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Calcium Sulphate Whiskers (CSW) an innovative material for civil engineering applications: A critical review of its preparation, characterization, current trends, and prospects

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Abstract

This paper provides a comprehensive review of calcium sulfate whiskers (CSW), focusing on their synthesis methods, morphological characteristics, and diverse applications across industries. Whiskers, known for their specific length-to-diameter ratio and ordered atomic structure, offer exceptional physical and mechanical properties, making them valuable reinforcements for various materials. Through a bibliometric analysis, a comprehensive overview of the existing research landscape within this field is provided, offering valuable insights into potential future research directions, and identifying promising avenues for further investigation The paper highlights the significance of controlling reaction parameters to achieve high-quality whiskers with desirable morphologies. Furthermore, the crystal symmetry and composition of calcium sulfate whiskers play crucial roles in determining the thermal behavior and performance of CSW. The multifaceted applications of CSW are extensively discussed, ranging from enhancing the mechanical properties of plastics, ceramics, and rubber to improving friction materials, paper production, and environmental filter materials. This review underscores the growing importance of CSW as versatile reinforcements and highlights the need for further research to optimize synthesis techniques and explore novel applications in emerging fields.

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For traditional materials, a polymer composite with high performance and large‐scale production is still the goal pursued by researchers. In our work, polyamide 6/calcium sulfate whiskers (PA6/SCW) composites were fabricated via melt‐compouding method. The calcium sulfate whisker based on gypsum mineral was used as reinforcement. After grafting silane coupling agent on the whisker surface, the whiskers showed a significant reinforcing effect in polyamide. The mechanics and tribology performance of the samples had been significantly inhanced. Based on the nucleation mechanism of lattice matching, calcium sulfate whiskers have obvious heterogeneous nucleation effect in PA6 matrix, while the crystallization period was slightly prolonged. This was caused by the network structure formed by the whiskers in the matrix, which impeded the free movement of polymer chain segments. In combination with the orientation degree of the molecular chains measured by the interdigital electrode, the reinforcing effect of the oriented PA6 specimens was derived from the orientation arrangement of the whiskers and the efficient load transfer. Highlights Mechanical and tribological properties of composite were significantly improved. The composite could achieved large‐scale production due to simple preparation. The whiskers had obvious heterogeneous nucleation effect in matrix. The reinforcing effect was derived from efficient load transfer from whiskers.
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Calcium sulfate whiskers (CSW) exhibit significant surface polarity and high surface energy, leading to aggregation when directly incorporated into polymer matrices, significantly affecting the mechanical properties of the composites. Sodium oleate (SO) was used to modify CSW, introducing –C=C– active groups on the surface, followed by in‐situ graft polymerization of sodium oleate‐modified CSW (SO‐CSW) with methyl methacrylate (MMA) to prepare CSW‐ g ‐PMMA composites. The modification mechanism of SO and PMMA on CSW, as well as the surface modification effects on CSW, were studied using DRIFT, XRD, XPS and TEM. The results show that oleic acid ions adsorb onto the surface of CSW in two forms. One is chemisorption, in which the oleic acid ions are adsorbed on the calcium ions site on the surface of CSW in a bridging way. The other is physical adsorption, oleic acid ions and dissolved calcium ions in the form of precipitation adsorbed on the outermost layer of CSW. After in‐situ polymerization, PMMA was successfully grafted onto the surface of CSW, and the thickness of the surface PMMA coating layer was approximately 67 nm. CSW‐ g ‐PMMA was used as a filler in the melt blending with polypropylene (PP) to prepare PP composites. CSW‐ g ‐PMMA mainly toughens and enhances the mechanical properties of PP composites in the form of crack bridging, crack deflection and pull‐out effect. Highlights The mechanism of sodium oleate modified calcium sulfate whisker (CSW) has been investigated. One is chemical adsorption that the oleic acid ions are adsorbed on the calcium ions site on the CSW surface in a bridge way. The other is physical adsorption that oleic acid ions and dissolved calcium ions are adsorbed in the form of precipitation on the outer layer of CSW. After the modification of sodium oleate, the active group –C=C– was introduced on the surface of CSW, and then the PMMA long chain polymer was grafted with an in‐situ polymerization to form a PMMA coating layer with a thickness of about 67 nm, which significantly improved the hydrophobicity of CSW surface. After modification with sodium oleate and PMMA grafting, the modified CSW significantly toughened and enhanced the mechanical properties of PP composites in the form of crack bridging, crack deflection and pull‐out effect.
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Development of lightweight plasters for mortar rendering utilizing Flue gas desulfurization (FGD) gypsum have been reported here. Lightweight plasters prepared using FGD gypsum and exfoliated vermiculite were characterized and studied in detail for interior wall applications. Different gypsum vermiculite plasters (GVP) with variable amounts of vermiculite were characterized by X-ray diffraction, scanning electron microscopy and thermal gravimetric analysis. The physicochemical and mechanical properties of all the samples are determined and considered to be efficient for interior applications. An optimum mix composition was selected based on its compressive strength, water absorption and porosity. Water absorption and porosity studies restrict the usage of GVP only to interior wall purposes. The acoustic performance of the materials revealed good sound absorption (α = 0.65). Plasters exhibit satisfactory durability under severe conditions of winter and summer weather. GVP shows excellent fire resistance under BS 476-1997 fire resistance classification with thermal conductivities (< 0.161 W/mK) much lower than standard building materials, which makes them fit for energy efficient insulation materials. These studies depict the efficient utilization of thermal power plant waste, FGD gypsum plaster in interior wall insulation for mortar rendering and can be further extended to exterior construction applications by reducing water absorption.
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In recent years, waste material recycling and reuse have attracted great interest as environmentally friendly modifiers to improve asphalt pavement performance. In this study, anhydrous calcium sulfate whiskers (ACSW), synthesized using phosphogypsum waste, and waste cooking oil (WCO), one of the most prevalent waste oils, were used together as modifiers to create an environmentally friendly asphalt mixture. In particular, WCO was used to compensate for the negative effects of ACSW on asphalt mixture performance at low temperatures. A variety of ACSW and WCO compound-modified asphalt mixtures were fabricated. High-temperature stability, medium-temperature fatigue, low-temperature anti-cracking, moisture susceptibility, repeated freeze–thaw, and long-term aging tests were conducted to comprehensively evaluate the pavement performance. Compared to the base asphalt mixture, the compound-modified asphalt mixtures were demonstrated to have better high- and low-temperature, moisture susceptibility, fatigue, anti-freezing, and anti-aging properties, especially for the 6%ACSW and 2%WCO compound-modified asphalt mixture. Therefore, the 6%ACSW and 2%WCO compound-modified asphalt mixture was ultimately selected for use in construction, as this mixture can meet the requirements for regions with cold winters and hot summers.
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In order to study the effect of calcium sulfate whiskers and calcium carbonate whiskers on the road performance of asphalt mixtures, the high temperature stability, low temperature performance, and the change of water stability, the effect of two whiskers with different dosage levels on the road performance of asphalt mixture was obtained. The test results show that the calcium carbonate whiskers can improve the high temperature stability and water stability of the asphalt mixture within a certain dosage level range, but the low temperature performance has no obvious improvement effect, and adding excessive calcium carbonate whiskers will reduce the low temperature performance. The addition of calcium sulfate whiskers can significantly improve the road performance of the asphalt mixture. Under the same dosage level, the effect of calcium sulfate whiskers in improving road performance is better than that of calcium carbonate whiskers. When the content of calcium sulfate whisker is 0.4%, the road performance improvement effect of asphalt mixture is the best.
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Calcium sulfate hemihydrate whiskers (CSHW), a multi-functional and high value-added building material, were prepared with flue gas desulfurization (FGD) gypsum by hydrothermal method, which could be a reasonable disposal of FGD gypsum. In order to obtain CSHW of a high aspect ratio, a series of manufacturing parameters such as reaction temperature, stirring speed, material–water ratio, and reaction time were investigated. The effect of stabilizing treatment and glycerol concentration on CSHW morphology were also studied by environmental scanning electron microscopy (ESEM) and statistical analysis. The results showed that the optimum preparing conditions of reaction temperature, stirring speed, water–material ratio, and reaction time were 160 °C, 200~300 rpm, 11:1 and 1 h, respectively. Furthermore, stabilizing treatment with octodecyl betaine was necessary for the preparation of CSHW. The final prepared whiskers had smooth surface, uniform morphology, a diameter of 260 nm, and a corresponding aspect ratio of 208.2. Moreover, the addition of glycerol reduced the activity of water, contributing to a lower reaction temperature and much smaller diameter.
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In order to better improve the practical engineering problems such as rutting, cracking, pit, and groove in highway engineering, the stability and strength characteristics of calcium sulfate whisker and polyester fiber in asphalt mixture are evaluated based on the road performance test. In this experiment, three different methods of the calcium sulfate whisker content and three different polyester fiber content were used to determine the best asphalt aggregate ratio to prepare different modified asphalt mixture samples. Its high temperature stability, water stability, and low-temperature performance were tested. The results show that the composite modifier of calcium sulfate whisker and polyester fiber can significantly improve the strength stability of asphalt mixture and make its high temperature stability reach 2900.3∼4230.7 Times/mm without reducing its low-temperature bending strength. The optimum content of calcium sulfate whisker is in the range of 2‰∼2.5‰ and polyester fiber is in the range of 2‰∼3‰. The rutting strength and Marshall stability reach the maximum.
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In order to study the effect of calcium sulfate whisker on the mechanical characteristics of Portland cement, the fluidity, initial setting time, final setting time, flexural strength and compressive strength of calcium sulfate whisker-reinforced cement with 0∼1.0% (mass fraction) were studied using experimental method. The results show that calcium sulfate whisker has little effect on the fluidity, initial setting time and final setting time. For the 28-day flexural strength and compressive strength of calcium sulfate whisker-reinforced cement, the optimum whisker content is 0.4% and 1.0% and the corresponding strength is increased by 19.54% and 35.84%, respectively. Compared with calcium carbonate whisker, the reinforcement effect of calcium sulfate whisker on the strength of Portland cement is better.
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This study presents and evaluates hybrid photopolymers applied to 3D printing. To obtain the hybrid system, epoxy acrylates (EA), epoxy resins and photoinitiators were compounded. As inorganic fillers are traditionally used to improve mechanical properties, calcium sulphate whiskers (CSW) coated with chitosan (CS) were modified by acryloyl chloride and the effects of the modification on hybrid photopolymers were investigated. The viscosity and volume shrinkage of the hybrid system were also evaluated. Fourier transform infrared measurements showed that hybrid photopolymers had a higher conversion. Meanwhile, the mechanical properties showed that the tensile strength of the cured samples containing 7% modified CSW was increased by 28% compared to that of pure resin. Additionally, the improvement of interface interaction between modified whiskers and hybrid photopolymers was observed by a scanning electron microscope. Dynamic mechanical analysis showed that the hybrid system, with the addition of 7% modified whiskers, exhibited a higher storage modulus and crosslinking density. This work, the first to use modified CSW on 3D printing to enhance the tensile strength of hybrid photopolymers, demonstrates that the photocured samples can contribute to high accuracy and resolution.
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Flue Gas Desulphurized Gypsum (FGDG) is a by-product generated by the thermal power industry to remove sulfur dioxide (SO 2) from flue gas emissions. This paper provides a comprehensive review of flue gas desul-phurized (FGD) gypsum production, property, and applications in the construction industry. In 2020, global FGD gypsum production reached an estimated 255 million tons, primarily in Asia (55%), followed by Europe (22%), North America (18%), and the rest of the world (5%). The paper discusses the chemical and physical properties of FGD gypsum, including its composition, crystal structure, particle size, and moisture content. It also reviews the various applications of FGD gypsum, such as in cement and concrete production, agriculture, and gypsum board manufacturing. In the construction industry, FGD gypsum is widely used as a raw material for producing gypsum products, such as plasterboard and cement, due to its high purity and low cost. Moreover, FGD gypsum is increasingly used in agriculture as a soil amendment, which can improve soil structure, increase water retention, and provide essential nutrients to plants. The potential for FGD gypsum to be reused and recycled is also discussed , as well as its environmental impact. FGD gypsum can be safely disposed of in landfills or used for backfilling, with appropriate measures to prevent the leaching of heavy metals and other contaminants. It also delves into the potential benefits of FGD gypsum for improving fire resistance, and acoustic properties, and reducing material waste. With its high purity level and lower cost compared to natural gypsum, FGD gypsum is a viable alternative that can transform the construction industry. This paper also discusses its major drawbacks, such as high SO 3 content, dissolution in water, and ettringite formation. This review suggests that there is potential to improve the properties of FGD gypsum further and increase its use as a sustainable construction material in the future.
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The fire resistance of a novel ultra-high performance concrete is critical to its application, while the evolution of its thermal properties with temperature plays a key role in its fire resistance. Therefore, this study presents an experimental investigation of the thermal performance of newly developed ultra-high performance concrete reinforced by cenosphere and multi-scale fibers (MSFUHPC) including carbon nanotubes, calcium sulfate whiskers, carbon fibers, steel fibers, and polyethylene fibers under high temperatures ranging from 25 • C to 800 • C. Meanwhile, scanning electron microscope, thermogravimetry analysis, and mercury intrusion poros-imetry tests are also conducted to study the thermal effect on the microstructure. Accordingly, the effect of multi-scale fibers and cenospheres on the thermal conductivity, volumetric specific heat, and thermal expansion of MSFUHPC are discussed. The results show that adding carbon nanotubes, carbon fibers, and hybrid fibers (steel fibers and polyethylene fibers) can increase thermal conductivity, and adding calcium sulfate whiskers and cenospheres exhibits an inverse effect at ambient temperature. In addition, the thermal expansion can be enlarged by incorporating carbon nanotubes or steel fibers and polyethylene fibers and restrained by adding carbon fibers. The influence of calcium sulfate whiskers and cenospheres on thermal expansion depends on their content. At high temperatures, the thermal conductivity shows an overall decreasing trend and the thermal conductivity of MSFUHPC is higher than that of its matrix and traditional UHPC, HPC, and NSC due to the reduction in the generation and propagation of cracks. By contrast, the volumetric specific heat shows an increasing trend with temperature. Besides, the thermal expansion of MSFUHPC is lower than that of UHPC, HPC, and NSC. The formulas of thermal parameters are fitted based on experimental data by considering the multi-scale fibers and temperatures, which can be used as input parameters for the numerical simulation of MSFUHPC structure under fire.
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Rapid chloride migration (RCM) test was used to determine the non-steady-state chloride migration coefficient (DRCM) of concrete mixed with calcium sulfate whisker (CSW) and basalt fiber (BF). Meanwhile, the microscopic morphology of CSW and BF in concrete was observed through scanning electron microscope (SEM), and the microstructural parameters were determined by the nuclear magnetic resonance (NMR) method. Results show that mixes with 3.0 kg/m³ CSW or BF alone outperformed in terms of chloride penetration reduction. However, by considering the strength factors comprehensively, the effect of enhancing concrete strength and chloride permeability resistance is better when 3.0 kg/m³ CSW and 4.5 kg/m³ BF are mixed. Binary problem optimal analysis results show that the optimal admixture dosage for improving the chloride impermeability and the optimum admixture dosage for enhancing the compressive strength (fc) of concrete are not consistent. According to SEM results, CSW is evenly distributed in the matrix and BF is wrapped by concrete matrix. The addition of BF and CSW to concrete reduced its porosity according to NMR tests. Increasing the content of BF would increase the contributive porosity (φ) of macro pores in concrete, while CSW could reduce the φ of macro pores. The DRCM has the strongest correlation with φ with a pore size of 100–1000 nm, followed by porosity.
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Decision-making in construction planning and scheduling is complex because of budget and resource constraints, uncertainty, and the dynamic nature of construction environments. A knowledge gap in the construction literature exists regarding decision-making frameworks with the ability to learn and propose an optimal set of solutions for construction scheduling problems, such as activity sequencing and work breakdown structure formulations under uncertainty. The objective of this paper is to propose a hybrid reinforcement learning–graph embedding network model that 1) simulates complex construction planning environments using agent-based modeling and 2) minimizes computational burdens in establishing activity sequences and work breakdown formations. Three case studies with practical construction scheduling problems were used to demonstrate applicability of the developed model. This paper contributes to the body of knowledge by proposing the hybridization of reinforcement learning and simulation approaches to optimize project durations with resource constraints and support construction practitioners in making project planning decision-making.
Article
As a byproduct of the industrial synthesis of phosphoric acid, there was a large quantity of phosphor gypsum (PG) accumulation, which needs enormous storage space and endangers the environment. The preparation of calcium sulfate whisker (CSW) from waste PG could solve the large amounts of PG accumulation and substitute the plant-based pulp fiber, reducing the pollution of wastewater in paper and pulp industries. In this work, the CSW, which length could reach 230 μm, was fabricated from the PG in a glycerol-water solution. It could be found that the glycerol-water volume ratio of 4:6 was the best ratio that could reduce the water activity and accelerate the phase transition from calcium sulfate dihydrate (CSD) to CSW. Meanwhile, the CSW, after calcination, could prepare the anhydrous calcium sulfate whisker (ACSW) with low solubility in water; when the ACSW substitution rate reached 50%, the property of the complex paper could still satisfy the standard of mechanical and optical for offset book paper.
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In order to improve the toughness of reactive powder concrete doped steel fibers, a novelty method of multi-scale toughening cement-based materials was investigated. The multi-scale synergistic toughening cement-based materials (nominated basic magnesium sulfate cement-based reactive powder concrete, BMSC based RPC) were designed and discussed for the first time by endogenous whiskers and doped steel fibers. Through orthogonal trials, curing regimes, sand packing and optimization of steel fibers dosage, the synergistic toughening at different microscales was realized by using the properties of endogenous whiskers (the 5·1·7 phase) and doped steel fibers. The results showed that the 28-day flexural strength and compressive strength of BMSC based RPC were 21.4 MPa and 120.5 MPa, respectively, when the steel fibers dosage was 2% (volume fraction). The preliminary research proved that the toughness of reactive powder concrete can be further improved under the multi-scale synergistic toughening effect of endogenous whiskers and doped steel fibers.
Article
Hemihydrate gypsum whiskers with high aspect ratios were prepared by using phosphogypsum as the raw material through the autoclaved method. The effects of additives containing different carboxyl groups, namely, sodium stearate, sodium citrate and EDTA-tetrasodium, on the average length and average aspect ratio of the hemihydrate gypsum whiskers were studied. The results show that hemihydrate gypsum whiskers with longer average length and larger average length–diameter ratio can be prepared by adding sodium stearate or EDTA-tetrasodium. The crystallisation behaviour of hemihydrate gypsum crystals with different crystal faces under different carboxyl additives was investigated through XRD, FTIR, XPS, and molecular dynamics simulation. XRD, FTIR, XPS results demonstrated that different carboxyl additives had less effect on promoting hemihydrate gypsum crystal growth along the C-axis direction at increasing number of carboxyl groups, and sodium citrate reacted with calcium ions, while sodium stearate and EDTA-tetrasodium did not react with calcium ions. Molecular dynamics simulation results manifested that the order of interaction energy between disparate carboxyl additives and hemihydrate gypsum crystal along the C-axis crystal plane was ΔE (EDTA-tetrasodium) < ΔE (sodium citrate) < ΔE (sodium stearate), indicating that the adsorption of various carboxyl additives on hemihydrate gypsum crystal along the C-axis crystal plane was related to the number of carboxyl groups.
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A novel way for transformation of flue gas desulfurization (FGD) gypsum, an industrial by-product to β-CaSO4·0.5H2O whiskers is investigated for calcium sulfate rich alternative construction material. As the consumption of gypsum products is increasing in construction industry and limited amount of natural gypsum is available, herein facile water treatment is used to generate fine tubular particles by recrystallization and growth of β-CaSO4·0.5H2O without any chemical for the first time. Remarkable properties of FGD gypsum are characterized with mean particle size of 42.875-48.759µm, bulk density of 1.08-1.21g/cm³ and specific gravity of 3.06-3.35. XRD, FTIR, WD-XRF, FESEM-EDS and HR-TEM analysis confirm the composition and morphology of the synthesized material. XRD pattern of treated FGD gypsum confirms the presence of highly crystalline β-CaSO4·0.5H2O. FESEM micrographs confirm the micro-needle type morphology with very high aspect ratio of 25:1.Results showed that magnetic stirring improves the removal efficacy of impurities. This study will open the perspective for development of FGD gypsum whiskers with very high aspect ratio and will pave the way for future investigation for a green alternate of natural gypsum for construction applications.
Article
Preparation of building gypsum from flue gas desulfurization (FGD) gypsum is of great value in effectively utilizing the FGD gypsum. An economical and effective process for preparing high-performance building gypsum from the FGD gypsum was proposed. CaO, as a crystal modifier, was added to adjust the quality of hemihydrate gypsum converted in the dehydration process of FGD gypsum. The effects of CaO addition, calcining temperature and holding time on the properties of building gypsum were investigated. The results show that the obtained hemihydrate gypsum mixing with water can prepare high-performance building gypsum after the FGD gypsum adding 3–5 wt% of CaO is calcined at 160–220 °C for 1.5–2 h. Considering the low-temperature calcination error and preparation cost, 3 wt% CaO is added to the FGD gypsum and the mixture is calcined at 200 °C for 1.5 h. The initial and final setting time of building gypsum prepared by this process is 4.183 and 6.75 min, and 2 h and 3 d flexural/compressive strengths reach 2.8/6.2 and 3.5/10.6 MPa, respectively. This research can provide useful information for large-scale reuse of solid waste FGD gypsum in the future.
Article
Blood clotting is a complex biochemical and biophysical process that leads to the formation of a stabilizing fibrin mesh. Fibrin polymerization is a necessary, multi-stage component of this process, and occurs on multiple temporal and spatial scales. These complexities make it difficult to predict how polymerization is affected by perturbations or under varying conditions. Mathematical modeling has been a fruitful approach to generating and testing novel hypotheses about this process. In this review, we focus on the historical context leading to current mathematical models of fibrin polymerization and discuss the contributions of biochemical interactions between thrombin, fibrin(ogen), and factor XIII. We highlight mathematical models that encompass multiple spatial and temporal scales (coarse-grain models, kinetic models, and models incorporating flow and transport effects). We also discuss the unique sets of challenges and benefits of each of these models and finally, we suggest directions for future focus.
Article
Calcium sulfate whisker (CSW)-reinforced paraffin (PA)/gypsum composites were prepared using PA as the phase change material, gypsum as the matrix, and CSW as the reinforcement. A universal testing machine, X-ray diffractometer, scanning electron microscope, and differential scanning calorimeter were used to determine the mechanical properties, thermal properties, phase composition, and microstructure of the composites. The results showed that the gypsum containing 15 wt% PA had the optimal properties for the PA/gypsum composites. The flexural and compressive strength values of the CSW-reinforced PA/gypsum composites were improved by 131% and 278%, respectively when the CSW content was 3.5 wt%. When the composites containing PA were treated by one thermal cycle, PA migrated into the micropores and the mechanical properties of the composites further improved.
Article
The asphalt concrete waterproofing layer (ACWL) has been used as a potential waterproofing structure for the subgrade in Chinese high-speed railway in cold regions. The waterproofing behavior and dynamic response of ACWL have been evaluated in the previously published studies. However, the research on the temperature and thermal stress distribution in ACWL track structure has not yet been studied. This research aims at investigating the strain distribution of ACWL under the temperature influence in winter. The field test was conducted to monitor the strain changes on the ACWL surface under ambient temperature. A 3D numerical model of track structure with ACWL was also developed based on the thermo-mechanical coupled method. The field test results indicated that the longitudinal strain on the ACWL surface changed periodically with the air temperature while the tensile strain increased with the decrease in temperature. The temperature analysis showed that the temperature change inside the track structure caused by the daily temperature change was mainly focused on the depth range from the track slab to the base plate. The maximum negative temperature gradient inside the track slab was reported to occur at 4:00 a.m in winter. The geotextile was further introduced to weaken the bond between the base plate and the ACWL. The calculations proved that the geotextile could effectively reduce the peak tensile strain on the ACWL surface at the joints of the base plates. 1 m − 2 m geotextile was recommended to be placed at both sides of the joint of the base plates.
Article
The high annual output and low utilization rate of flue gas desulfurization (FGD) gypsum cause acute damage to the environment in China. FGD gypsum board is a potential product, but its wide application is limited due to its poor mechanical properties and water resistance. Here, anhydrous calcium sulfate whisker (ACSW) and calcium carbonate whisker (CW) were chosen to improve the properties of FGD gypsum. The effects of two types of whiskers on the working performance, mechanical properties, and water resistance of FGD gypsum were analyzed and compared. The results show that the incorporation of whiskers reduced the processability of the FGD gypsum slurry. The mechanical properties of FGD gypsum improved greatly because the whiskers can increase the contact points between the crystals and fill the pores in FGD gypsum. The 7-day dry flexural strength of FGD gypsum with ACSW and CW increased by 46.99% and 33.07%, respectively. ACSW was better for enhancement than CW due to its good adhesion with gypsum. However, ACSW was not as effective as CW in improving the water resistance, because the structure of ACSW will be destroyed in a humid environment. This research focuses on improving the mechanical and further utilization of FGD gypsum, which also provides a theoretical basis for improving the performance of FGD gypsum.
Article
Complex jarosite waste was obtained from zinc metal hydrometallurgical process, which contained gypsum and ammonium jarosite (NH4Fe3(SO4)2(OH)6). The influence of impurity ions (Fe3+ and NH4+) on the calcium sulfate hemihydrate (HH) morphology was studied using pure gypsum as the raw material, respectively. HH crystals with a high aspect ratio were obtained without the impurity ions. The diameter increased and the aspect ratio of the HH decreased, while the addition of iron sulfate and ammonia sulfate increased. Ammonium iron (NH4+) can be removed by using calcium oxide to decompose the ammonium jarosite in the waste and then to wash the sediment with tap water. The sediment (calcified jarosite sediment) mainly contained CaSO4·2H2O and Fe(OH)3. The influence of cultivating time on HH crystals growth was researched by using the sediment as raw materials. The diameter of the whiskers increased, while the hydrothermal time increased. The whiskers were obtained, with high a aspect ratio (10–60), large diameter (1–4 µm) and smooth surface, after the sediment was treated at 140 °C for 6 h in pH = 5 solution.
Article
Abrasion resistance is a critical parameter in the durability of cement-based materials. Calcium carbonate (CaCO3) whiskers are added into the mortar to enhance the abrasion resistance. Herein, two standards (GB/T 16925-1997 and JC/T 421-2004) were employed to evaluate the abrasion resistance of the mortar modified with CaCO3 whiskers (MMCW). The mortar abrasion resistance was characterized using the abrasion parameters such as abrasion resistance depth (D), abrasion loss (G), mass loss (ML), and abrasion resistance indices (Iα). These results indicate that the abrasion resistance of MMCW was markedly improved, and the abrasion resistance of MMCW increased with the rise in CaCO3 whiskers content. By adding different content of CaCO3 whiskers into mortar, the compressive strength (fcu) enhanced from 2.4% to 10.2%, as compared to that of the plain mortar. The bridging effect of CaCO3 whiskers prevented the cracks development, while the filling effect of CaCO3 whiskers refined the large pores. The calcium hydroxide (CH) orientation improvement effect and the dilution effect of CaCO3 whiskers limited the growth of CH hydration product of mortar, enhanced the compactness of mortar, and ultimately, improved the abrasion resistance of MMCW. An excellent linear relationship was established between the abrasion resistance and compressive strength. By comparing the experimental data with the data from the literature, the prediction model of the relative abrasion resistance parameters was obtained, which can provide a reference for studying the abrasion resistance of different cementitious materials.
Article
Hemihydrate calcium sulfate whiskers (HH-CSWs) were hydrothermally synthesized in a sulfuric acid solution at 120 °C for different holding times (20, 40, and 60 min). The phase structures and morphologies were characterized by XRD and SEM, respectively. The XRD pattern of the sample under 60 min was refined via the Rietveld fitting method. The structure models of the HH-CSW sample under a 60-min holding time was established based on Rietveld fitting results. No difference in the positions of diffraction peaks was determined. The as-prepared holding time increased the intensity and aspect ratio of the diffraction peaks of both samples. In the prepared HH-CSW structure, Ca–O polyhedron is a 12-sided polyhedron similar to that in the gypsum structure; the Ca atom is located in two positions in the one-unit cell; the H2O channel along the c-axis similar is O-shaped and bigger than that in hexagonal and monoclinic CaSO4·0.5H2O structures. Therefore, although the prepared HH-CSW crystals’ structure are similar to that of the reported hexagonal and monoclinic CaSO4·0.5H2O structures, they are not the same. The formation mechanism of HH-CSW from flue gas desulfurization (FGD) gypsum is discussed based on the analysis of gypsum structure.
Article
Calcium sulfate whiskers were prepared under hydrothermal conditions by a one-step method using semi-dry flue gas desulfurization ash with iron ore sintering as the raw material. The effects of temperature, time, speed, and other factors on the calcium sulfate whiskers morphology and growth rule were systematically studied in a high-pressure hydrothermal reaction system. The results demonstrate that the presence of calcium carbonate inhibits high-efficiency desulfurization ash oxidation at low temperatures and affects calcium sulfate whiskers growth. The calcium sulfite content in flue gas desulfurization ash increased to 81.17% when glacial acetic acid was used as the raw material for pickling. In the supersaturated hydrothermal system, the one-step preparation of calcium sulfate whiskers from flue gas desulfurization ash was successful at a reaction temperature of 140 °C, time of 30 min, initial pressure of 2 MPa, initial solid–liquid ratio of 1:30, and rotation speed of 300 r/min. A medium solution purification link was added to create calcium sulfate whiskers products in the hydrothermal reaction system. Calcium sulfate whiskers products with an average diameter of less than 0.8 μm and average aspect ratio of over 140 were obtained at a purification temperature of 60 °C, purification agent concentration of 0.6 mmol/g, temperature of 140 °C, time of 30 min, and speed of 100 r/min.
Article
α-and β-hemihydrate calcium sulfate (HH), the most important calcium sulfate phases, are usually achieved by autoclave hydrothermal reaction and calcination in air, respectively. Herein, we report an investigation on preparation of HH from dihydrate calcium sulfate(DH) in a salt-mediated glycerol-water solution. We found that both α-HH and β-HH in forms of apparently single crystals could be obtained at 110 ℃ only by tuning the concentrations of glycerol, Na2EDTA, and NaCl. TG-DSC, XRD, SEM, and Raman spectroscopy were used to identify and characterize the product crystals. The results indicate that the high relative supersaturation of HH could be the key factor determining the generation of β-HH crystal in solution. Glycerol and NaCl generally decrease the water activity, enhance the crystallization kinetics, and therefore allow the presence of a metastable HH phase (especially for compact α-HH bulk crystal). But too high concentration of glycerol (i.e., ≥70%) and NaCl (i.e., ≥0.2 M) would be favorable for β-HH crystallization with obviously defective crystal morphology due to the dramatically increased local relative supersaturation. Na2EDTA could retard the recrystallization but well regulate the morphology of HH bulk crystals.
Article
In this paper, the solid waste desulfurization gypsum produced by coal-fired power plants was used as raw material to prepare calcium sulfate whiskers with high application prospects. Calcium sulfate whiskers with uniform morphology and high aspect ratio can be prepared by hydrothermal method in sulfuric acid solution. A new process of desulfurization gypsum activated by high-energy grinding to reduce the reaction temperature and sulfuric acid concentration was developed. Through the comparison of product morphology, the best grinding time was determined to be 3.5 h. The mechanism of desulfurization gypsum through physical-chemical coupling to reduce energy consumption was clarified. The activation of desulfurization gypsum by grinding and the acidic environment provided by the sulfuric acid solution made the calcium sulfate solution reached rapid saturation and accelerated the nucleation rate. By calculating the conversion and crystallization rate of calcium sulfate whiskers, it was found that there was obvious "autocatalytic" kinetic characteristics during the crystallization process.
Article
The controllable transformation of flue gas desulfurization (FGD) gypsum to calcium sulfate hemihydrate (HH) whiskers under facile, readily monitored, and cost-effective hydrothermal conditions could play a vital role in the preparation of high quality HH whiskers and improve our understanding of the transformation process. This work assessed the conversion of FGD gypsum to HH whiskers in 5×10–4 mol/L H2SO4 and 1.5 wt% CuCl2 at 120 ºC while determining the effects of temperature as well as H2SO4 and CuCl2 concentrations on transformation kinetics. The preparation of HH whiskers was found to involve a solution-mediated transformation from the dihydrate (DH) to the α-HH. This transition was determined to proceed via a dissolution crystallization mechanism, the rate of which was controlled by nucleation and growth of the HH whiskers. An autocatalytic kinetic model was established based on variations in the HH whiskers mole fraction over time, and this model accurately fit the experimental data with R² = 0.990. Increasing the temperature or H2SO4 concentration accelerated the transformation by modifying the super-saturation and water activity in the reaction solution, while increasing the CuCl2 concentration had the opposite effect. The hydrothermal conditions had an important effect on the transformation from FGD gypsum to HH whiskers.
Article
The temperature required for growth of mullite whiskers in heat-resistant adhesive was decreased to 700 °C based on solid-liquid-solid (S-L-S) mechanism. The bonding strength was increased by 23.1% (700 °C) ∼ 60.7% (1200 °C), and the maximal 32.3 MPa was achieved after sintered at 1200 °C. Moreover, the whiskers-grown adhesive also displayed better anti-damage capacity, and multiple grading fractures occurred during the test procedure, which was mainly due to the pull-out mechanism.
Article
On the one hand, solvers for the propositional satisfiability problem (SAT) can deal with huge instances composed of millions of variables and clauses. On the other hand, Constraint Satisfaction Problems (CSP) can model problems as constraints over a set of variables with non-empty domains. They require combinatorial search methods as well as heuristics to be solved in a reasonable time. In this article, we present a technique that benefits from both expressive CSP modeling and efficient SAT solving. We model problems as CSP set constraints. Then, a propagation algorithm reduces the domains of variables by removing values that cannot participate in any valid assignment. The reduced CSP set constraints are transformed into a set of suitable SAT instances. They may be simplified by a preprocessing method before applying a standard SAT solver for computing their solutions. The practical usefulness of this technique is illustrated with two well-known problems: a) the Social Golfer, and b) the Sports Tournament Scheduling. We obtained competitive results either compared with ad hoc solvers or with hand-written SAT instances. Compared with direct SAT modeling, the proposed technique offers higher expressiveness, is less error-prone, and is relatively simpler to apply. The automatically generated propositional satisfiability instances are rather small in terms of clauses and variables. Hence, applying the constraint propagation phase, even huge instances of our problems can be tackled and efficiently solved.
Article
Nowadays, hybrid fiber reinforced concrete is being considered for structural applications due to its enhanced mechanical properties compared to concrete without fibers/plain concrete. In this work, the mechanical properties of new kind of hybrid fiber reinforced concrete, i.e. CaCO3 whisker-steel fiber-basalt fiber reinforced concrete (CSBFRC) with various basalt fibers percentages are studied. All the fiber-reinforced concretes are compared with that of plain concrete (PC). The CaCO3 whisker, steel and basalt fiber lengths are 20–30 μm, 35 mm and 12 mm, respectively. Steel fibers and CaCO3 whiskers contents are 0.32% and 0.9%, by volume, respectively. Various basalt fiber contents of 0.34%, 0.68%, 1.02% and 1.36%, by volume, are added. For each batch, cylinders and beam-lets are cast and tested under respective compressive, splitting tensile and flexural load as per ASTM standards. Stress-strain curves and load-deflection curves are obtained. Strengths, energy absorptions and toughness indices are determined against for each type of loading. The scanning electron microscopy (SEM) analysis is performed to reveal the behavior (interfacial bonding) of CaCO3 whiskers, basalt fibers and steel fibers. It is concluded that, with increasing content of basalt fibers up to 0.68%, there is an increase in the mechanical properties of hybrid fiber reinforced concrete and CSBFRC4 is found to be an optimum. However, beyond 0.68%, the mechanical properties of CSBFRC decrease with an increase in the basalt fiber content. The resistance against cracking provided by hybrid fibers is observed by SEM images.
Article
In order to investigate the influence of calcium sulfate whisker (CSW) on the high temperature performance of asphalt binder, two types of CSW, calcium sulfate hemihydrates whisker (HCSW) and calcium sulfate anhydrous whisker (ACSW), were used. The rheological properties and thermal stability performance of CSW modified asphalt were studied using Dynamic shear rheomter (DSR) and Thermogravimetric (TG). The results show that two CSWs can enhance the rutting resistance but weaken the temperature sensitivity of asphalt binder. The effect of ACSW on the high rheological properties is more significantly than that of HCSW. In addition, the thermal stability performance of ACSW modified asphalt is better than that of HCSW modified asphalt.
Article
Calcium carbonate whisker is an emerging class of inorganic fiber having tremendous properties and vast applications. Calcium carbonate whisker is extensively used as a filler due to its tremendous properties including low price, simple preparation requirements and ease of availability. As a filler, it increases the physical and chemical properties such as heat resistance, strength and modulus. Its unique properties are seeking the attention of researchers for utilizing it as a filler in various applications. In this paper, different basic preparation techniques of calcium carbonate whisker such as metathesis reaction, sol-gel, carbonation, gravity crystallization and urea hydrolysis are described briefly. In addition to this, the review focus on research advancements achieved in the utilization of calcium carbonate whisker in different applications such as friction materials, paper making, reinforcing composite materials, cement, mortar and concrete. Calcium carbonate whisker is mostly used for enhancing the properties of cementitious materials. In this review, a special emphasis is given to the role of whisker in cementitious materials such as cement, concrete and mortar.
Article
Three kinds of calcium sulfate whisker (CSW) and two types of asphalt binders were used to prepare the modification asphalt by mechanical stirring. The influence of the types and contents of CSW on the high temperature performance, deformation resistance property, temperature susceptibility performance, viscosity-temperature characteristic, rheological property and low temperature performance of asphalt binder were respectively analyzed by softening point test, ductility test, cone penetration test, Brookfield rotational viscosity test, dynamic shear rheometer (DSR) and bending beam rheometer (BBR). In addition, the interaction mechanism of CSW with asphalt binder was investigated by the Attenuated total reflection-Fourier transform infrared spectroscopy (ATR-FTIR), X-ray diffraction (XRD) analyzer and Scanning electron microscope (SEM). The results indicate that three CSWs can significantly improve the high temperature performance, rheological property and deformation resistance property of two asphalt binders, but will weaken their temperature susceptibility performance, viscosity-temperature characteristic and low temperature performance to some extent when CSWs contents range from 7 wt% to 11 wt%. The effect of CSWs on the performances of asphalt binder is related to the types of CSW and the kinds of base asphalt, respectively. The results of ATR-FTIR, XRD and SEM tests indicate that the interaction between CSWs and asphalt binder is physical adsorption and interleaving process.