Materials

15 pages, 2575 KiB

Open AccessArticle

Mechanical and Pore Properties of Foam Concrete Under Salt Erosion Environment

by Weihong Huang, Jiankun Liu, Qinyuan Shi and Weiwei Niu

Materials 2025, 18(12), 2810; https://doi.org/10.3390/ma18122810 (registering DOI) - 15 Jun 2025

This study investigates the evolution of the macro- and micro-scale properties of foamed concrete under different saline environments, including sulfate, chloride, and composite salt conditions. The research focuses on the changes in compressive strength, pore structure, and hydration products of the material. Through [...] Read more.

This study investigates the evolution of the macro- and micro-scale properties of foamed concrete under different saline environments, including sulfate, chloride, and composite salt conditions. The research focuses on the changes in compressive strength, pore structure, and hydration products of the material. Through full-immersion tests and compressive strength measurements, combined with microstructural characterization techniques such as mercury intrusion porosimetry (MIP) and thermogravimetric analysis (TG), the deterioration mechanisms of foamed concrete under salt attack are systematically explored. The results indicate that Sulfate ions exhibit the most aggressive erosion effect, and the presence of chloride ions can produce a “passivation” effect which partially mitigates the damage caused by sulfate ions. Moreover, increasing the material density and incorporation of mineral admixtures contributes to pore structure refinement, significantly enhancing resistance to salt attack. These findings provide a theoretical basis for the practical application of foamed concrete under a complex salt erosion environment. Full article

(This article belongs to the Special Issue The Advanced Development in Concrete Materials: Properties and Construction Techniques)

11 pages, 5307 KiB

Open AccessArticle

Improving Energy Storage Properties of Barium Zirconate Titanate Ceramics via Defect Dipole Engineering

by Zhiyi Wang, Zhengchao Qin, Si Gao, Hongjuan Zheng, Jin Luo, Yunfei Liu and Yinong Lyu

Materials 2025, 18(12), 2809; https://doi.org/10.3390/ma18122809 (registering DOI) - 15 Jun 2025

Abstract

Lead-free ceramic materials have been widely studied since dielectric capacitors became a key component for energy storage. In this work, we adopted defect dipole engineering and improved the energy storage performance of barium zirconate titanate (BZT) ceramics by doping them with MnO₂ [...] Read more.

Lead-free ceramic materials have been widely studied since dielectric capacitors became a key component for energy storage. In this work, we adopted defect dipole engineering and improved the energy storage performance of barium zirconate titanate (BZT) ceramics by doping them with MnO₂. With the increase in Mn content, the hysteresis loop changed from a conventional loop to a pinned hysteresis loop, resulting in a decrease in remnant polarization (P_r). When x = 0.02, the recoverable energy storage density (W_rec) reached 0.1561 J/cm² @ 40 kV/cm, a 59% increase from undoped BZT. Further, XPS and EPR analyses confirmed that many oxygen vacancies were generated. We also performed SEM and TEM characterization and observed the microstructures. These results are consistent with theories suggesting that the formation of the pinned hysteresis loop is attributable to oxygen vacancies and defect dipoles. Full article

(This article belongs to the Special Issue Performance Testing and Application of Ferroelectric/Piezoelectric Ceramics)

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13 pages, 2780 KiB

Open AccessArticle

Development of a Bio-Inspired Aerogel with Robust Sustainability and Thermal Insulation Performance

by Zhao Zhang, Ying Wang, Maiyong Zhu and Songjun Li

Materials 2025, 18(12), 2808; https://doi.org/10.3390/ma18122808 (registering DOI) - 15 Jun 2025

Abstract

The aim of this study was to develop a practical aerogel with robust sustainability and thermal insulation performance. Inspired by the muscular movement of creatures, this target was achieved by developing a tendon-like composite aerogel. The composite aerogel provided comprehensive reinforcement due to [...] Read more.

The aim of this study was to develop a practical aerogel with robust sustainability and thermal insulation performance. Inspired by the muscular movement of creatures, this target was achieved by developing a tendon-like composite aerogel. The composite aerogel provided comprehensive reinforcement due to the comparable composition and compatibility, which made the robust sustainability and thermal insulation performance feasible. The results showed that the prepared aerogel exhibited the desired sustainability and thermal insulation performance, manifesting better abrasion resistance (increased by up to 12.3 times) and lower thermal conductivity (decreased by 25.9%) in contrast to the counterpart in the absence of the tendon. The adoption of the tendon-inspired design also ameliorated the physicochemical properties, including the higher specific surface area (2673.64 m²·g⁻¹), porosity (94.12%) and hydrophobic properties (138.1°). The robust sustainability and thermal insulation performance of the prepared aerogel, coupled with the better physicochemical properties, indicate the prospect of developing sophisticated aerogels for applications, particularly outdoor applications involving a rapid change of heat. Full article

(This article belongs to the Section Soft Matter)

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15 pages, 4047 KiB

Open AccessArticle

Effect of a Recycled Phosphogypsum Modifier on the Performance of High-Content Phosphogypsum Cementing Materials

by Jiuyang Lian, Chiqiu Wu, Zhonghe Shui and Wei Lyu

Materials 2025, 18(12), 2807; https://doi.org/10.3390/ma18122807 (registering DOI) - 14 Jun 2025

Abstract

Phosphogypsum, a byproduct of phosphate fertilizer production, represents a significant environmental concern due to its large-scale production and low utilization rates. Although preparing phosphogypsum-based cementitious materials offers a potential solution to these issues, high-content phosphogypsum cementitious systems encounter significant technical barriers, including long [...] Read more.

Phosphogypsum, a byproduct of phosphate fertilizer production, represents a significant environmental concern due to its large-scale production and low utilization rates. Although preparing phosphogypsum-based cementitious materials offers a potential solution to these issues, high-content phosphogypsum cementitious systems encounter significant technical barriers, including long setting durations and insufficient early-age strength development, thereby restricting their practical implementation. Hence, this research developed innovative modifiers through an environmentally friendly low-temperature thermal activation process (100–160 °C) utilizing recycled phosphogypsum aggregates and circumventing the substantial carbon emissions associated with conventional modification approaches. Systematic characterization demonstrated that the dehydration phase modifier synthesized at 120 °C (DH120) exhibited optimal phase composition, resulting in a 35.7% enhancement in its 14-d compressive strength (9.8 MPa vs. 7.2 MPa for the control) and an 11.3% reduction in its initial setting time (27.5 vs. 31.0 h for the control). Microstructural characterization by low-field nuclear magnetic resonance and X-ray diffractometry revealed that DH120 effectively enhanced refinement of the pore structure (37.7% mesopore volume reduction) and promoted the ettringite crystallization kinetics. This work establishes a sustainable framework for utilizing industrial byproducts in cementitious material systems. Full article

(This article belongs to the Special Issue Characterization and Optimization of Cement-Based Materials)

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20 pages, 5267 KiB

Open AccessArticle

Effect of Hot Isostatic Pressure on the Microstructure Evolution of Ti-22Al-25Nb Alloy Formed by Selective Laser Melting

by Jingjun He, Haiou Yang, Linhao Huang, Jingyu Man, Yuhan Wu and Xin Lin

Materials 2025, 18(12), 2806; https://doi.org/10.3390/ma18122806 (registering DOI) - 14 Jun 2025

Abstract

The density of SLMed (Selective Laser Melting) Ti-22Al-25Nb alloy was improved through hot isostatic pressing (HIP) treatment, and the influence of HIP and solution aging on the microstructure of Ti-22Al-25Nb alloy in the as-deposited state was examined. The results indicate that following (1100 [...] Read more.

The density of SLMed (Selective Laser Melting) Ti-22Al-25Nb alloy was improved through hot isostatic pressing (HIP) treatment, and the influence of HIP and solution aging on the microstructure of Ti-22Al-25Nb alloy in the as-deposited state was examined. The results indicate that following (1100 °C + 300 MPa)/3 h-HIP, the specimen densities have risen to 99.71%, porosity has markedly decreased, and internal flaws have been eradicated. Microstructural analysis reveals a significant presence of GBα₂ (GB, Grain Boundary) along grain boundaries, with GBLO + α₂ (GBL, Grain Boundary Lath; O, Orthorhombic) laths extending parallel from the grain boundaries into the intragranular region. Additionally, a limited number of cross or snowflake O + α₂ lath clusters and acicular O phases are precipitated within the B2 (B, Body-centered cubic) phase in the HIPed state, characterized by isotropic and linear grain boundaries. The GBLα₂ and GBLO exhibit two growth modes: sympathetic nucleation and interfacially unstable nucleation. During the solid solution treatment following HIP, as the solid solution temperature rises, the acicular O phase, GBLO, lath O phase, lath α_2, and GBα₂ sequentially dissolve, increasing the volume fraction of the B2 phase. After HIP, the aging microstructure is primarily characterized by the proliferation of the acicular O phase precipitated from the B2 phase and retaining the lath O phase in a solid solution. The precipitation of GBLO in the original solid solution is suppressed, and the GBLα₂ in the original solid solution partially decomposes into rimO, resulting in coarse grain size and significant internal decomposition of α₂. Following solution treatment and aging at 920 °C, the proliferation of the acicular O phase enhances ductility, resulting in ideal overall characteristics with a yield strength (YS) of 760.81 MPa, ultimate tensile strength (UTS) of 869.32 MPa, and elongation (EL) of 2.683%. This study demonstrates that the HIP treatment and the modification of solution aging parameters can substantially increase the density and refine the microstructure of Ti-22Al-25Nb alloy, hence enhancing its mechanical properties. Full article

(This article belongs to the Section Metals and Alloys)

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15 pages, 17068 KiB

Open AccessArticle

Comparison of Microstructure and Mechanical Properties of Ultra-Narrow Gap-Welded and Submerged Arc-Welded Q355E HSLA Steel

by Youqi Wang, Renge Li, Qingnian Wen, Wenkai Xiao, Shang Wu, Xian Zhai and Fuju Zhang

Materials 2025, 18(12), 2805; https://doi.org/10.3390/ma18122805 (registering DOI) - 14 Jun 2025

Abstract

Reasonable welding methods are of great significance for optimizing the microstructure and ensuring the mechanical properties of welded joints. In this study, ultra-narrow gap welding (UNGW) and submerged arc welding (SAW) were employed to weld Q355E high-strength low-alloy (HSLA) steel thick plates, and [...] Read more.

Reasonable welding methods are of great significance for optimizing the microstructure and ensuring the mechanical properties of welded joints. In this study, ultra-narrow gap welding (UNGW) and submerged arc welding (SAW) were employed to weld Q355E high-strength low-alloy (HSLA) steel thick plates, and the microstructure and mechanical properties of the welded joints were systematically characterized. The UNGW welded joint exhibits superior comprehensive mechanical properties: a room-temperature tensile strength of 664 MPa with 43.1% elongation at fracture, along with higher microhardness and enhanced impact performance at −40 °C, all of which significantly outperform SAW welded joints. This advantage primarily stems from the faster cooling rate during UNGW, which promotes the formation of beneficial acicular ferrite in the joint microstructure. This study provides theoretical support and technical guidance for welding HSLA steel thick plates. Full article

(This article belongs to the Section Metals and Alloys)

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24 pages, 7255 KiB

Open AccessArticle

Application of Flotation Tailings as a Substitute for Cement in Concrete Structures for Environmental Protection and Sustainable Development—Part I: Sulfide Neutralization

by Vanja Đurđevac, Novica Staletović, Lidija Đurđevac Ignjatović, Violeta Jovanović, Nikola Vuković and Vesna Krstić

Materials 2025, 18(12), 2804; https://doi.org/10.3390/ma18122804 (registering DOI) - 14 Jun 2025

Abstract

Flotation tailings (FT), as a product of the exploitation and processing of copper ore, represent a significant environmental and health risk due to the high content of heavy metals and sulfide compounds. Contemporary concepts of sustainable development and circular economy increasingly emphasize the [...] Read more.

Flotation tailings (FT), as a product of the exploitation and processing of copper ore, represent a significant environmental and health risk due to the high content of heavy metals and sulfide compounds. Contemporary concepts of sustainable development and circular economy increasingly emphasize the need for rational use of resources and minimization of all types of waste, including mining waste. In this context, the reuse of flotation tailings in the construction industry represents a significant step towards closing the material flow in the mining and construction sectors. In order to reduce the negative impact of FT on the environment, the possibility of its application as a substitute for a portion of cement in the production of concrete was investigated. The main challenge is to reduce the negative impact of sulfides, originating from sulfide compounds, in order to achieve the desired concrete quality. Limestone aggregates of different size fractions (0/4, 4/8, 8/16 mm) were used for sulfide neutralization. Pyrite concentrate was used as a sulfide source, which together with FT provides the mixtures FT-7, FT-14, FT-25, and FT-40, with sulfur contents of 7.56, 13.84, 25.02, and 39.82%, respectively. FT mixtures were used as a substitute for Portland cement (PC) in the preparation of concrete. Test methods included XRD (X-ray diffraction), XRF (X-ray fluorescence), SEM (scanning electron microscopy), LP (leaching procedure), TCLP (toxicity characterization leaching procedure), assessment of acid eluate generation potential (AP—acid potential, NP—neutralization potential, and NNP—net neutralization potential), NEN (determination of heavy metals in cured concrete eluate), and UCS (uniaxial compressive strength of cured concrete). The results showed that the chemical characteristics of FT, as well as the chemical and mechanical properties of hardened concrete, allow the efficient use of these tailings in concrete mixes, which significantly utilizes FT, reduces the generation of mining waste, and contributes to the reduction of the negative impact on the environment and achieving sustainable development in mining. Full article

(This article belongs to the Special Issue Reaction Mechanism and Properties of Cement-Based Materials (2nd Edition))

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33 pages, 10001 KiB

Open AccessFeature PaperArticle

Epoxy Adhesive Materials as Protective Coatings: Strength Property Analysis Using Machine Learning Algorithms

by Izabela Miturska-Barańska and Katarzyna Antosz

Materials 2025, 18(12), 2803; https://doi.org/10.3390/ma18122803 (registering DOI) - 14 Jun 2025

Abstract

This study analyzed the mechanical properties of epoxy adhesive materials used as functional coatings, focusing on how physical modifications impact their microstructure and strength. Compositions based on Epidian 5, 53 and 57 resins were cured using TFF, Z-1, or PAC curing agents and [...] Read more.

This study analyzed the mechanical properties of epoxy adhesive materials used as functional coatings, focusing on how physical modifications impact their microstructure and strength. Compositions based on Epidian 5, 53 and 57 resins were cured using TFF, Z-1, or PAC curing agents and modified with various fillers: mineral (CaCO₃ calcium carbonate), active (activated carbon filler, CWZ-22), and nanostructured (montmorillonite, ZR-2) fillers. The best results were achieved with calcium carbonate (10–20 wt%) in Epidian 5 or 53 resins cured with TFF or Z-1, yielding tensile strength up to 64 MPa, compressive strength up to 145 MPa, and bending strength up to 123 MPa. Activated carbon and nanofillers showed moderate improvements, particularly in more flexible matrices. To support property prediction, machine learning algorithms were applied and successfully modeled the mechanical behavior based on composition data. The most accurate models reached R² values of 0.93–0.95 for compression and bending strength. While the models for compression and bending strength demonstrated high accuracy, the tensile strength model yielded lower predictive performance, indicating that further refinement and expanded input features are necessary. Shapley analysis further identified curing agents and fillers as key predictive features. This integrated experimental and data-driven approach offers an effective framework for optimizing epoxy-based coatings in industrial applications. Full article

(This article belongs to the Special Issue Manufacturing, Characterization and Modeling of Advanced Materials)

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22 pages, 2868 KiB

Open AccessReview

Review of Research Progress on Dry Granulation Technology for Blast Furnace Slag

by Hecheng Hu, Tuo Zhou, Ye Li, Bing Xia, Man Zhang, Nan Hu and Hairui Yang

Materials 2025, 18(12), 2802; https://doi.org/10.3390/ma18122802 (registering DOI) - 14 Jun 2025

Abstract

Blast furnace slag, a high-temperature molten by-product generated during the ironmaking process in the metallurgical industry, has garnered significant attention for its resource utilization technologies. Compared to the traditional water-quenching method, dry granulation offers notable advantages. This paper systematically compares and analyzes the [...] Read more.

Blast furnace slag, a high-temperature molten by-product generated during the ironmaking process in the metallurgical industry, has garnered significant attention for its resource utilization technologies. Compared to the traditional water-quenching method, dry granulation offers notable advantages. This paper systematically compares and analyzes the performance parameters of three typical dry treatment processes: mechanical crushing, air-quenching granulation, and centrifugal granulation. It reveals that the centrifugal granulation process demonstrates substantial technical superiority in key metrics, such as particle size distribution uniformity, particle morphology optimization, and heat recovery efficiency. Building on this, this study provides a comprehensive review of the current state of centrifugal granulation technology, from both experimental and simulation perspectives. Additionally, the combined processes of centrifugal granulation and air quenching can fully exploit the synergistic benefits of each technology, thereby enhancing overall efficiency. However, the wind’s cooling effect can lead to the premature solidification of molten slag when it splits into liquid filaments, resulting in slag wool. To address this, this paper proposes a centrifugal granulation device equipped with a windbreak board, which facilitates temperature zoning. This approach prevents premature solidification in the liquid filament region while ensuring the timely cooling and solidification of slag particles, offering a novel technical solution for optimizing centrifugal granulation in metallurgical solid waste resource utilization. Full article

(This article belongs to the Special Issue Nonconventional Technology in Materials Processing-3rd Edition)

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12 pages, 2449 KiB

Open AccessArticle

Investigation of Current Effect of Suspended Graphene Pressure Sensor

by Haowei Mi, Run Qi, Pengcheng Li, Ningning Su and Junqiang Wang

Materials 2025, 18(12), 2801; https://doi.org/10.3390/ma18122801 (registering DOI) - 14 Jun 2025

Abstract

The current effect of passive devices is crucial for device testing. The current effect of a suspended graphene pressure sensor in the range of 0–2 mA is studied in this paper. The results show that the resistance of graphene films and the piezoresistive [...] Read more.

The current effect of passive devices is crucial for device testing. The current effect of a suspended graphene pressure sensor in the range of 0–2 mA is studied in this paper. The results show that the resistance of graphene films and the piezoresistive effect of devices exhibit stable performance within the current threshold range of 400 μA and 300 μA, respectively. Auger electron spectroscopy and Raman spectroscopy tests indicate that the resistance of graphene increases first and then decreases at high current intensity, resulting from the electrostatic adsorption of oxygen atoms in the initial phase of electrification and the Joule-induced desorption in the later phase. This study presents guiding significance for the electrical testing of suspended graphene devices. Full article

(This article belongs to the Special Issue Advances in the Manufacturing of Optical Materials, in Optical Sensing, and in Material Performance Analysis)

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9 pages, 189 KiB

Open AccessArticle

Pilot Study of the Shade Matching of Biomimetic Composite Resins in Posterior Dental Restorations: Randomised Clinical Trial

by Cristina Rico-Romano, Dina Aslimani Amar, Valentin Doucept, Rosa M. Vilariño-Rodríguez, Pablo Garrido-Martínez, Vanessa Gutierrez-Vargas and Jesús Mena-Álvarez

Materials 2025, 18(12), 2800; https://doi.org/10.3390/ma18122800 (registering DOI) - 14 Jun 2025

Abstract

Cosmetic restorative dentistry focuses on restoring teeth affected by caries or trauma using materials that mimic natural teeth in shape, texture, and color. Composite resins, particularly nanofilled composites, are widely used due to their superior mechanical and aesthetic properties. Accurate tooth color selection [...] Read more.

Cosmetic restorative dentistry focuses on restoring teeth affected by caries or trauma using materials that mimic natural teeth in shape, texture, and color. Composite resins, particularly nanofilled composites, are widely used due to their superior mechanical and aesthetic properties. Accurate tooth color selection is crucial, and methods include visual (shade guides) and instrumental (spectrophotometers, colorimeters, and intraoral scanners). Newer biomimetic composites, such as Admira Fusion 5 and Clearfil Majesty ES-2 Universal, simplify shade selection through advanced optical technologies. A randomized clinical study involving 30 patients compared the color-matching accuracy of two biomimetic composite resins: Admira Fusion 5 (Voco) and Clearfil Majesty ES-2 Universal (Kuraray). The study utilized the Vita Easyshade spectrophotometer and the Primescan intraoral scanner. Patients were treated following standardized protocols, and shade accuracy was evaluated pre- and post-restoration using Cohen’s Kappa index. Admira Fusion 5 showed higher shade-matching accuracy, with good agreement between pre- and post-restoration measurements using both instruments. Clearfil Majesty ES-2 Universal demonstrated lower reproducibility in shade matching, particularly in posterior teeth, with lower agreement in pre- and post-tests. Instrumentation Comparison: Primescan showed slightly better performance than Easyshade, but both provided comparable results. In conclusions, universal composites may not always achieve optimal shade matching in posterior teeth. Layered composites provide better color adaptability. While digital instruments enhance shade accuracy, combining them with visual methods yields the best clinical outcomes. Further research with larger sample sizes is needed to improve shade-matching techniques in aesthetic restorative dentistry. Full article

(This article belongs to the Special Issue Advances in Dental Implants and Prosthetics Materials)

27 pages, 27663 KiB

Open AccessArticle

Spark Plasma Sintering and Electrospark Deposition of High Entropy Alloys with Elemental Variation

by Ciprian Alexandru Manea, Laura Elena Geambazu, Ileana Mariana Mateș, Delia Pătroi, Gabriela Beatrice Sbârcea, Dorinel Tălpeanu, Jan Přikryl, Gifty B. Oppong and Augustin Semenescu

Materials 2025, 18(12), 2799; https://doi.org/10.3390/ma18122799 (registering DOI) - 13 Jun 2025

Abstract

A novel processing route of producing CrFeNiMo, Co_0.5CrFeNiMo, and Al_0.5CrFeNiMo high-entropy alloy coatings was proposed in this work. Pre-alloyed HEAs were consolidated by spark plasma sintering (SPS) in order to fabricate electrodes for electrospark deposition (ESD) coatings on carbon [...] Read more.

A novel processing route of producing CrFeNiMo, Co_0.5CrFeNiMo, and Al_0.5CrFeNiMo high-entropy alloy coatings was proposed in this work. Pre-alloyed HEAs were consolidated by spark plasma sintering (SPS) in order to fabricate electrodes for electrospark deposition (ESD) coatings on carbon steel substrates. Investigations were performed to observe aspects such as phase composition and stability, contamination level, homogeneity, elemental distribution, and microstructural integrity. The results indicated phase stability and lower porosity when increasing the SPS temperature by 50 °C for all cases, with tetragonal distortion related to the HEAs’ severe lattice distortion core effect. The coating surface analysis indicated that a continuous and compact coating was obtained, correlated with the ESD layering count, but microfissures were present after 6 layers were applied due to the reduced ductility combined with rapid cooling under Ar atmosphere. The chemical integrity of both the sintered samples and the coatings was preserved during the processing, revealing a uniform elemental distribution with no contaminations or impurities present. The results indicated successful HEA sintering and deposition, highlighting the potential of the combined SPS-ESD process for high-performance material fabrication with possible applications in aggressive environments. Full article

(This article belongs to the Section Metals and Alloys)

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12 pages, 689 KiB

Open AccessArticle

Analysis of Key Material Parameters of Evaporator Wicks and Working Fluids for a Loop Heat Pipe Operating in the Temperature Range of 500–700 K

by Paweł Szymański, Piotr Radomski, Jae-Ho Jeon and Dariusz Mikielewicz

Materials 2025, 18(12), 2798; https://doi.org/10.3390/ma18122798 (registering DOI) - 13 Jun 2025

Abstract

This study presents a preliminary evaluation of candidate wick material and working fluid for a flat-loop heat pipe (F-LHP) designed to operate within the temperature range of 500–700 K. The selection process considered key thermal and physical parameters, including thermal conductivity, chemical compatibility [...] Read more.

This study presents a preliminary evaluation of candidate wick material and working fluid for a flat-loop heat pipe (F-LHP) designed to operate within the temperature range of 500–700 K. The selection process considered key thermal and physical parameters, including thermal conductivity, chemical compatibility between wick and fluid, capillary pressure generation, pressure drop across the wick structure, and structural integrity at elevated temperatures. A range of metallic and ceramic wick materials, along with suitable high-temperature working fluids, were reviewed and compared based on performance metrics and practical availability. Special attention was given to oxidation and corrosion resistance, capillary performance, and thermal stability under elevated-temperature conditions. Nine different porous wicks with distinct materials and microstructures—differing in pore size, porosity, and permeability—were analyzed in combination with seven different working fluids. The analysis focused on determining which combinations generated the highest capillary pressure and which exhibited the lowest flow resistance due to external flow, thereby enhancing the LHP’s performance. Based on these results, the study identifies the most effective wick–fluid pairings for F-LHP applications, offering an optimal balance of thermal performance and long-term reliability. These findings provide a foundation for further experimental validation and the development of prototypes. Full article

(This article belongs to the Section Materials Physics)

17 pages, 2710 KiB

Open AccessArticle

The Effects of Sand Incorporation on the Pore Structure, Strength, and Fractal Characteristics of Alkali-Activated Slag Cementitious Materials

by Yuchen Ye, Zhenyuan Gu, Yi Wang, Ying Sun, Chenhui Zhu and Jie Yang

Materials 2025, 18(12), 2797; https://doi.org/10.3390/ma18122797 - 13 Jun 2025

Abstract

Sand content plays a critical role in regulating the structural compactness and strength development of alkali-activated slag cementitious materials. In this study, three types of specimens—pure slag paste, standard sand mortar, and fine sand mortar—were prepared to investigate the effects of sand incorporation [...] Read more.

Sand content plays a critical role in regulating the structural compactness and strength development of alkali-activated slag cementitious materials. In this study, three types of specimens—pure slag paste, standard sand mortar, and fine sand mortar—were prepared to investigate the effects of sand incorporation on pore structure and fractal characteristics. Mechanical properties, pore structure, and micro-morphology were systematically evaluated at different curing ages. Mercury intrusion porosimetry (MIP) was employed to measure porosity, pore size distribution, and the threshold pore diameter, while fractal dimensions were calculated to quantify pore complexity and compactness. The results showed that the pure slag paste achieved the highest compressive strength at all ages but posed environmental concerns due to high resource consumption. In contrast, sand-incorporated mortars exhibited stable strength development and continuous pore structure refinement. Notably, the use of fine sand in Group C reduced slag content by approximately 5.6% compared to Group A, contributing to lower CO₂ emissions and enhanced sustainability. Fractal analysis revealed a strong correlation between fractal dimension, pore compactness, and compressive strength. A higher fractal dimension indicated a more complex and interconnected pore network, promoting matrix densification. At 90 days, Group C achieved the highest fractal dimension and lowest porosity, attributed to the micro-filling effect of fine sand, which facilitated the formation of a denser and more continuous gel network. These findings provide a theoretical foundation for the multiscale characterization of alkali-activated cementitious systems and support the design of more sustainable mix formulations. Full article

(This article belongs to the Special Issue Innovative Construction Materials for Sustainable and Greener Applications)

18 pages, 2815 KiB

Open AccessArticle

Evaluation of Crack Resistance Performance of Semi-Flexible Pavement Materials

by Songqiang Chen, Jianfei Zheng, Xi Wu and Lufan Li

Materials 2025, 18(12), 2796; https://doi.org/10.3390/ma18122796 - 13 Jun 2025

Abstract

Semi-flexible pavement (SFP) materials have garnered extensive application and research attention owing to their exceptional deformation resistance. The crack resistance of SFP materials constitutes a critical aspect of their road performance. This study conducts a comprehensive analysis of the crack resistance of SFP [...] Read more.

Semi-flexible pavement (SFP) materials have garnered extensive application and research attention owing to their exceptional deformation resistance. The crack resistance of SFP materials constitutes a critical aspect of their road performance. This study conducts a comprehensive analysis of the crack resistance of SFP materials through material characterization and structural mechanical response evaluation. To assess the cracking behavior of SFP materials across the entire temperature spectrum, three experimental methodologies were employed: low-temperature flexural tensile testing, indirect tensile testing, and semi-circular bending tensile testing. Experimental findings reveal that SFP materials exhibit superior crack resistance compared to SMA-13 under ambient and elevated temperature conditions, while demonstrating inferior performance relative to SMA-13 in low-temperature environments. Through a comparative analysis of structural mechanical responses between SMA-13 and SFP pavements, it was determined that the implementation of a single-layer SFP material can reduce pavement tensile strain by 30–50%. This investigation provides comprehensive insights into the crack resistance characteristics of SFP materials and offers valuable guidance for material selection in pavement structural design. Full article

(This article belongs to the Section Construction and Building Materials)

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17 pages, 938 KiB

Open AccessArticle

Geopolymer Modified with Insoluble Calcite and Various Silica Fumes Originated from Different Manufacturing Processes

by Yong Xu, Xiaonan Wang, Lilin Yang, Yang Liu, Tong Gao, Han Li, Yukai Wang, Ning Xie, Jing Meng, Jinping Ou and Wenshou Wang

Materials 2025, 18(12), 2795; https://doi.org/10.3390/ma18122795 - 13 Jun 2025

Abstract

It has been proven that silica fume (SF), which is a by-product from the manufacturing of single-crystal silicon, is beneficial for enhancing the mechanical properties, durability, and workability of geopolymers, as it can be quickly dissolved and form silicate-based cementitious phases in alkaline [...] Read more.

It has been proven that silica fume (SF), which is a by-product from the manufacturing of single-crystal silicon, is beneficial for enhancing the mechanical properties, durability, and workability of geopolymers, as it can be quickly dissolved and form silicate-based cementitious phases in alkaline environments. However, the reinforcement mechanism of SF on geopolymer remains unclear due to the chemical complexity of geopolymer and the variety of SF types. Additionally, the solubility of calcite in an alkali environment is quite limited, and thus the formation of the amorphous calcium-based gels will be thwarted due to the lack of soluble calcium ions. Most importantly, with the development of the single-crystal industry, the amorphous silica content, crystallinity, and trace elements of SF itself have changed, which blocks the understanding of the activation mechanism of geopolymers combined with SF and insoluble calcite. To unveil the underlying modification mechanisms of SF on geopolymer materials along with insoluble calcite, in this study, two types of SF were used as the fly ash replacement in a fly ash/limestone system to prepare geopolymer materials. The reinforcement effect significantly depends on the SF types even with similar particle size and chemical compositions. The results indicate that the mechanical properties of geopolymer materials modified with SFs are not only governed by the ratio and contents of Si, Ca, Al, and Mg in SFs but also depend on the crystallinity and activity of the SFs. The hydration products could be varied according to the reaction environment. The research results not only contribute to the optimization design and application of geopolymer materials but also pave new pathways for the upcycling use of solid wastes such as SF, low-grade fly ash, or even other aluminosilicate solid wastes to achieve sustainable development. Full article

(This article belongs to the Special Issue Geopolymers and Alkali-Activated Materials: Preparation and Properties)

20 pages, 917 KiB

Open AccessArticle

The Influence of Different Parameters for the Removal of Pb and Zn Ions on Unmodified Waste Eggshells

by Elena Petronela Bran, Oana-Irina Patriciu, Luminița Grosu, Irina-Claudia Alexa, Brîndușa Bălănucă, Adrian-Ionuț Nicoară and Adriana-Luminița Fînaru

Materials 2025, 18(12), 2794; https://doi.org/10.3390/ma18122794 - 13 Jun 2025

Abstract

The need to use environmentally friendly and cost-effective methods to remove heavy metals from wastewater is a permanent concern worldwide. Eggshells have been indicated as a worthy biosorbent for the adsorption of heavy metals due to their bioavailability and composition. In the present [...] Read more.

The need to use environmentally friendly and cost-effective methods to remove heavy metals from wastewater is a permanent concern worldwide. Eggshells have been indicated as a worthy biosorbent for the adsorption of heavy metals due to their bioavailability and composition. In the present study, the absorption capacity of untreated chicken (CEs) and quail (QEs) eggshells for the removal of Pb and Zn ions from aqueous solutions was evaluated at room temperature and 40 °C, using four types of agitation systems: classical and orbital agitation and ultrasonic and microwave-assisted activation. The monitoring of aqueous solutions was performed by electrochemical and spectro-analytical (AAS) procedures before and after the adsorption process. FTIR and RAMAN spectroscopy, SEM-EDAX microanalysis, and X-ray diffraction were used to investigate the characteristics of eggshell samples post-exposure to Pb²⁺ or Zn²⁺. For any type of agitation and temperature, the CEs were able to induce more than 65% removal efficiency for lead and over 80% in the case of zinc. Concerning the Zn removal efficiency of QEs, notable results were recorded when microwaves were applied (>90%) and at 40 °C for orbital shaking and ultrasound (>80%). The results of the present study may offer new and valuable information for the optimal removal of Pb²⁺ and Zn²⁺ using eggshells, thus contributing to the sustainable management of waste through the recycling of this type of biomaterial. Full article

(This article belongs to the Special Issue Adsorption Materials and Their Applications (2nd Edition))

14 pages, 2266 KiB

Open AccessArticle

Solid-State Transformation (S_total = 0, 1, and 2) in a Ni²⁺ Chelate with Two tert-Butyl 5-(p-Biphenylyl)-2-pyridyl Nitroxides

by Masataka Mitsui and Takayuki Ishida

Materials 2025, 18(12), 2793; https://doi.org/10.3390/ma18122793 - 13 Jun 2025

Abstract

A novel S = 1/2 paramagnetic chelating ligand tert-butyl 5-(p-biphenylyl)-2-pyridyl nitroxide (bppyNO) and its S = 1 nickel(II) ion complex [Ni(bppyNO)₂Br₂] were synthesized. X-ray crystallography revealed a 2p–3d–2p heterospin triad, with half of the molecule being [...] Read more.

A novel S = 1/2 paramagnetic chelating ligand tert-butyl 5-(p-biphenylyl)-2-pyridyl nitroxide (bppyNO) and its S = 1 nickel(II) ion complex [Ni(bppyNO)₂Br₂] were synthesized. X-ray crystallography revealed a 2p–3d–2p heterospin triad, with half of the molecule being crystallographically independent. A relatively planar chelate geometry with the torsion angle ϕ(Ni-O-N-C_2py) = −10.6(5)° at 300 K becomes significantly out-of-plane distorted with ϕ = −46.9(8) and 26.1(11)° at 90 K accompanied by disorder at the oxygen site. The torsion angle changes, Δϕ = 36° and 37°, are among the largest reported for related compounds. Magnetic measurements indicate gradual and incomplete spin transition-like behavior around 143(2) K. A three-state model involving an intermediate-spin (S_total = 1) state is proposed to explain non-zero χ_mT plateau in a low-temperature region. Density functional theory calculations using the determined structures support the proposed mechanism. Furthermore, geometry optimizations assuming S_total = 0, 1, and 2 are in good agreement with the present model. Full article

(This article belongs to the Special Issue From Molecular to Supramolecular Materials)

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13 pages, 10443 KiB

Open AccessArticle

Influence of Post-Weld Heat Treatment on the Performance of UHSS Joints

by Mustafa Tümer, Alptekin Kısasöz, Florian Pixner and Norbert Enzinger

Materials 2025, 18(12), 2792; https://doi.org/10.3390/ma18122792 - 13 Jun 2025

Abstract

Ultra-high strength steel (UHSS) contributes significantly to lightweight design, environmental compatibility and lower fuel consumption. However, it is essential to maintain excellent mechanical properties in terms of structural integrity, strength and ductility after the applied welding process. In this study, the effect of [...] Read more.

Ultra-high strength steel (UHSS) contributes significantly to lightweight design, environmental compatibility and lower fuel consumption. However, it is essential to maintain excellent mechanical properties in terms of structural integrity, strength and ductility after the applied welding process. In this study, the effect of post-welding heat treatments on the welding of UHSS S1100MC was investigated in order to compensate for the deterioration in toughness that occurred as a result of joining by electron beam welding. Electron beam welding (EBW) provides high energy density and therefore relatively low heat input compared to arc welding. However, the narrow fusion zone (FZ) and heat-affected zone (HAZ) may have insufficient toughness values due to rapid cooling of the joint. In order to protect the relationship between strength and toughness, both the material and the joint were subjected to heat treatment at 500, 650 and 750 °C temperatures for 2 h and were cooled in the furnace. Microstructural characterization and mechanical testing, namely hardness, Charpy impact and tensile tests, were performed to correlate the influence of post-weld heat treatment on the microstructural formation and the corresponding mechanical properties. While the material and the joint maintained their hardness values at 500 °C of around 412 ± 15 HV_0.2, there was an approximately 8% decrease in hardness to 378 ± 18 HV_0.2 at 650 °C. At 750 °C, it dramatically lost its high hardness properties, resulting in a low 178 ± 9 HV_0.2. However, direct quenching from the austenitic temperature resulted in fresh martensite, which provided both the required strength and toughness values in the EBW joint. With a hardness of 437 HV_0.2, a tensile strength of 1345 MPa and a fracture elongation of more than 9%, superior mechanical properties could be achieved. Full article

(This article belongs to the Section Metals and Alloys)

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