Particle Packing Research Articles

Synergistic effect of nano silica on carbonation resistance of multi-blended cementitious mortar

Confiscation of alkaline buffer in a blended cementitious system surges the risk of carbonation. Understanding the carbonation mechanism and kinetics of multi-blended cementitious systems in correspondence to microstructural properties is the need of the hour. In this context, the change in the microstructure of binary, ternary, and quaternary blended cementitious mortar mix comprising of fly ash or/and ultra-fine fly ash or/and nano-silica upon accelerated carbonation (3.5% CO2; 70% RH) was studied. All multi-blended mixes were proportioned using modified Andreasen and Andersen particle packing theory. Permeable porosity and carbonation parameters such as carbonation depth, rate of change in compressive strength, and carbonation shrinkage were measured. Further, qualitative/quantitative estimation of carbonation phases was done using characterization techniques such as TGA and FTIR. In control mix with solely OPC, the reaction of CO2 with calcium-bearing phases showed chemo-mechanical changes leading to 18% improvement in strength at 30 days of exposure. The optimized multi-blended cementitious systems with nano-silica exhibited higher resistance to carbonation kinetics. Phase assemblages quantified through TGA within depth of carbonation imply a negligible concentration of portlandite (CH). However, mixes without nano-silica exhibited a significant reduction in bound water content associated with C–S–H/AFt/AFm phases and intensified the precipitation of calcium carbonate (CaCO3) phase. Asymmetric stretching band of C–O–C at 1424 cm−1 corresponding to calcite phase measured using FTIR validates the outcomes of TGA.

The emergence of Airy stress function in two-dimensional disordered packings of particles

The packing of hard-core particles in contact with their neighbors is a statically determinate problem. The probability functional method allows deriving the full system of equations for the stress tensor components analytically. For an isotropic and homogeneous two-dimensional packing, we derive the classical Euler–Cauchy and Navier equations; the latter allows expressing the stress tensor of the system in terms of the Airy stress function.

Locally Resolved Simulation of Gas Mixing and Combustion Inside Static and Moving Particle Assemblies

AbstractResolving the flow field within the voids between particles of reacting granular assemblies is essential to obtain correct conversion rates. Due to the associated computational cost, a high spatial resolution cannot be employed on the device‐scale. A combination of a locally resolving and an averaging method is suggested. Namely, the so‐called blocked‐off method is used to resolve the voids between particles within prescribed regions, while the volume‐averaged method is applied in the remaining domain. For a nitrogen jet injected into a regular simple cubic packing of spherical particles passed by a cross‐flow of air, the jet dispersion is too low when applying the volume‐averaged method for the whole computational domain. The same is also observed in a reacting methane jet flowing into a bed of arbitrarily packed particles with air cross‐flow, where the locally resolved approach predicts significantly larger local reaction rates.

Impact of mica on geotechnical behavior of weathered granitic soil using macro and micro investigations

The micaceous weathered granitic soil (WGS) is frequently encountered in civil engineering worldwide, unfortunately little information is available regarding how mica affects the physico-mechanical behaviors of WGS. This study prepares reconstituted WGS with different mica contents by removing natural mica in the WGS, and then mixes it with commercial mica powders. The geotechnical behavior as well as the microstructures of the mixtures are characterized. The addition of mica enables the physical indices of WGS to be specific combinations of coarser gradation and high permeability but high Atterberg limits. However, high mica content in WGS was found to be associated with undesirable mechanical properties, including increased compressibility, disintegration, and swelling potential, as well as poor compactability and low effective frictional angle. Microstructural analysis indicates that the influence of mica on the responses of mixtures originates from the intrinsic nature of mica as well as the particle packing being formed within WGS. Mica exists in the mixture as stacks of plates that form a spongy structure with high compressibility and swelling potential. Pores among the plates give the soil high water retention and high Atterberg limits. Large pores are also generated by soil particles with bridging packing, which enhances the permeability and water-soil interactions upon immersion. This study provides a micro-level understanding of how mica dominates the behavior of WGS and provides new insights into the effective stabilization and improvement of micaceous soils.

Effect of Particles Dispersion State and Packing Ability of the Slurry on the Density of Green Body for Ceramics Wet Shaping Processes

It is important to control and characterize the particles dispersion and aggregation state in a slurry for ceramics wet forming processes. In these processes the particle concentration of the slurry must be high, thus, it is desired to directly evaluate the particles dispersion state for dense slurries without dilution. In this paper, we discuss the relationship between the density of green body fabricated in tape casting process and the slurry characteristics measured without dilution, such as apparent viscosity (relative viscosity) and the packing fraction of sediment. It was demonstrated that the slurry with the highest packing fraction of sediment yields the green sheet with the highest density, while the slurry with the lowest apparent viscosity (relative viscosity) does not necessarily yield the green sheet with the highest density. It was indicated that the key of slurry characterization for controlling the density of green sheet should be considering the change of the particles dispersion and aggregation state due to thickening and dehydrating of the slurry in tape casting process.

Dehydration‐driven mass loss from packs of sintering hydrous silicate glass particles

AbstractGlass sintering involves the densification of packs of particles and the expulsion of the interparticle pore gas. The pore space begins as a convolute interconnected interparticle network, and ends as distributed isolated bubbles; two configurations that are separated by the percolation threshold. Here, we perform experiments in which (i) the particles are initially saturated in H2O at 871 K, and (ii) they are then heated non‐isothermally at different rates to temperatures in excess of 871 K. In step (ii), H2O becomes supersaturated and the particles diffusively lose mass as they sinter together. We use thermogravimetry to track the loss of mass with time. We find that the mass loss is initially well predicted by solutions to Fick's second law in spherical coordinates with the appropriate material and boundary conditions. However, as the sintering pack crosses the percolation threshold at a time predicted by sintering theory, we find that the mass loss deviates from simple diffusional solutions. We interpret this to be the result of an increase in the diffusion distance from the particle‐scale to the scale of the sintering pack itself. Therefore, we conclude that the open‐ to closed‐system transition that occurs at the percolation threshold is a continuous, but rapid jump for diffusive and other transport properties. We use a capillary Peclet number Pc to parameterize for this transition, such that at low Pc diffusive equilibrium is achieved before the sintering‐induced transition to closed system, whereas at high Pcthere is a “diffusion crisis” and disequilibrium may be maintained for longer relative timescales that depend on the system size.

Void fraction for random loose packing of multi-size mixed cylindrical particles

Most particles in industrial applications are mixed cylindrical particles with the same diameter and different aspect ratios (ARs). In this study, the void fraction of packed bed with binary, ternary, quaternary, and quintuple mixtures of cylindrical particles is explored by discrete element method (DEM). The concept of equivalent AR is proposed for predicting void fraction of packing of mixed cylindrical particles based on previous research. Results show that (1) the correlation between equivalent AR and void fraction is higher when ARVave is used, and the prediction error is only −0.57%–6.21%. (2) Better prediction results can be obtained using ARNave in some cases, such as for packing of mixed cylindrical particles with small AR intervals and wide distribution. However, the prediction results are unsatisfactory in the case of larger AR intervals and cylindrical particles with AR = 1. (3) The R2 can be increased to 0.983 when corrected by multiplying by 96.87%.

Exploring new solvent mixtures for near-net shaping of Ultra-High Temperature Ceramics via colloidal processing

Suspensions of Ultra-High Temperature Ceramic (UHTC) powder with various organic solvents and co-solvent mixtures were prepared with the intention of verifying and understanding the relationship between interparticle interactions, flow behavior, and resulting particle packing. These relationships can be used to predict optimal solvent choices in colloidal processing of UHTCs to guarantee suspension stability through an adequate balance of the interparticle forces in suspension. Four organic solvents were selected: cyclohexane (CYC), tetrahydrofuran (THF), acetone (ACE) and N,N-dimethylformamide (DMF). The theoretical performance of each suspension, based on interparticle interaction energies, was compared to the empirical results. Detailed flow behavior was studied through rheological measurements and particle packing was measured by Archimedes’ method. The relationship between interparticle forces, viscosity and particle packing is discussed. Other contributing factors such as the performance of the dispersing agent were also considered and surface characteristics were reported using X-ray Photoelectron Spectroscopy (XPS), providing a modification to the adsorption parameter in the repulsive model. Low viscosity slurries were obtained at approximately 40% solid loading. CYC and THF demonstrated the ability to be effective co-solvents and produce the highest suspension stability with 100% CYC. These findings will help to produce UHTC complex shape component with minimal flaws during processing that can withstand extreme, high temperature oxidizing environments after densification.

A diffused-interface model for the lyophilization of a packed bed of spray-frozen particles

Spray freeze-drying is particularly suitable for the preservation of biopharmaceuticals as it involves gentle drying and can easily be integrated with continuous manufacturing strategies. This process is still an evolving application, and its potential is often being explored experimentally. However, experimental methods are expensive and time-consuming. Therefore, much effort is currently focused on the development of mathematical models to understand the basic mechanisms and hence lay the foundation for analysis and experimentation. Even though a few models were proposed in the past, all of them presented various flaws and failed in describing the process behavior. We propose a multiscale approach, which is able to reproduce the structure of a packing of spray-frozen particles and extract detailed pore-scale geometrical features, informing the final vial-scale drying and heat transfer simulation. This latter step is the main innovation here presented, a new model that is based on the concept of a diffused interface and describes the process in a more accurate way.

Scalable Hierarchically Structured Materials from a Multiscale Particle System Enabled by Microscaffolds.

Structural hierarchy is the key to manufacturing multiscale particle-based composite materials. A novel manufacturing method was developed to generate scalable hierarchical structures in concrete. The new method used 3D-printed microscaffolds to interact with the multiscale particle packing in concrete, resulting in a structured lightweight composite material. The size of internal members can vary by more than two orders of magnitude, to adapt to different applications. Based on compression tests and microstructural investigation by optical microscope and quantitative nanomechanical mapping, we found that the new material is 63.93% more efficient in energy absorption capacity compared with traditional lightweight concrete. Our experimental trials also showed that introducing structural hierarchy can reduce the consumption of cementitious material in the system by up to 14% and significantly reduce the use of scaffolds. The method could be applied to a board spectrum of multiscale particle-based materials, such as dental cement and bone implant materials, to improve material performance and efficiency in medical and construction applications.

Optimization of concrete mix design based on three-level separation distance of particles

This study proposed a modified mix design approach for producing concrete with reduced cement content and CO2 emissions. Three key design parameters: BSD (binder separation distance), FSD (fine aggregate separation distance) and CSD (coarse aggregate separation distance), from paste, mortar and concrete level are suggested based on the particle packing theory. The potential synergistic effects of BSD, FSD and CSD on fresh behavior, compressive strength and binder efficiency of designed concrete are evaluated. The results show that by considering the effect of particle specific area and particle size distribution, three proposed parameters exhibit better eco-efficiency and high correlations with concrete properties. The experiments together with numerical analysis show that the correlations coefficient of BSD, FSD and CSD together on slump, flow rate and compressive strength are 0.979, 0.923 and 0.913, respectively, indicating that these three values can be used together to tailor the properties of concrete. It is found that BSD is the most important parameter in determining the mechanical property, with a high correlation of 0.97. While FSD and CSD together are more sensitive on fresh behaviors. In addition, a prediction model based on machine learning was established, the errors between the measured and predicted values are within 10% and the binder efficiency in mixtures designed with three scale separation distances were increased by 7% in general, which validates the feasibility of using particle separation distances as key design parameters, and the applicability of this prediction method.

Microstructure and strength of microporous MgO refractory aggregates with nano-sized pores

ABSTRACT In this work, microporous MgO refractory aggregates with nano-sized pores were prepared by in situ decomposition method using Mg(OH)2 as raw material. The effects of firing temperatures (1600–1700°C) and compacting pressures (100–200 MPa) on the microstructures and properties of microporous MgO refractory aggregates were thoroughly studied. The results showed that the microporous MgO refractory aggregates contained two types of pore structures, which were intra-particle pores with pore sizes of 180.0–230.0 nm formed by in situ decomposition of Mg(OH)2 and inter-particle pores with pore sizes of 1.5–3.0 μm derived from particle packing between Mg(OH)2 pseudomorph particles, respectively. Besides, the firing temperatures had a great influence on the intra-particle pore size and microcrystallite size. And the compacting pressures not only influenced the intra-particle pore size via packing behaviors but also affected the inner firing behaviors of the pseudomorph particles due to the increase in H2O vapor pressure. Overall, at a compacting pressure of 150 MPa and firing temperature of 1650°C, the sample had the best comprehensive performance with a bulk density of 1.92 g/cm3, a compressive strength of 11.9 MPa, an apparent porosity of 45.0%, a relative aggregate tube strength of 25.2% and a median pore size of 262.3 nm.

Powder Spreading Mechanism in Laser Powder Bed Fusion Additive Manufacturing: Experiments and Computational Approach Using Discrete Element Method.

Laser powder bed fusion (LPBF) additive manufacturing (AM) has been adopted by various industries as a novel manufacturing technology. Powder spreading is a crucial part of the LPBF AM process that defines the quality of the fabricated objects. In this study, the impacts of various input parameters on the spread of powder density and particle distribution during the powder spreading process are investigated using the DEM (discrete element method) simulation tool. The DEM simulations extend over several powder layers and are used to analyze the powder particle packing density variation in different layers and at different points along the longitudinal spreading direction. Additionally, this research covers experimental measurements of the density of the powder packing and the powder particle size distribution on the construction plate.

Effect of Cement and Bitumen Emulsion on the Drying and Film Formation of Bitumen Emulsion Paste

To obtain a bitumen emulsion paste with a good bitumen film structure and rapid drying rate, the effect of cement content, cement type, and emulsion type on the drying and film formation of bitumen emulsion paste was studied. The drying and film formation mechanism of emulsion was employed to study the drying curve of bitumen emulsion paste. Results indicate that the drying and film formation process of bitumen emulsion paste can be divided into three stages. The solid volume fraction of the beginning of the final stage (ϕ2) and the maximum particle volume fraction (ϕm) are proposed to evaluate the drying and film formation quality of bitumen emulsion paste. Higher values of ϕ2 and ϕm indicate a denser particle packing state and lower residual water content of the final stage, which are beneficial to the drying and film formation quality of bitumen emulsion paste. The ϕ2 and ϕm of bitumen emulsion paste differ slightly with the increasing content of ordinary portland cement. However, these quantitative indexes vary significantly with changing bitumen emulsions with different values of ϕ2 and ϕm. Compared with ordinary portland cement, sulfoaluminate cement can lead to a noticeable decrease in ϕ2 and ϕm; thus, its addition is harmful to the film structure of bitumen emulsion paste. Overall, to obtain a bitumen emulsion paste with a good bitumen film structure and rapid drying rate, a bitumen emulsion with good drying and film formation quality is recommended.

A nonlinear particle packing model for micro-aggregate

The nonlinear packing model for micro-aggregate was constructed to predict the actual packing density of micro-aggregate based on the nonlinear packing model for granular soil. The characteristics, differences and applications of five common calculation models of packing density were discussed. The correlation coefficients of the particle packing model based on granular soil were modified to derive packing density relationship for the micro-aggregates. The average relative error of the measured results is less than 0.4% and the root mean square error is within 3.2 × 10−3, which means the validity of the derived relationship is verified.

Application of mixture design of experiments to the development of alkali-activated composites based on chamotte and waste glass

Alkali-activated binders have been indicated as possible replacement for Portland Cement with environmental benefits. However, an adequate dosage of raw materials and alkalis content is mandatory to obtain alkali-activated materials with suitable physical and mechanical characteristics. The aim of this work is the application of mixture design of experiments to determine optimal dosages for pressed alkali-activated composites based on two precursors: chamotte (C) and waste glass (WG). Fine sand (S) was used as aggregate. Ten mixtures were obtained from the experimental design, and cylindrical samples were cast by uniaxial pressing. The alkalis/precursor ratio was kept constant for all the mixtures. After curing, the physical, mechanical and microstructural characteristics of the specimens were assessed. The application of desirability function revealed that the optimal composition was the one with higher content of sand (50%), 30% chamotte and 20% waste glass. The addition of sand contributed to reduce porosity and increase the particle packing, promoting the increase of mechanical strength. Regarding compressive strength, almost all the mixtures met the requirement for solid bricks. This study demonstrates the potential of using mixture design of experiments as an efficient tool to determine the proper composition and predict the behavior of waste-based alkali-activated materials.

Compactness matters: Improving Bayesian optimization efficiency of materials formulations through invariant search spaces

Would you rather search for a line inside a cube or a point inside a square? Physics-based simulations and wet-lab experiments often have symmetries (degeneracies) that allow reducing problem dimensionality or search space, but constraining these degeneracies is often unsupported or difficult to implement in many optimization packages, requiring additional time and expertise. So, are the possible improvements in efficiency worth the cost of implementation? We demonstrate that the compactness of a search space (to what extent and how degenerate solutions and non-solutions are removed) affects Bayesian optimization search efficiency. Here, we use the Adaptive Experimentation (Ax) Platform by Meta™ and a physics-based particle packing simulation with eight or nine tunable parameters, depending on the search space compactness. These parameters represent three truncated log-normal distributions of particle sizes which exhibit compositional-invariance and permutation-invariance characteristic of formulation problems (e.g., chemical formulas, composite materials, alloys). We assess a total of four search space types which range from none up to both constraint types imposed simultaneously. In general, the removal of degeneracy through problem reformulation (as seen by the optimizer’s surrogate model) improves optimization efficiency. The code is hosted at https://github.com/sparks-baird/bayes-opt-particle-packing. We recommend that optimization practitioners in the physical sciences carefully consider the trade-off between implementation cost and search efficiency before running expensive optimization campaigns.

Investigation of spherical and non-spherical binary particles flow characteristics in a discharge hopper

The particulate flow characteristics of granular particles including mung beans, wooden spheres, wooden cylinders, wooden cubic, plastic spheres, plastic cylinders, and their binary mixtures in a flat bottom hopper were studied experimentally using high-speed high-resolution camera recordings. An image processing method based on the color threshold was developed to calculate the area ratio for binary mixtures. The mass discharge rates, the residue inclination angles, and the changes in edge height and center height with time were also investigated. In addition, the effects of different initial packing patterns on the particulate flow, mixing and segregation behaviors were analyzed and compared. It is interesting to find that the shape of particles and the initial binary particle packing patterns have significant effects on the discharging characteristics. The results of this study provide helpful guidance for industrial applications and provide experiment data for computational model validation.

A benchmark study of different numerical methods for predicting rock failure

Nowadays, with many available numerical methods developed in rock mechanics, researchers have always focused on the parameter calibration of the numerical model but ignored the predictive capability of these methods. A comparative study of nine commonly used numerical methods was performed for predicting rock failure through international cooperation organized by the Discontinuous Deformation Analysis (DDA) commission of the International Society for Rock Mechanics (ISRM). Two steps of numerical modelling were conducted including a calibration procedure from given experimental results and a numerical prediction for benchmark tests with these calibrated parameters for three types of rocks. Through the comparison between different numerical and experimental results, the inherent weaknesses and strengths of different numerical methods in terms of predicting rock failure were identified and analysed. The influence of human intervention in terms of parameter selection is even more significant than the choice of different numerical methods. Some potential factors (i.e., different boundary conditions, heterogeneity of rock material, strength parameters, particle packing, and failure criteria) that may occur in numerical and physical tests were further discussed. Through the comparison of different failure criteria, we found the selection of rock failure criterion might be the major factor that affected the predictive capability of numerical methods, and the nonlinear failure model (the Hoek–Brown criterion) showed the superiority in the prediction of rock fracturing subjected to complex stress conditions. This comparative work also enlightens the significance of a high-quality calibration process and the future advancement of rock failure criteria.

Recycling of steel slag powder in green ultra-high strength concrete (UHSC) mortar at various curing conditions

The large quantity of steel slag deposit has caused great environmental pressure. This study aims to recycle steel slag as binder in the production of green ultra-high strength concrete (UHSC) subjected to different curing regimes (i.e. standard and steam curing). The fresh behaviors, mechanical properties, microstructure and ecological value of the UHSC are explicitly assessed with the help of the Funk and Dinger particle packing model. The results show that the inclusion of steel slag powder (SSP) increases the workability (up to about 13%) and wet packing density of UHSC. The use of SSP results in a decrease of early compressive strength, because the SSP inhibits cement hydration at early stage. The steam curing can minimize this negative impact of SSP on the early compressive strength (>120 MPa). Besides, the increase of SSP dosage results in a reduction in the flexural strength for both early and later ages, but steam curing reduces the decline ratio. The high temperature curing not only promotes the hydration process of both cement and SSP, but also accelerates the pozzolanic reaction of silica fume, resulting in a lower porosity (4.78%) of UHSC with SSP. Finally, the ecological assessment shows that the recycling SSP as a replacement of cement in the UHSC system can reduce energy consumption and carbon emissions. Therefore, the results in this study indicate the feasibility of utilizing SSP in the fabrication of sustainable UHSC products with excellent performance.