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Nowadays, the circular economy is driving the construction industry towards greater sustainability for both environmental and financial purposes. One prominent area of research with significant contributions to circular economy is the reuse of steel from decommissioned structures in new construction projects. This approach is deemed far more efficient than ordinary steel recycling, due to the fact that it contributes towards reducing both the cost of the new project and the associated carbon emissions. Along these lines, the feasibility of utilizing steel wind turbine towers (WTTs) as part of a new structure is investigated herein, considering that wind turbines are decommissioned after a nominal life of approximately 25 years due to fatigue limitations. General principles of structural steel reuse are first presented in a systematic manner, followed by two case studies. Realistic data about the geometry and cross-sections of previous generation models of WTTs were obtained from the Greek Center for Renewable Energy Sources and Savings (CRES), including drawings and photographic material from their demonstrative wind farm in the area of Keratea. A specific wind turbine was selected that is about to exceed its life expectancy and will soon be decommissioned. Two alternative applications for the reuse of the tower were proposed and analyzed, with emphasis on the structural aspects. One deals with the use of parts of the tower as a small-span pedestrian bridge, while the second addresses the transformation of a tower section into a water storage tank. Several decision factors have contributed to the selection of these two reuse scenarios, including, amongst others, the geometric compatibility of the decommissioned wind turbine tower with the proposed applications, engineering intuition about the tower having adequate strength for its new role, the potential to minimize fatigue loads in the reused state, the minimization of cutting and joining processes as much as possible to restrain further CO₂ emissions, reduction in waste material, the societal contribution of the potential reuse applications, etc. The two examples are briefly presented, aiming to demonstrate the concept and feasibility at the preliminary design level, highlighting the potential of decommissioned WTTs to find proper use for their future life. Full article

To reduce the carbon footprint of the concrete industry and promote a circular economy, this study explores the reuse of waste materials such as glass powder (GP) and nitrile rubber (NR) fibres in concrete. However, the inclusion of these waste materials results in lower compressive strength compared to conventional concrete, limiting their application to non-structural elements. To overcome this limitation, this study adopts the concept of confined concrete by developing concrete-filled steel tube (CFST) stub columns. In total, twelve concrete mix variations were developed, with and without steel tube confinement. GP was utilised at replacement levels of 10–30% by weight of cement, while NR fibres were introduced at 0.5% and 1% by volume of concrete. The findings demonstrate that the incorporation of GP and NR fibres leads to a reduction in compressive strength, with a compounded effect observed when both materials are combined. Steel confinement within CFST columns effectively mitigated the strength reductions, restoring up to 17% of the lost capacity and significantly improving ductility and energy absorption capacity. All CFST columns exhibited consistent local outward buckling failure mode, irrespective of the concrete mix variations. A comparison with predictions from existing design codes and empirical models revealed discrepancies, underscoring the need for refined design approaches for CFST columns incorporating sustainable concrete infill. This study contributes valuable insights into the development of eco-friendly, high-performance structural systems, highlighting the potential of CFST technology in facilitating the adoption of waste materials in the construction sector. Full article

Laser beam powder bed fusion of metals (PBF-LB/M, or simply L-PBF) has emerged as one of the most competitive additive manufacturing technologies for producing complex metallic components with high precision, design freedom, and minimal material waste. Among the various categories of additive manufacturing processes, L-PBF stands out, paving the way for the execution of part designs with geometries previously considered unfeasible. Despite offering several advantages, parts with overhang features require the use of support structures to provide dimensional stability of the part. Support structures achieve this by resisting residual stresses generated during processing and assisting heat dissipation. Although the scientific community acknowledges the role of support structures in the success of L-PBF manufacturing, they have remained relatively underexplored in the literature. In this context, the present work investigated the impact of laser power and scanning speed on the dimensioning, integrity and tensile strength of single-walled block type support structures manufactured in AISI 316L stainless steel. The method proposed in this work is divided in two stages: processing parameter exploration, and mechanical characterization. The results indicated that support structures become more robust and resistant as laser power increases, and the opposite effect is observed with an increment in scanning speed. In addition, defects were detected at the interfaces between the bulk and support regions, which were crucial for the failure of the tensile test specimens. For a layer thickness corresponding to 0.060 mm, it was verified that the combination of laser power and scanning speed of 150 W and 500 mm/s resulted in the highest tensile resistance while respecting the dimensional deviation requirement. Full article

Polypropylene (EPP) concrete offers advantages such as low density and good thermal insulation properties, but its relatively low strength limits its engineering applications. Waste steel fibers (WSFs) obtained during the sorting and processing of machining residues can be incorporated into EPP concrete (EC) to enhance its strength and toughness. Using the volume fractions of EPP and WSF as variables, specimens of EPP concrete (EC) and waste steel fiber-reinforced EPP concrete (WSFREC) were prepared and subjected to cube compressive strength tests, splitting tensile strength tests, and four-point flexural strength tests. The results indicate that EPP particles significantly improve the toughness of concrete but inevitably lead to a considerable reduction in strength. The incorporation of WSF substantially enhanced the splitting tensile strength and flexural strength of EC, with increases of at least 37.7% and 34.5%, respectively, while the improvement in cube compressive strength was relatively lower at only 23.6%. Scanning electron microscopy (SEM) observations of the interfacial transition zone (ITZ) and WSF surface morphology in WSFREC revealed that the addition of EPP particles introduces more defects in the concrete matrix. However, the inclusion of WSF promotes the formation of abundant hydration products on the fiber surface, mitigating matrix defects, improving the bond between WSF and the concrete matrix, effectively inhibiting crack propagation, and enhancing both the strength and toughness of the concrete. Full article

Geogrids are frequently utilized in engineering for reinforcement; yet, they are vulnerable to construction damage when employed on coarse-grained soil subgrades. In contrast, waste tire grids are more appropriate for subgrade reinforcement owing to their rough surfaces, integrated steel meshes, robust transverse ribs, extended degradation cycles, and superior durability. Based on the limit equilibrium theory, this study developed formulae for calculating the internal and external frictional resistance, as well as the end resistance of waste tires, to ascertain the interface bearing properties and calculation techniques of waste tire grids. Based on this, a mechanical model for the ultimate pull-out resistance of waste-tire-reinforced soil was developed, and its validity was confirmed through a series of pull-out tests on single-sided strips, double-sided strips, and tire grids. The results indicated that the tensile strength of one side of the strip was approximately 43% of that of both sides, and the rough outer surface of the tire significantly enhanced the tensile performance of the strip; under identical normal stress, the tensile strength of the single-sided tire grid was roughly nine times and four times greater than that of the single-sided and double-sided strips, respectively, and the grid structure exhibited superior anti-deformation capabilities compared to the strip structure. The average discrepancy between the calculated values of the established model and the theoretical values was merely 2.38% (maximum error < 5%). Overall, this research offers technical assistance for ensuring the safety of subgrade design and promoting environmental sustainability in engineering, enabling the effective utilization of waste tire grids in sustainable reinforcement applications. Full article

This study developed a full solid waste-based cementitious material (ISWs-CM) using steel slag (SS), ground granulated blast furnace slag (GGBFS), phosphorus slag (PS), carbide slag (CS), and desulfurized gypsum (DG) to completely replace cement. A two-layer optimization strategy, combining three chemical moduli and simplex lattice experiments, was employed to determine the proportion and to investigate the impact of proportions on the uniaxial compressive strength of mortar. As an application case, the ISWs-CM with the optimal proportion was employed to stabilize aeolian sand, and its effectiveness as a cement substitute and the underlying mechanisms were investigated. The results indicated that the ISW proportion that maximized the strength of the mortar was SS:GGBFS:PS:CS = 5:20:20:40. The strength of the mortar was enhanced when the proportion of GGBFS exhibiting the highest reactivity was increased and also increased initially and then decreased with an increase in CS when the dosage of GGBFS was fixed. The aeolian sand stabilized by ISW-CM exhibited higher strength than that stabilized with cement. The greater number and variety of hydration products resulted in denser connections and encapsulation of sand particles, which highlights the synergistic effect of ISWs and the potential of ISW-CM as a cement replacement across diverse applications including aeolian sand stabilization. Full article

Most industrial processes depend on heat, electricity, demineralized water, and chemical inputs, which themselves are produced through energy- and resource-intensive industrial activities. In this work, acetic acid (AA) production from syngas (CO, CO₂, and H₂) fermentation is explored and compared against a thermochemical fossil benchmark and other thermochemical/biological processes across four main Key Performance Indicators (KPI)—electricity use, heat use, water consumption, and carbon footprint (CF)—for the years 2023 and 2050 in Portugal and France. CF was evaluated through transparent and public inventories for all the processes involved in chemical production and utilities. Spreadsheet-traceable matrices for hotspot identification were also developed. The fossil benchmark, with all the necessary cascade processes, was 0.64 kg CO_2-eq/kg AA, 1.53 kWh/kg AA, 22.02 MJ/kg AA, and 1.62 L water/kg AA for the Portuguese 2023 energy mix, with a reduction of 162% of the CO₂-eq in the 2050 energy transition context. The results demonstrated that industrial practices would benefit greatly from the transition from fossil to renewable energy and from more sustainable chemical sources. For carbon-intensive sectors like steel or cement, the acetogenic syngas fermentation appears as a scalable bridge technology, converting the flue gas waste stream into marketable products and accelerating the transition towards a circular economy. Full article

Soil acidification, salinization, and heavy metal pollution pose serious threats to global food security and sustainable agricultural development. Biochar, with its high porosity, large surface area, and abundant functional groups, can effectively improve soil properties. However, due to variations in feedstocks and pyrolysis conditions, it may contain potentially harmful substances. Industrial wastes such as fly ash, steel slag, red mud, and phosphogypsum are rich in minerals and show potential for soil improvement, but direct application may pose environmental risks. The co-application of biochar with these wastes can produce composite amendments that enhance pH buffering capacity, nutrient availability, and pollutant immobilization. Therefore, a review of biochar-industrial waste composites as soil amendments is crucial for addressing soil degradation and promoting resource utilization of wastes. In this study, the literature was retrieved from Web of Science, Scopus, and Google Scholar using keywords including biochar, fly ash, steel slag, red mud, phosphogypsum, combined application, and soil amendment. A total of 144 articles from 2000 to 2025 were analyzed. This review summarizes the physicochemical properties of biochar and representative industrial wastes, including pH, electrical conductivity, surface area, and elemental composition. It examines their synergistic mechanisms in reducing heavy metal release through adsorption, complexation, and ion exchange. Furthermore, it evaluates the effects of these composites on soil health and crop productivity, showing improvements in soil structure, nutrient balance, enzyme activity, and metal immobilization. Finally, it identifies knowledge gaps as well as future prospects and recommends long-term field trials and digital agriculture technologies to support the sustainable application of these composites in soil management. Full article

The Scalable and Expeditious Additive Manufacturing (SEAM) process is an advanced additive manufacturing (AM) technique that relies on the optimization of metal powder suspensions to achieve high-quality 3D-printed components. This study explores the critical role of dispersants in enhancing the performance of stainless steel (SS) 420 metal powder suspensions for the SEAM process by improving powder loading, recyclability, flowability, and consequent final part density. The addition of dispersant allows for increased powder contents while preserving stable rheological properties, thereby enabling higher powder loading without compromising the rheological characteristics required in the SEAM process. Previously, our team implemented a two-step printing strategy to address the segregation issues during printing. Nonetheless, the semi-cured layer was not recyclable after printing, resulting in a significant amount of waste in the SEAM process. This, in turn, leads to a considerable increase in material costs. On the other hand, the addition of a dispersant has been shown to enhance suspension stability, enabling multiple cycles of reuse. This novel approach has been demonstrated to reduce material waste and lower production costs. The enhanced flowability guarantees uniform suspension spreading, resulting in defect-free layer deposition and superior process control. Moreover, the dispersant’s ability to impede particle agglomeration and promote powder loading contributes to the attainment of a 99.33% relative density in the final sintered SS420 parts, thereby markedly enhancing their mechanical integrity. These findings demonstrate the pivotal role of dispersants in refining the SEAM process, enabling the production of high-density, cost-effective metal components with superior material utilization and process efficiency. Full article

Ground granulated blast-furnace slag (GGBS), a waste product generated during steel production, can be added as a substitute for cement in concrete to mitigate the environmental impact of the cement and steel industries. However, the use of GGBS is limited because it decreases the early strength development of cement or concrete. This study evaluated the performance of incorporating synthesized C-S-H nanoparticles to enhance the compressive strength, early hydration, and microstructure of cement composite. The synthesized C-S-H nanoparticles were produced at standard atmospheric pressure and room temperature. Heat of hydration, X-ray diffraction, and thermogravimetric analyses were conducted to investigate the hydration and mechanical properties of the cement containing the C-S-H nanoparticles. Further, mercury intrusion porosimetry was conducted to examine the pore structures. The experimental finding demonstrated that adding C-S-H nanoparticles accelerated the early hydration progress in the cement composites, thereby increasing their initial compressive strength. Full article

The utilization of steel slag (SS) in construction materials represents an effective approach to improving its overall recycling efficiency. This study incorporates SS into a conventional ground granulated blast-furnace slag (GGBS)–fly ash (FA)-based binder system to develop a ternary system comprising SS, GGBS, and FA, and investigates how this system influences the static mechanical properties of ultra-high-performance geopolymer concrete (UHPGC). An axial point augmented simplex centroid design method was employed to systematically explore the influence and underlying mechanisms of different binder ratios on the workability, axial compressive strength, and flexural performance of UHPGC, and to determine the optimal compositional range. The results indicate that steel slag has a certain negative effect on the flowability of UHPGC paste; however, with an appropriate proportion of composite binder materials, the mixture can still exhibit satisfactory flowability. The compressive performance of UHPGC is primarily governed by the proportion of GGBS in the ternary binder system; an appropriate GGBS content can provide enhanced compressive strength and elastic modulus. UHPGC exhibits ductile behavior under flexural loading; however, replacing GGBS with SS significantly reduces its flexural strength and energy absorption capacity. The optimal static mechanical performance is achieved when the mass proportions of SS, GGBS, and FA are within the ranges of 9.3–13.8%, 66.2–70.7%, and 20.0–22.9%, respectively. This study provides a scientific approach for the valorization of SS through construction material applications and offers a theoretical and data-driven basis for the mix design of ultra-high-performance building materials derived from industrial solid wastes. Full article

Recycling plastics waste into concrete represents one of the possible approaches for its valorization, offering both economic and environmental benefits. Although numerous studies have explored the mechanical properties of concrete with plastics waste, its durability performance remains largely unexplored. In this context, this study aims to assess the electrochemical behavior of rebars embedded in reinforced concrete modified by partially replacing natural aggregates with recycled plastics, comparing their behavior to that of conventional concrete. The corrosion of reinforcing steel bars was evaluated by wet and dry cycles (w/d) in calcium chloride solutions, monitoring corrosion potential and potentiostatic polarization resistance, and recording electrochemical impedance spectroscopy (EIS) and polarization curves. In addition, the chloride diffusion tendency and the mechanical performances were assessed in unreinforced samples. The findings indicate that in environments with lower chloride concentrations, concrete with plastic granules provides good protection against rebar corrosion. Although the mechanical results of the studied mixes confirmed that incorporating plastic granules as aggregates in the concrete matrix causes a reduction in compressive strength, as known in the literature, the modified concrete also exhibits improved post-cracking behavior, resulting in enhanced ductility and fracture toughness. Full article

The utilization of steel slag (SS), fly ash (FA), and ground granulated blast furnace slag (GGBFS) as soil additives in construction represents a critical approach to achieving resource recycling of these industrial by-products. This study aims to activate the SS-FA-GGBFS composite with a NaOH solution and Na₂CO₃ and employ the activated solid waste blend as an admixture for lateritic clay modification. By varying the concentration of the NaOH solution and the dosage of Na₂CO₃ relative to the SS-FA-GGBFS composite, the effects of these parameters on the activation efficiency of the composite as a lateritic clay additive were investigated. Results indicate that the NaOH solution activates the SS-FA-GGBFS composite more effectively than Na₂CO₃. The NaOH solution significantly promotes the depolymerization of aluminosilicates in the solid waste materials and the generation of Calcium-Silicate-Hydrate and Calcium-Aluminate-Hydrate gels. In contrast, Na₂CO₃ relies on its carbonate ions to react with calcium ions in the materials, forming calcium carbonate precipitates. As a rigid cementing phase, calcium carbonate exhibits a weaker cementing effect on soil compared to Calcium-Silicate-Hydrate and Calcium-Aluminate-Hydrate gels. However, excessive NaOH leads to inefficient dissolution of the solid waste and induces a transformation of hydration products in the modified lateritic clay from Calcium-Silicate-Hydrate and Calcium-Aluminate-Hydrate to Sodium-Silicate-Hydrate and Sodium-Aluminate-Hydrate, which negatively impacts the strength and microstructural compactness of the alkali-activated solid waste composite-modified lateritic clay. Full article

Ultra-high-performance concrete (UHPC) reinforced with recycled tire steel fibers (RTSFs) was studied to evaluate its mechanical properties and cracking behavior. Using acoustic emission (AE) monitoring, researchers tested various RTSF replacement rates in compression and flexural tests. Results revealed a clear trend: mechanical properties initially improved then declined with increasing RTSF content, peaking at 25% replacement. AE analysis showed distinct patterns in energy release and crack propagation. Signal timing for energy and ringing count followed a delayed-to-advanced sequence, while b-value and information entropy changes indicated optimal flexural performance at specific replacement rates. RA-AF classification demonstrated that shear failure reached its minimum (25% replacement), with shear cracks increasing at higher ratios. These findings demonstrate RTSFs’ dual benefits: enhancing UHPC performance while promoting sustainability. The 25% replacement ratio emerged as the optimal balance, improving strength while delaying crack formation. This study provides insights into the mechanism by which waste tire steel fibers enhance the performance of UHPC. This research provides valuable insights for developing eco-friendly UHPC formulations using recycled materials, offering both environmental and economic advantages for construction applications. Full article

Iron-rich bauxite residue (red mud) is a hazardous alkaline solid waste produced during the production of alumina from high-iron bauxite, which poses severe environmental challenges due to its massive stockpiling and limited utilization. In this study, metallic iron was recovered from high-iron red mud using the smelting reduction process. Thermodynamic analysis results show that an increase in temperature and sodium oxide content, along with an appropriate mass ratio of Al₂O₃ to SiO₂ (A/S) and mass ratio of CaO to SiO₂ (C/S), contribute to the enhancement of the liquid phase mass fraction of the slag. During the smelting reduction process of high-iron red mud, iron recoveries for low-alkali high-iron red mud and high-alkali high-iron red mud under optimal conditions were 98.14% and 98.36%, respectively. The metal obtained through reduction meets the industrial standard for steel-making pig iron, which is also confirmed in the pilot-scale experiment. The smelting reduction process of high-iron red mud can be divided into two stages, where the reaction is predominantly governed by interfacial chemical reaction and diffusion control, respectively. The apparent activation energy of high-alkali high-iron red mud is lower than that observed for low-alkali high-iron red mud. The reduced slag can be used as a roadside stone material or cement clinker. This proposed method represents a sustainable process for the comprehensive utilization of high-iron red mud, which also promotes the minimization of red mud. Full article