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Abstract The movement of sand dunes in desert railways has various harmful effects, which reduces operational safety and speed and causes great maintenance and renewal costs. To encounter the movement of sand dunes, different traditional and modern methods are implemented, none of which covers completely the problems, especially the ballast solidification and line closuring as the major ones. In this paper, a new system called "humped slab track" is introduced, in which the ballast solidification problem is obviated with the application of ballast-less superstructure. The problem of track closuring with sand dunes is also mitigated by means of elevating the rails higher, using uncases reinforced concrete called "humps", and letting the sands traverse from the generated superstructure space between the humps and beneath the rails, just like fluid. To show the efficiency of the proposed system, the simulation of sand movements from the track section was performed using two-phase analysis in the FLUENT finite element software.

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In this paper, the important characteristics of solidification including supercooling degree, solidification time, nucleation temperature, phase change temperature which affecting on efficiency are experimentally studied. A purposely designed experimental device was used to investigate the solidification characteristics of titania nanofluid (0.01%wt. 0.02% wt. and 0.04%wt.). The results evidently reveal that adding titania nanoparticles to Deionized water as a base fluid can reduce the time of solidification, phase change temperature and supercooling degree. By adding 0.04% wt. titania nanoparticles, the solidification time, phase change temperature and supercooling degree are reduced by 70%, 18%, 69% while nucleation temperature is enhanced by 29%. Thus, the time of solidification is more affected by adding nanoparticles than other solidification characteristics. Further, the experimental results show that nanofluid heat flux is higher than that of base fluid. Also a comparison of Fuzzy logic modelling and experimental results for liquid fraction is studied. The results reveal that the fuzzy logic modelling is a reliable and powerful technique for predicting the liquid transient fraction. From the results it is also concluded that extremely low concentration of titania have low average error.

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Applications of hollow spherical particles in industry and in thermal spraying process have been developed in recent years. Despite dense droplets, in hollow droplets, the volume changes of the gas play an important role in the dynamics of impact and the shape of the formed splats. In plasma thermal spraying, impact velocities of particles to the surface is in the range of 50 m/s-300 m/s, therefore, changes in pressure and volume of the trapped gas, is important. In this research, impact of hollow droplet on a flat surface and its solidification has been simulated. Volume of fluid model for compressible flows at real thermal spraying condition is used while the impact velocities in the range of 50 m/s-300 is considered. In a few moments after the impact of droplet on the surface, a pressure wave is formed in the air. This wave, increase the vorticity in vicinity of interface of two fluid, which has a great effect on shaping the formed splats. Simulations showed that shape of formed splats vary with velocities in the range of 50 m/s-300 m/s. In higher velocities, the surface of the formed splat is more porous.

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In this research, the impact of a completely molten hollow droplet and a semi-molten hollow droplet on a surface is simulated numerically. At first, the production process of hollow particles from the agglomerated particles is addressed analytically. By this model, one can predict the particle diameter, solid core diameter and shell thicknesses of produced particle. The results of this section show that hollow particle may hardly develop at small initial porosity values (p=0.2). Then, the collected data from analytical model is used as input data for numerical simulation. In the numerical model, the central solid core was assumed to be a fluid with high viscosity. Due to high impact velocity, volume and density changes of the trapped gas inside droplet are important. Therefore the compressible form of governing equations is used. The results show that the hydrodynamic and solidification behavior of a completely molten droplet and a semi-molten droplet during impact process are different. In the semi-molten state, the central solid core prevents the formation of a counter jet. For this reason, a hollow semi-molten droplet is solidified faster than a completely molten hollow droplet. The overall time of solidification in the completely molten state is 35 μs and the corresponding time for semi-molten state, is 12 μs. Moreover the splat of a semi-molten hollow droplet is more continues compared with a completely molten droplet

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Abstract: Introduction: Solidification and stabilization of heavy metal contaminants is recognized as the technology to prevent transfer of contaminants to the lower layers of soil and groundwater. A noticeable increase in distribution of heavy metal contaminants in recent years highlights the importance of effective methods for engineering disposal of industrial wastes. The most important challenge ahead of this endeavor is perhaps the determination of right framework and mechanism of action. Precise mechanism of mobility of contaminants can be grasped by gaining accurate and comprehensive understanding about system behavior and evaluating it from the nano- and micro-structure perspectives. Nano- and micro-sized clay particles can be used effectively as adsorbents of many contaminants (e.g. heavy metal ions and organic compounds) in sewage and wastewater. Moreover, as clay soils have high cation exchange capacity (CEC), they provide appropriate conditions for cation exchange and create considerable capacity to retain heavy metal contaminants. In spite of conducting extensive studies on stabilizing contaminants by the use of cement, inadequate attentions have been paid to microstructure study of interaction process of clay particles, heavy metal ions, and cement, specifically in cement hydration process in different time intervals. Based on this, the present research aims to study the interaction process of clay particles, heavy metal contaminants, and cement over time from the perspective of microstructure. This include the investigation of the effect of presence of heavy metal on cement hydration process and formation of nano-structure calcium silicate hydrate (C-S-H). Material and method: In this study, the behavioral tests were conducted on natural clay soil collected from the Qazvin Plain, Iran. The purpose of this selection was to determine geotechnical-environmental properties and contaminant adsorption-retention capability of samples of natural clay with average specific surface area and CEC and the effects of natural clay on the solidification and stabilization process. The majority of experiments of this study were conducted based on ASTM standards and geotechnical-environmental test guidelines of McGill University (Canada). Density and pH of clay samples were determined in accordance with ASTM, D854 and ASTM, D4972 standards. Soil carbon content was determined by titration. Specific surface area (SSA) of the soil was measured using EGME solution. The cation exchange capacity (CEC) of the soil was determined using 0.1 M barium chloride solution. Meanwhile, different concentrations of heavy metal contaminant (zinc) and different percentages of Portland cement were added to natural clay. The interaction process was analyzed experimentally by examining pH changes and evaluating microstructure study (XRD). Result and discussion: According to laboratory results obtained in this study, the high specific surface area of C-S-H nanostructure improves the adsorption characteristics and leads to better filling of pores. It also improves the retention capability by decreasing the mobility of heavy metal contaminants via encapsulation of their ions (solidification). The results show that formation of C-S-H nanostructure improves absorption features due to high specific surface area and decreases mobility of the heavy metal ions through their encapsulation (solidification). In addition, the presence of the heavy mental contaminant (zinc) reduces formation of C-S-H nanostructure so that the presence of 25 cmol/kg-soil of heavy metal ion (zinc) decreases peak intensity of C-S-H nanostructure about 160 CpS.

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Stabilization/solidification (S/S) has emerged as a cost-effective method for treating a variety of wastes, particularly heavy metal (HM) contaminated soils. Among the many available fixing agents, Portland cement (PC) has been used extensively for the remediation of contaminated sites. However, there are significant environmental and technical impacts associated with PC application. Thus, the present research was conducted to address the efficacy of cement and nano-clay mixture in enhancing the S/S process. In so doing, artificially contaminated soils were first prepared by mixing kaolinite with zinc (Zn) at levels of 0 to 2%. Afterward, tow type of nano-clay (Na-Montmorillonite and Na-Cloisite), cement and cement/nano-clay (CNC) were separately added to the sample, and then, a set of macro and micro level experiments including batch equilibrium, pH, toxicity characteristic leaching procedure (TCLP), unconfined compression strength (UCS), X-ray diffraction (XRD) and energy dispersive X-ray (EDX) analyses were carried out at various curing periods (1, 7 and 28 days) to assess the effectiveness of the additives. The results obtained show that the addition of nano-clay can increase the HM retention capability of soil; however, this may be partly lost when the treated soil are subjected to acidic TCLP solution. In addition, with increasing the HM content, due to the decrease in buffering capacity of system and the restructuring of the clay particles, the soil remediation potential at presence of nano-clay is decreased considerably. It was found that the application of sole cement may significantly enhance the HM retention capacity of soil. But in this case, the physicochemical reactions of Zn ions with cement could hinder and/or reduce the generation of hydration products phases such as calcium silicate hydrate (CSH) and calcium aluminate hydrate (CAH), resulting in the degradation of cementation structure-bonding of S/S matrix, as clearly confirmed by the formation of calcium zincate and the diminution in the cementitios compounds peak intensity in the XRD patterns of cement-treated soils. Therefore, the leaching characteristics and the mechanical properties of the S/S material with sole cement are adversely affected by increasing the amount of HM ions. As a result, a large quantity of cement (20 wt% per one percent of HM) and a long time of curing (≈ 28 days) should be employed to meet the full needs of HM immobilization in contaminated soil and give the EPA-acceptable UCS value (≥ 0.35 MPa). The TCLP and XRD test results indicate that the cement/nano-clay combination can expedite the S/S process and alleviate the deleterious influences of metal ions and acidic attack on the stabilized sample. The EDX analyses also support the increase in the development of hydration reactions and the formation of cementing materials in the presence of CNC, providing the enhancement of binding capacity that will lead to the greater strength (up to 50%) in comparison to cement application. Hence, the CNC binary system is more efficient in modifying the contaminated soil with a lower amount of binder (to about 40%) and shorter curing ages (by nearly 4 times) than that of the sole cement. Overall, it is concluded that the cement/nano-clay mixture can be utilized as an effective S/S amendment and CNC content of 15 wt% per 1% of HM can successfully remediate the contaminated soil after 7 days of curing.

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In recent years, the use of nano-materials in different engineering and science projects has increased. The study of the impact of nano-materials in combination with other civil engineering constituents in different geotechnical and geo-environmental engineering projects is very common. This study is aimed to investigate the mechanism of cadmium retention in the process of cement based solidification/stabilization of cadmium contaminated bentonite in the presence of nano-silica. The mechanism of contaminant retention is investigated with the evaluation of cadmium and nano-silica behaviour with change in pH of the environment, adsorption, TCLP results, and evaluation of XRD experimental achievements. The bentonite sample for this research is taken from Iran-Barit Company. To establish the availability of silica ions for interaction with cement and bentonite at different pH, a series of solubility experiments of nano-silica at different pH levels were performed. The results of solubility experiments show that as the pH increases to the alkaline range, the solubility of nano-silica noticeably increases. This fact proves that at the high range of pH due to the use of cement, the required pH conditions for solubility of nano-silica will be provided. Therefore, there will be more possibility for the formation of CSH component. Cadmium nitrate was used to contaminate the bentonite sample for the experimental part. For this purpose, bentonite samples were mixed with 10, 30, and 50 cmol/kg-soil of cadmium nitrate in the electrolyte soil ratio of 20:1. Then, these samples were shaken for two hours in every 24 hours. This process was repeated for 96 hours. After this equilibrium step, the soil suspension was centrifuged. After drying these laboratory contaminated samples, they were solidified/stabilized with different percentages of cement and nano-silica. The results of this paper indicate that the contaminant adsorption and retention of cadmium by bentonite is less than that of adsorption for zinc and lead. The achieved results of TCLP experiments for solidified/stabilized samples with different percentages of cement indicate that the EPA criteria for TCLP experiment which emphasizes for test performance after 28 days, is not suitable for solidification and stabilization of cadmium. In fact, a longer period is necessary to achieve equilibrium and stable results. Furthermore, the results show that due to the low adsorption of cadmium by bentonite and due to the noticeable reduction of pH at the presence of cadmium ions, the required percentages of cement for solidification/stabilization of cadmium contaminated bentonite is much more than the required quantity of cement for other heavy metal contaminated bentonite samples. In addition, the results of XRD experiments show that the pozzolanic interaction process is more efficient in the presence of nano-silica. Furthermore, based on the results of TCLP experiments, the formation of CSH in the presence of nano-silica contributes to the contaminant retention by solidification/stabilization of cement based cadmium contaminated bentonite. Finally, according to the results of this study, in solidified/stabilized samples by mixtures of cement and nano-silica, it is shown that due to the contribution of silica ions in pozzolanic interactions, the solidification is the governing phenomenon for the prevention of heavy metal leachate from solidified/stabilized samples.
 

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In recent years the use of nano materials in engineering projects has significantly increased. In fact, the impact of nano materials as a component in composite materials is one of the new horizons in engineering science. On the other hand, existence of heavy metal contaminated soils is one of the common problems in geo-environmental projects all around the world. In the process of retention of heavy metals by clayey soils, the pH of soil solution plays a significant role. In fact, an increase in pH of soil pore fluid causes a noticeable increase in contaminant retention. On the other hand, the use of additives in soil can increase the contaminant retention as well. In comparison with other additives such as cement, polymers do not require a long curing conditions. In addition, they have a positive impact on permeability of compacted soil, in which their presence decrease the soil permeability. In spite of several researches on the interaction process of clay minerals and polymer, there are very limited researches on the interaction process of polymer-clay minerals-heavy metal contaminant. Therefore, the main objective of this paper is to investigate the role of polyacrylamide polymer in retention of heavy metals in bentonite.
To achieve the above mentioned objective, at the first step, the buffering capacity of polymer treated bentonite samples were measured. In this series of experiments, different concentrations of nitric acid from 0.002 to 0.02 molar were prepared. Then, 4 grams of bentonite and bentonite treated with different concentrations of polymer were poured in centrifuged tubes. Then, 40 cc of nitric acid were added to each centrifuge tube. After equilibrium period, the pH of soil suspension was measured and reported. In the second step of this research, two different series of experiments were performed. In the first series of experiments of this research, bentonite treated by polymer were exposed to different concentrations of heavy metal contaminants. In the second series of experiments, contaminated bentonite samples were treated by different percentages of polymer. The contaminant retention of these samples was investigated by performance of sets of batch equilibrium experiments. The achieved results indicate that the polymer treated bentonite sample (with 3% polymer), after exposure to 200 cmol/kg-soil has shown 19% increase in contaminant retention in comparison to bentonite sample. However, the addition of 3% polymer to contaminated bentonite with 200 cmol/kg-soil lead nitrate has shown 72% increase in contaminant retention in comparison to contaminant retention of bentonite sample. Based on the achieved experimental results it is concluded that there are three phases in heavy metal contaminants in interaction process of bentonite-polymer-heavy metal. These phases include retention by double-layer of clay, the contaminant retention in micro pores of clay minerals which are solidified by polymer, and the contaminate retention capability of polymer. According to the achieved results, the solidification effect of polymer has more contribution to the contaminant retention than the polymer buffering capacity. This proves that post-solidification of contaminated bentonite is a practical method for efficient prevention of contaminant transport in clayey soils. 
 

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The presence of heavy metal contaminants in clays causes changes in soil properties, which increases the risk of contaminant transport into the clay layer. Various techniques have been developed recently to remediate contaminated soil. These methods include electrokinetics, biological treatment, and immobilization. Among them, cement-based stabilization/solidification technique is very common. Generally, cement-based systems are frequently used because of their affordability and great durability. This method uses the two mechanisms of chemical stabilization and physical solidification to retain heavy metals and decrease the mobility of contaminants. Even though several researches have been performed to address the different aspects of cement-based solidification/stabilization, there has been a lack of research on the stabilization/solidification of heavy metals in the presence of two different heavy metal ions. Therefore, the objective of this study is to determine to what extent the pH variations affect lead and cadmium retention and release in single and double-component cement-based stabilization/solidification systems. To accomplish the aforementioned objective, first, the retention capacity of lead and cadmium, in cement-based S/S of contaminated bentonite in single and double-component heavy metal systems is investigated. The effect of pH on the stabilization/solidification process was determined by conducting a series of XRD tests in order to investigate and differentiate the stabilization and solidification mechanisms. In the next step, the amount of released cadmium and lead ions during the TCLP test was determined in single and double-component systems in order to investigate the effect of pH and the simultaneous presence of lead and cadmium in the release of these heavy metals. According to the achieved results, the maximum retention capacities of lead and cadmium occur in the pH ranges of 8.5 to 11 and 10 to 12, respectively. Furthermore, retention in the double-component system is always lower than that of the single-component system, assuming a similar initial concentration. In addition, XRD results illustrate that the intensity of the peaks of cadmium and lead chemical compounds has increased for the samples their pHs take place in the safe zones. This indicates an increase in the contribution of the stabilization mechanism. According to the results of this paper, at a similar contaminant concentration, the intensity of the C-S-H peak increases with an increase in cement percentages. This indicates progress in the contribution of the solidification mechanism in the retention of heavy metals. Still, the release of these heavy metals is always lower than the maximum allowable value reported by the US EPA in the above-mentioned pH ranges. Moreover, the results of this research show that the amount of released cadmium and lead in the double-component system is more than that of the single-component system, assuming similar initial concentrations. Based on the definition of the safe zone, in the defined pH range, the contaminant is retained during the TCLP test mainly by chemical stabilization (precipitation). In fact, in the above range, an increase in the amount of cement has not shown a significant effect on the amount of heavy metal desorption. This finding is supported by the results of retention tests.

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Heavy metal pollutants containing lead have consistently been major sources of environmental contamination over the past decades. Human and industrial activities have directly or indirectly led to the introduction of substantial amounts of lead-based pollutants into soil and groundwater. The Solidification/Stabilization (S/S) technique using cement, by significantly reducing the mobility and solubility of lead in soil, serves as an effective tool for remediating lead-contaminated soils. Conversely, the heavy metal pollutant lead significantly affects the setting time of cement, and the setting time directly impacts the efficiency of cementitious compounds. Consequently, understanding the interaction between lead and cement is of paramount importance. In this regard, the present study aims to investigate the influence of the heavy metal lead on the setting time and microstructural interaction of lead and cement. To achieve this, lead nitrate solution with concentrations of 0, 10,25,50, 100, 250 and 500 kg/cmol-solid, was added to cement. The effect of lead on the hydration process and setting time of cement was examined through setting time tests, X-ray diffraction (XRD) analysis, scanning electron microscopy (SEM) images, and leachability analysis (TCLP). According to the research results, the precipitation and chemical complexation of the heavy metal lead in the form of Pb(OH)₂ and Pb-C-S-H delayed the cement hydration process, extended the initial and final setting times of cement paste, and immobilized and solidified lead pollution effectively. By adding 25 kg/cmol-solid lead nitrate, the initial setting time of cement increased from 65 minutes to 155 minutes. Microstructural results demonstrated that cement effectively interacted with heavy metal lead up to a concentration of 100 cmol/kg-solid during the Solidification/Stabilization (S/S) process, keeping pollutant levels within permissible limits for soil.
 

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The solidification/stabilization of bentonite and heavy metals is among the conventional methods in geo-environmental projects. Among the various methods used for the solidification/stabilization process, cement-based systems are widely used due to their relatively low cost, availability, and environmental compatibility. Cement-based solidification/stabilization technology is an attractive option for managing heavy metal contaminants and facilitating final transportation and containment, thereby reducing contaminant emissions to the environment. The efficiency of solidification/stabilization technology can be improved through certain modifications. The objective of this paper is to determine the effect of substituting calcium carbonate on improving the solidification/stabilization process of bentonite and heavy metals towards reducing cement consumption.
To achieve this goal, samples of bentonite containing 100 cmol/kg-soil concentration of lead nitrate with different compositions of cement and calcium carbonate were solidified/stabilized. To determine the appropriate concentration of added contamination to the soil, a series of tests for heavy metal retention using the soil suspension equilibrium method, based on EPA standards, has been conducted. These tests were performed on bentonite suspensions at heavy metal lead concentrations ranging from 0 to 250 cmol/kg-soil. In EN197 standard, two types of Portland limestone cement are introduced with the names II/A-L and II/B-L, containing 6 to 20 percent and 21 to 35 percent calcium carbonate, respectively (EN197-1, 2000). Based on this, in the present study, up to 25 percent by weight of calcium carbonate is used as a substitute for ordinary Portland cement, and the combination of cement and calcium carbonate as a binder is used. The mechanism of contaminant retention was evaluated through XRD, TCLP, pH, and UCS tests. In this study, the amount of immobility of heavy metals after stabilization and solidification process using the Toxicity Characteristic Leaching Procedure (TCLP) based on EPA-1311 method has been evaluated. In the first stage of the aforementioned experiment, the solidified/stabilized contaminated sample was adjusted to pH 8.2 with a 0.1 molar hydrochloric acid solution and prepared as a suspension with an S:W ratio (solid:water) of 1:20. All suspensions were continuously shaken for 18 hours using a mechanical shaker, and after measuring the pH of the samples and centrifuging them, the liquid phase was separated and the contaminant concentration was measured using a GBC932 AB Plus atomic absorption spectrophotometer. Unconfined compressive strength (UCS) can be used as a criterion for assessing hydration reaction progress. In this study, samples were subjected to curing for 7 and 28 days in a closed system and placed in a humid chamber at 23 degrees Celsius and 95% humidity according to ASTM D1633-17 standard, with a uniform density of 1.85 g/cm³ for testing unconfined compressive strength. Furthermore, X-ray diffraction (XRD) analysis was utilized to investigate the microstructure of the samples and the progress of the cement hydration process and its interaction with contaminated clay minerals.
According to the results of this study, replacing 15% calcium carbonate instead of cement preserves the necessary conditions for the establishment of stabilization and solidification mechanisms. For instance, for a sample containing 1.4% optimum moisture, the desorption amount of lead ions in the TCLP test is equal to 2 milligrams per liter, and the uniaxial resistance of the sample is equal to 1.45 MPa, meeting both EPA standards. In fact, the achieved results indicate that substituting up to 15% calcium carbonate instead of ordinary Portland cement not only reduces cement consumption but also improves the contaminant retention capability in the cementitious solidification process. The reason for the improvement in these conditions is attributed to the simultaneous role of calcium carbonate filling and nucleation alongside the increase in the range of carbonate compound sedimentation.