7.0 km/s) impact testing campaign was conducted using Ø 2 mm, 95% Al projectiles onto A357 aluminium alloy stochastic foams with 4–5 mm typical pore dimensions, or alternatively diamond cubic periodic structures of cast AlSi12 with a 6 mm lattice parameter, in order to assess their performance as shielding material against orbital impacts. Either 0.15 mm Al foil or multi-layer insulation (MLI) was used as a front bumper material. It was found that the periodic structures failed to retain the impact debris for any incident angle of impact between 0° and 12°, at least in part due to ricochets and/or spalled material finding its way through open straight channels within the periodic structure. A porous material architecture traversed by no open, straight path is thus required for proper impact protection. Stochastic foams satisfy this criterion and indeed were found to stop the debris. Depending on bumper configuration, stochastic foams gave comparable performance to that of simple Whipple shield designs at 1.3–1.9 × the areal weight. We suggest that a finer pore structure with respect to the projectile diameter should yield a higher impact absorption per areal weight. As an auxiliary result, it was found that MLI as a front bumper was less efficient in fragmenting the projectile compared to Al foil of similar areal weight. In conclusion, open porosity stochastic foams are a promising material as sandwich panel cores for space applications, as they may reduce the need for a dedicated shield, so long as the small debris produced by the impact can be isolated from the satellite systems.

Micropore Structure of Cement-Stabilized Gold Mine Tailings

Mine tailings have often to be stabilized by mixing them with cementing agents. In this study, the pore structure of gold tailings stabilized with Portland cement was evaluated by means of mercury intrusion porosimetry. The investigation was conducted on samples prepared with different fractions of tailings and cement as well as on samples activated with elevated temperature curing and chemical (CaCl2) addition. It was observed that all mixed samples exhibit a mono-modal pore size distribution, indicating that the cement-stabilized tailings are characterized by a single-porosity structure. The results also showed that the higher fraction of tailings and cement leads to a dense and finer pore structure. The total porosity of mixture samples decreases with increasing curing temperature and CaCl2 concentration due to the acceleration of hydration reaction.

Manufacturing of γ-LiAlO2 matrix for molten carbonate fuel cell by high-energy milling

Molten carbonate fuel cells (MCFCs) are promising high temperature power generating devices. However, unlike solid oxide fuel cells (SOFCs) they utilize a liquid electrolyte which must be immobilized in a porous matrix.In this paper, a slurry composition for lithium aluminate (γ-LiAlO2) matrix was developed and green matrices were subsequently formed by the tape casting method. In order to achieve the desired structure of the matrix (pore size, porosity) γ-LiAlO2 powder was milled in a planetary ball mill for 18 h with a solvent, dispersant and defoamer. After this step, other ingredients were added, including a binder and plasticizer to obtain optimal rheology of the slurry. Cell tests confirmed optimal performance of the matrix compared to the third party reference γ-LiAlO2 matrices. Burned out matrix was characterized by scanning electron microscopy (SEM) and laser diffraction in order to determine the γ-LiAlO2 powder particle size and morphology. The results show that high-energy milling enabled a fine pore structure and high specific surface area of the matrix to be obtained in a relatively short time, compared to conventional fabrication routes. The matrix structure obtained within this study is suitable for high performance operation of MCFC.

Application of organic- and nanoparticle-modified foams in foamed concrete: Reinforcement and stabilization mechanisms

In this study, an ultra-stable foam used for foamed concrete was fabricated by modifying the gas–liquid interface by coupling the effects of an organic surfactant and nanoparticles. A serial of techniques was employed to study the products and microstructure development of foam and foamed concrete, respectively. The results show that nano-silica and hydroxypropyl methylcellulose can slow the coalescence and disproportionation of the bubbles by adsorbing at the air bubble surface and increasing the viscosity of cell wall paste, thus preventing gas transfer and hindering physical drainage between the gaseous and liquid phases. Also, a more homogeneous and finer pore structure is generated with the inclusion of the organic surfactant and nanoparticles. The pozzolanic reaction between the nano-silica and Ca(OH)2 results in an increased C-S-H content and the densification of the cell wall structure. In addition, the stability of foamed concrete with modification and relevant stabilization mechanisms were also investigated.

Is Physisorption Useful for Fine Pore Structure Control? Control of Pore Structure and Properties of SBA-15 by Paraffin Physisorption

A simple method for controlling the pore structure and properties of ordered mesoporous silica (OMS) by the physisorption of organic molecules is reported. The paraffin-modified OMSs were characterized by N2 gas and water vapor adsorption isotherms, small-angle X-ray scattering, and infrared spectroscopy. We found that paraffin first adsorbed on the pore surface defects, resulting in a smooth OMS pore surface; thus, the paraffin controlled the pore structure. In addition, the surface hydrophobicity increased with increasing paraffin filling fraction.

Hydration characteristics and structure formation of cement pastes containing metakaolin

Metakaolin (MK) is one of the most effective mineral admixtures for cement-based composites. The deposits of kaolin clays are wide-spread in the world. Metakaolin is comparable to silica fume as an active mineral admixture for cement-based composites. In this paper, the rheological and mechanical properties of cement paste containing metakaolin are investigated. The effect of MK is more evident at “tight” hydration conditions within mixtures with low water-cement ratio, provided by application of superplasticizers. The cement is replaced with 0 to 15% metakaolin, and superplasticizer content ranged from 0 to 1.5% by weight of cementitious materials (i.e. cement and metakaolin). An equation is derived to describe the relationship between the metakaolin and superplasticizer content and consistency of pastes. There is a linear dependence between metakalolin content and water demand. Second-degree polynomial describe the influence of superplasticizer content. The application of SP and MK may produce cement-water suspensions with water-retaining capacity at 50-70% higher than control suspensions. The investigation of initial structure forming of cement pastes with SP-MK composite admixture indicates the extension of coagulation structure forming phase comparing to the pastes without additives. Crystallization stage was characterized by more intensive strengthening of the paste with SP-MK admixture comparing to the paste without admixtures and paste with SP. Results on the porosity parameters for hardened cement paste indicate a decrease in the average diameter of pores and refinement of pore structure in the presence of metakaolin. A finer pore structure associated with an increase in strength. X-ray analysis data reveal a growing number of small-crystalline low-alkaline calcium hydrosilicates and reducing portlandite content, when MK dosage increases. Scanning electron microscopy (SEM) data confirm, that hardened cement paste containing MK has crystalline structure with dominance of partially crystalized hydrosilicates and gel-like formations.

Effect of natural carbonation on the pore structure and elastic modulus of the alkali-activated fly ash and slag pastes

The aim of this paper was to investigate the effect of natural carbonation on the pore structure, and elastic modulus (Em) of alkali-activated fly ash (FA) and ground granulated blast furnace slag (GBFS) pastes after one year of exposure in the natural laboratory conditions. The chemical changes due to carbonation were examined by X-ray diffraction (XRD), scanning electron microscope/energy-dispersive X-ray (SEM−EDX) and attenuated total reflectance Fourier transformed infrared spectroscopy (ATR-FTIR). Subsequently, the pore structure and Em of the degraded material were tested by mercury intrusion porosimetry (MIP), nitrogen (N2) adsorption, and nanoindentation.The chemical degradation of alkali-activated pastes due to natural carbonation is showed to be dependent on the GBFS content and their pore structure development. It was found that the pure alkali-activated GBFS paste was not carbonated at all within the tested period due to fine gel pore structure. On the other hand, carbonation of the gel in the pastes consisting FA and GBFS generated significant mineralogical and microstructural changes. The extensive decalcification of the gel was reflected in the increase of nanoporosity. Consequently, the Em of the carbonated pastes decreased.This study suggests that the degradation of alkali-activated FA and GBFS pastes due to carbonation may be accurately evaluated through micromechanical properties measurements rather than only by testing alkalinity of the pore solution and corrosion of reinforcement such as commonly studied carbonation effect in the ordinary Portland cement (OPC)-based materials.

Preparation of hierarchical porous metals by two-step liquid metal dealloying

A hierarchical porous metal has been prepared by two-step liquid metal dealloying using an alloy precursor containing two soluble components and two metal baths. After the first dealloying step, one of the soluble components is dissolved and a coarse porous structure is formed. After a shorter dealloying step in another melt, the other soluble component is dissolved and a fine porous structure is formed within the previously formed coarse ligaments. The resultant porous metal presents a hierarchical morphology with a bimodal pore distribution, resulting in a higher porosity and surface-area-to-volume ratio than what can be formed by one-step dealloying methods.

Control of the Structure of Porous Glass-Ceramic Material

The kinetics of pore formation in pressed parts of the system cullet – clay – colemanite, fired with different rates of heating, was investigated. It was found that the measured heating rates (2.5 – 3 K/min) bring the processes occurring during firing closer to equilibrium and make it possible even at 680°C to obtain glass ceramic materials with uniform fine porous structure and satisfactory properties and to preserve the shape of the parts.

Adsorption of perfluorooctane sulfonate (PFOS) on corn straw-derived biochar prepared at different pyrolytic temperatures

The adsorption behavior of perfluorooctane sulfonate (PFOS) on biochar produced from corn straw at varying pyrolytic temperatures (i.e., 250, 400, 550, and 700 °C, BC250–700, respectively) was investigated. High pyrolytic temperatures (>400 °C) resulted in the increase of some fine-pore structures, larger surface area (SA) and higher aromaticity of biochar. The adsorption kinetics of PFOS fitted well to a pseudo-second order model while the adsorption isotherms followed a Langmuir adsorption isotherm model (R2>0.949). The adsorption capacity increased with the pyrolytic temperature with the highest adsorption capacity (i.e., 169.30mg/g) being obtained on BC 700. The adsorption of PFOS on the biochar samples was mainly governed by hydrophobic and electrostatic interactions. The hydrophobicity enhancement of biochar pyrolyzed at high temperatures is one of the important factors contributing to PFOS adsorption. The capacity of biochar for adsorbing PFOS decreased with the increase of solution pH. The adsorption of PFOS on the biochar samples is a spontaneous and endothermic process.

Slow Pyrolysis Magnetization of Hydrochar for Effective and Highly Stable Removal of Tetracycline from Aqueous Solution

Although biochar has been intensively studied as an inexpensive adsorbent for diverse organic pollutants in aqueous solution, synchronously achieving high adsorption capacity, separability, and stability is still a challenge. Herein, we partially addressed this issue via an integrated activation and pyrolytic magnetization of sawdust hydrochar, during which the surface area of magnetic activated sawdust hydrochar (M-SDHA) increases from 1.7 to 1710 m2 g–1, and the weight loss decreases from 70 to 5% at 700 °C. Correspondingly, the maxmium adsorption capacity of M-SDHA toward tetracycline (TC) reaches 423.7 mg g–1 and remains constant at pH 5–9. Multiple characterizations show that the fine pore structure and surface functional groups of M-SDHA were maintained during the pyrolysis magnetization process, which is responsible for the high adsorption capacity. In the pyrolysis magnetization process, the FeCl3 was reduced to Fe3O4 which endowed M-SDHA with magnetism and may simultaneously improve the thermosta...

Facile fabrication of mechanical monolithic polyamide aerogels via a modified sol–gel method

The good-formability polyamide (PA) aerogel via a modified sol–gel technology was facilely fabricated by using melamine and isophthaloyl chloride with the acid-binging agent followed by CO2 supercritical drying. The synthesis procedure was straightforward and simple, relying on no nitrogen-based protective atmosphere. The influences of various parameters, such as the amount of acid-binging agent, reaction temperature and precursor concentration, on the aerogel structure were discussed. The structural properties of PA aerogels were characterized by the scanning electron microscopy, transmission electron microscopy, Brunauer–Emmett–Teller methods and Fourier transform infrared spectroscopy. The results indicated that the PA aerogel had a typical three-dimensional porous structure. Due to the moderate mechanical property of the PA aerogel, the aerogel had potential for the use in construction and building materials. We reported that the good-formability polyamide (PA) aerogel via a modified sol–gel technology was facilely fabricated by using melamine and isophthaloyl chloride with the acid-binging agent followed by CO2 supercritical drying. The synthesis procedure was straightforward and simple, relying on no nitrogen-based protective atmosphere. The dimethylacetamide (DMAC) as the acid-binging agent could indeed remove the hydrochloric acid to accelerate the cross-linking degree of the PA prepolymer and improve the formation of a fine porous structure of the PA aerogel. Thus, the specific surface area of the PA aerogel after the addition of acid-binging agent could be enhanced as high as 391 m2/g, higher than that (282 m2/g) of the one without the DMAC. Furthermore, it also showed the influences of various parameters, such as reaction temperature and precursor concentration, on the aerogel structure. According to the structure analysis, the results indicated that the PA aerogel had a typical three-dimensional structure with various porous size. Due to the moderate mechanical property, the PA aerogel had potential for use in construction and building materials.

Pore Structure Evolution and Its Effect on Strength Development of Sulfate-Containing Cemented Paste Backfill

In this study, the effects of the initial sulfate content on the properties of cemented paste backfill (CPB) made from coarse tailings has been investigated via mercury intrusion porosimetry. The combined effects of the sulfate content and curing time on the total porosity, pore size distribution, and unconfined compressive strength of the produced material were discussed. It was found that the specimens with an initial sulfate content of 5000 and 35,000 ppm exhibited higher unconfined compressive strength, while the resulting fine porous structures characterized by pore radii of 10–400 and 1–10 μm significantly improved the mechanical properties of the CPB. In addition, an increase in the curing time decreased the overall pore volume in the radius range of 1–400 μm but increased the pore volume at pore radii less than 1 μm.

The Role of Phosphorus Slag in Steam-Cured Concrete

Steam curing is an effective method to increase the hydration degree of binder containing phosphorus slag. The role of phosphorus slag in steam-cured concrete was investigated by determining the hydration heat, hydration products, nonevaporable water content, pore structure of paste, and the compressive strength and chloride ion permeability of concrete. The results show that elevated steam curing temperature does not lead to new crystalline hydration products of the composite binder containing phosphorus slag. Elevating steam curing temperature enhances the early hydration heat and nonevaporable water content of the binder containing phosphorus slag more significantly than increasing steam curing time, and it also results in higher late-age hydration degree and finer pore structure. For steam-cured concrete containing phosphorus slag, elevating curing temperature from 60°C to 80°C tends to decrease the late-age strength and increase the chloride permeability. However, at constant curing temperature of 60°C, the steam-cured concrete containing phosphorus slag can achieve satisfied demoulding strength and late-age strength and chloride permeability by extending the steam curing duration.

Pressureless sintering and gas flux properties of porous ceramic membranes for gas applications

The preparation and characterization of kaolin based ceramic membranes using styrofoam (STY) and sawdust (SD) as pore formers have been prepared by mechano-chemical synthesis using pressureless sintering technique with porogen content between (0–20) wt% by die pressing. Pellets were fired at 1150 °C and soaking time of 4 h. The membranes cast as circular disks were subjected to characterization studies to evaluate the effect of the sintering temperature and pore former content on porosity, density, water absorption and mechanical strength. Obtained membranes show effective porosity with maximum at about 43 and 47% respectively for membranes formulated with styrofoam and sawdust porogens but with a slightly low mechanical strength that does not exceed 19 MPa. The resultant ceramic bodies show a fine porous structure which is mainly caused by the volatilization of the porogens. The fabricated membrane exhibited high N2 gas flux, hence, these membranes can be considered as efficient for potential application for gas separation by reason of the results shown in the gas flux tests.

On drying shrinkage in alkali-activated concrete: Improving dimensional stability by aging or heat-curing

The problem of excessive drying shrinkage in alkali-activated concrete (AAC) is well-documented in the literature. The magnitude of drying shrinkage is often three or more times that in portland cement concrete. This study investigates the effects of binder type, activator concentration, strength, age, and curing method on the manifestation of drying shrinkage in alkali-activated fly ash and slag cement concrete. Early-age shrinkage strains in excess of 1200με (0.12 percent strain) are observed in AAC. This is attributed to delayed hydration, microstructure refinement, and strength development. The resulting damage is far more significant than in portland cement concrete. Shrinkage and resulting damage are greatly reduced when specimens are dried at later age and after heat-curing. Alkali-activated slag cement concrete is more sensitive to water loss than portland cement or alkali-activated fly ash concrete. This results from a finer pore structure in alkali-activated slag binders.

Experimental studies and thermodynamic modeling of the carbonation of Portland cement, metakaolin and limestone mortars

The carbonation of Portland cement, metakaolin and limestone mortars has been investigated after hydration for 91days and exposure to 1% (v/v) CO2 at 20°C/57% RH for 280days. The carbonation depths have been measured by phenolphthalein whereas mercury intrusion porosimetry (MIP), TGA and thermodynamic modeling have been used to study pore structure, CO2 binding capacity and phase assemblages. The Portland cement has the highest resistance to carbonation due to its highest CO2 binding capacity. The limestone blend has higher CO2 binding capacity than the metakaolin blends, whereas the better carbonation resistance of the metakaolin blends is related to their finer pore structure and lower total porosity, since the finer pores favor capillary condensation. MIP shows a coarsening of the pore threshold upon carbonation for all mortars. Overall, the CO2 binding capacity, porosity and capillary condensation are found to be the decisive parameters governing the carbonation rate.

Carbon nanotube (CNT) and nanofibrillated cellulose (NFC) reinforcement effect on thermoplastic polyurethane (TPU) scaffolds fabricated via phase separation using dimethyl sulfoxide (DMSO) as solvent

Although phase separation is a simple method of preparing tissue engineering scaffolds, it suffers from organic solvent residual in the scaffold. Searching for nontoxic solvents and developing effective solvent removal methods are current challenges in scaffold fabrication. In this study, thermoplastic polyurethane (TPU) scaffolds containing carbon nanotubes (CNTs) or nanofibrillated cellulose fibers (NFCs) were prepared using low toxicity dimethyl sulfoxide (DMSO) as a solvent. The effects of two solvent removal approaches on the final scaffold morphology were studied. The freeze drying method caused large pores, with small pores on the pore walls, which created connections between the pores. Meanwhile, the leaching and freeze drying method led to interconnected fine pores with smaller pore diameters. The nucleation effect of CNTs and the phase separation behavior of NFCs in the TPU solution resulted in significant differences in the microstructures of the resulting scaffolds. The mechanical performance of the nanocomposite scaffolds with different morphologies was investigated. Generally, the scaffolds with a fine pore structure showed higher compressive properties, and both the CNTs and NFCs improved the compressive properties of the scaffolds, with greater enhancement found in TPU/NFC nanocomposite scaffolds. In addition, all scaffolds showed good sustainability under cyclical load bearing, and the biocompatibility of the scaffolds was verified via 3T3 fibroblast cell culture.