Reuse of Retired Wind Turbine Blades in Civil Engineering

Abstract

The rapid growth of the wind energy sector has led to a rising number of retired wind turbine blades (RWTBs) globally, posing significant environmental and logistical challenges for sustainable waste management. Handling enormous RWTBs at their end of life (EoL) has a significant negative impact on resource conservation and the environment. Conventional disposal methods, such as landfilling and incineration, raise environmental concerns due to the non-recyclable composite material used in blade manufacturing. This study explores the upcycling potential of RWTBs as innovative construction materials, addressing both waste reduction and resource efficiency in the construction industry. By exploring recent advancements in recycling techniques, this research highlights applications such as structural components, lightweight aggregates for concrete, and reinforcement elements in asphalt pavements. The key findings demonstrate that repurposing blade-derived materials not only reduces landfill dependency but also lowers carbon emissions associated with conventional construction practices. However, challenges including material compatibility, economic feasibility, and standardization require further investigation. This study concludes that upcycling wind turbine blades into construction materials offers a promising pathway toward circular economy goals. To improve technical methods and policy support for large-scale implementation, it recommends collaboration among different fields, such as those related to cementitious and asphalt materials.

1. Introduction

Wind power systems are a pivotal part of renewable energy, providing a straightforward strategy to effectively reduce the carbon emissions of the power industry. Based on the 2023 Statistical Yearbook of World Energy, global wind power generation reached 2104.8 billion kWh in 2022, reflecting a 13.5% year-over-year increase [1]. In addition, China leads the world in wind energy, with 221 GW of installed capacity, accounting for over one-third of the total installed capacity globally. This figure far surpasses that of the US (96.4 GW) and Germany (59.3 GW). China also operates the world’s largest onshore wind farm, which has a capacity of 7965 MW and is five times larger than the US’s second-largest wind farm (1548 MW), solidifying its dominance in both scale and individual project size [1]. Regarding wind turbine blade (WTB) waste, this implies that by 2031, the initial amount of WTB waste is expected to be around 60 tonnes [2]. The intended service lifespan of wind turbine blades is approximately 20 to 25 years. Most of the blades produced in China are therefore likely to be decommissioned after 15 years [3]. Even worse, the accumulative WTB waste generated in China between 2035 and 2044 will be about 74,000 tonnes [4]. Currently, landfill is the primary method to handle plastic waste in China, with 14% being incinerated, 30% being recycled, 20% being dumped into the environment, and the remaining 36% being landfilled [5]. As a result, these manipulations certainly hinder the rational use of composite materials and lead to environmental pollution. To achieve environmental benefits and sustainable development globally, the efficient way of handling WTB waste is through recycling and reusing, allowing wind energy to rapidly achieve true “clean energy”.

Tremendous recycling technologies have been developed for thermoset composite materials, mainly including three scenarios: mechanical recycling (shredding, grinding, and cement kiln co-disposal), chemical recycling (solvent decomposition and supercritical fluids), and thermal recycling (pyrolysis, fluidized bed, and microwave pyrolysis) [3]. Undoubtedly, mechanical recycling offers a low-cost solution to recycle glass fibers (GFs) or carbon fibers (CFs) from retired wind turbine blades (RWTBs). Although it is a cost-efficient and low-energy consumption method, the recycled fiber-reinforced polymer (rFRP) always faces some risks of reducing mechanical properties (size and tensile strength) [6]. On the other hand, highly flexible strength fibers were successfully separated from WTBs through chemical recycling. This method seems to be a high-performance approach to acquiring fiber-reinforced polymer (FRP). However, there is still a challenge related to decreasing the cost of chemical recycling to produce clean fiber-reinforced polymer (GFRP). Compared to GFRP, chemical recycling is typically considered more suitable for recycling carbon fiber-reinforced polymer (CFRP) because of the expensive cost of virgin CFRP [1]. Additionally, another considerably favorable recycling method is pyrolysis, which uses heat to decompose the resin into small organic molecules in an inert gas environment (nitrogen or no oxygen). Unfortunately, its process is always accompanied by significant energy consumption. As a consequence, to obtain environmental benefits and develop a low-cost method, a significant amount of work needs to be performed.

The GFRP and CFRP from RWTB can be recycled using diverse recycling technologies, leading to different characteristics. Thus, recycled GFRP and CFRP can be reused in various applications. As reported in [7]. Recycled fiber-reinforced polymer (rFRP) is extensively utilized as a filler or fiber in the production of new FRPs and artificial wood products. In addition, the recycled GFRP and CFRP can be optimally capitalized as building materials road materials (asphalt materials), which are typically used in large-volume engineering programs that consume large amounts of natural materials, such as cement and concrete. To minimize natural aggregate extraction in concrete, substituting coarse aggregates, which comprise the majority of typical concrete mixes, presents the most viable approach. Some researchers have reported that recycled FRPs used as an aggregate in concrete can enhance sustainability. In a previous study [8], the mechanical behavior of concrete mixtures containing CFRP with three various size distributions, as well as CFRP/cement mass ratios of 0, 0.05, 0.075, and 0.1, was investigated. The results showed that the need for additional water increases to maintain workability due to the incorporated CFRP particles, regardless of their size, and the flexural strength and compressive strength rose as the CFRP content increased. However, a diminishing trend was observed in large-sized recycled particles (≥21.78 mm²). This indicated that it is difficult to use recycled FRPs as a coarse aggregate due to their anisotropic behavior. If we can prevent coarse particles from exhibiting anisotropic behavior, it could be possible to use them as a fine aggregate. There have been several longitudinal studies involving the use of FRPs as a fine aggerate, which have reported that although some durability-related properties were boosted, a considerable loss of mechanical performance occurred in incorporation content over 15% [9,10]. In addition, previous research [11] has established that the incorporation of glass fiber-reinforced polymer (GFRP) used as filler shows significant potential in asphalt materials. However, it requires a reasonable mass ratio of filler to asphalt, which usually does not exceed 0.8:1, to achieve optimal low-temperature properties. Regrettably, there have been few studies summarizing the applications of FRPs in construction, particularly in cementitious and asphalt materials.

Hence, this paper first evaluates various management strategies, focusing on their effectiveness in waste handling by examining recycling processes available in the literature and identifying existing research gaps. This study finds that mechanical recycling dominates GFRP treatment due to cost advantages, whereas chemical recycling shows potential for higher-value recovery. Crucially, CFRP pyrolysis proves to be more economical than virgin production. However, all methods still require further optimization for energy efficiency and cost reduction. Second, we discuss the direct applications in architecture, coastal protection, and infrastructure. Additionally, the use of recycled FRPs in the production of cementitious materials and asphalt matrices is reviewed to explore whether FRPs improve the mechanical performance of the matrix, as well as to understand the mechanisms between recycled FRPs and the matrix. We have found that the incorporation of recycled FRPs can significantly enhance the tensile strength of the matrix. Nevertheless, significant efforts are needed to optimize the use of recycled FRPs while ensuring that mechanical performance and durability do not decline. Lastly, this report reviews the current uses for recycled FRPs and discusses some unresolved challenges in these areas, highlighting issues with current recycling processes and paving the way for the broader use of recycled materials in construction.

2. Current Treatment Strategies for Retired Wind Turbine FRP Materials

A large number of general WTBs are generally composed of GFs and high-grade resins. Additionally, as the requirement for wind power grows, longer blades with greater weights have raised widespread concerns [12]. To achieve weight reduction in WTBs, CFs can be employed as reinforced fiber in selected structural components of the blades, which are typically longer than 45 m [13] due to their superior mechanical properties, specifically their light weight and high strength. Despite the elevated cost of CFs in comparison with GFs, the significant weight-saving capability will shed light on increasing applications for CFs in the wind power sector. The increasing amount of CFPR waste generated from EoL wind turbines up to 2050 has been estimated, as reported in Figure 1 [14]. Moreover, the production of CFs originating from virgin materials is a relatively high-energy consumption process (198–595 MJ/kg) that generates significant pollution, especially when compared with CF recovery [15]. The recovery cost for CFs is approximately USD 5/kg, which is far lower than the original CF price at USD 33/kg. Therefore, CFPR and GFRP are both extensively recycled, as argued by [13].

2.1. Mechanical Recycling

2.1.1. Shredding and Grinding

Mechanical recycling technology is a processing method that transforms waste materials into renewable resources through physical processing methods (such as cutting and crushing), achieving material recycling. Therefore, high-speed grinding operations for particle size distribution reduction between 50 μm and 10 mm are utilized. Research indicates that mechanical recycling technology is currently the only commercially viable solution for processing decommissioned wind turbine blades. As illustrated in Figure 2 [16], this technology converts retired blades into granular materials, recycled fibers, and powder products of varying particle sizes through physical processing methods such as cutting, crushing, and grinding, thereby facilitating the effective reuse of waste resources [17].

The economic viability of fiber-reinforced polymer (FRP) recycling is primarily influenced by the cost of the shredding process, with the particle size of the FRP waste significantly determining the performance of recycled products. Depending on particle size, recycled FRP materials can be utilized in various applications: particles larger than 25 mm × 25 mm are suitable for building materials, cardboard production, lightweight cement boards, and sound-absorbing materials; particles ranging from 3.2 to 9.5 mm can be used as fillers for roof asphalt, bulk molding compound (BMC), and concrete, as well as for road repair materials; particles smaller than 60 μm are appropriate for use as fillers in sheet molding compound (SMC), BMC, and thermoplastic composites [18].

Compared with the landfilling and incineration methods, it has been demonstrated that mechanical recycling plays a non-trivial role in recycling GFRC and CFRC blades. This method is widely used to recycle GFRCs in various applications [19,20]. It has been revealed [21] that mechanical recycling methods (grinding and reincorporation) are used to recycle GFRCs. The study indicated that replacing 5% to 30% of the weight of virgin fibers (VFs) with recycled fibers (RFs) is advisable in the production of composites to achieve optimal flexible properties. Supporting this view, researchers [22] have attempted to recycle glass fiber-reinforced polymers (GFRPs) from wind turbine blade waste by using shredded composites and glass fibers. The authors subsequently utilized these recyclates to produce new composites by varying the weight percentage or substituting some of the virgin glass fibers with recycled materials. However, the mechanism of this phenomenon has not been unveiled yet. Taken together, these studies support the notion that GFRPs produced from waste through mechanical recycling can be widely utilized across various fields.

2.1.2. High-Voltage Fragmentation

High-Voltage Fragmentation (HVF) technology was initially applied in the field of mineral processing at Imperial College London in 1986, primarily for the separation and extraction of valuable minerals from ores [23]. This technology is based on the electrodynamic fragmentation mechanism, where a plasma channel forms between electrodes when the current rise time is less than 500 nanoseconds. Since the dielectric breakdown strength of solid materials is lower than that of the surrounding water medium, the plasma channel focuses on the sample to be processed. During this process, localized temperatures can reach 104 K, with pressure ranging between 109 and 1010 Pa, thereby generating shock waves within the sample. These shock waves propagate along the weak interfaces of material phases, causing dissociation at the interfaces and achieving material separation [24]. The machine is shown in Figure 3 [25,26].

Given HVF’s capability to achieve selective separation by utilizing multi-phase material interfaces, its potential application value in the recycling field has been extensively explored since the early 21st century. In the field of composite materials, HVF technology has already become a major research focus, aiming to study the liberation of fibers from the matrix. Comparative studies on the treatment of thermoset composites (GFRP) using HVF and mechanical shredding have shown that fibers obtained through HVF exhibit a broader length distribution range, along with slightly higher Young’s modulus and tensile strength [27]. Subsequent research has further confirmed these findings and conducted a comparative analysis of the energy consumption of both methods. The data indicate that the average energy consumption of HVF is 2.6 times that of mechanical shredding, with an average energy conversion efficiency of 18.1% [28]. While experimental results demonstrate promising energy-saving benefits, we recognize that several challenges remain for the industrial-scale adoption of High-Voltage Fragmentation technology. Key practical limitations include (1) the scalability of electrode systems for continuous operation, (2) material compatibility issues in heterogeneous waste streams, and (3) the current capital expenditure requirements for high-voltage infrastructure. These factors currently need to be addressed in our ongoing industry collaboration, focusing on throughput optimization and cost–benefit analysis for commercial deployment.

2.2. Chemical Recycling

Chemical recycling means that the composite (such as epoxy resin, carbon fiber, and glass fiber) of the retired wind turbine blades (RWTBs) is processed into reused and valuable chemical products. Chemical recycling mainly includes solvent dissolution and supercritical fluids [19]. The dissolution process is often called solvolysis. The solvents generally utilized to degrade epoxy resin include tetralin, supercritical/subcritical fluids [23,29], nitric acid [30], and supercritical water. Supercritical fluid commonly highlights the relatively special state in which the temperature and pressure of the fluid exceed its inherent critical temperature and critical pressure [19]. As mentioned in the critical condition (threshold value), the rate of mass transfer with low viscosity and high diffusion constants for all kinds of materials is notably accelerated in supercritical fluids [22].

Supercritical fluids (SCFs), particularly supercritical water (SCW) and supercritical alcohols, also serve as promising media for recovering fibers, including glass and carbon fibers, as well as resin [31]. Recent research [7,32] has investigated the recycling of CFRPs from WTBs under two conditions: one with supercritical water at temperatures between 250 °C and 300 °C and a pressure ranging from 4 to 27 MPa, and the other with supercritical alcohols at temperatures varying from 300 °C to 450 °C and a pressure of 15–17 MPa. Surprisingly, only 2–10% degradation of the mechanical strength was found in the recovered carbon fiber [33]. However, these recent studies were conducted in labs, and it is still a challenging task to separate these materials from each other within the RTWB structure in industrial settings. Furthermore, study [34] indicates that improved chemical recycling technology for samples from two unknown RWTB materials has been used to separate resins from GFs in an efficient two-step approach. The optimal reaction conditions are obtained through a two-step process in which EoL glass fiber-reinforced polymer (GFRP) waste from wind turbine blades is first heated to 270 °C in ethylene glycol for 16 h, followed by a second heating to 300 °C in a mixture of water, 1-propanol, and KOH for 3 h. Although this approach improved the recycling method for extracting GFRPs from RWTBs, challenges persist concerning chemical recycling involving the solvolysis of mixed waste streams of GFRPs. The reason is that a longer reaction time is caused by thick and dense materials, which can decrease the mechanical strength of fibers.

In addition to reagent categories, processing time, and reaction time, the chemical recycling process was also certainly affected by catalyst concentration. Current research [35] uses ionic liquid as a catalyst at atmospheric pressure to depolymerize polyethylene terephthalate (PET) with ethylene glycol. The result revealed that the conversion of PET increased with a growing quantity of catalyst. The degraded products may be more easily dissolved or dispersed using specific solvents, thereby accelerating the overall decomposition process. However, a challenge always exists due to the complete separation of the applied catalyst salt from the purification of the depolymerized products [32].

2.3. Thermal Recycling

To date, multiple distinct types of thermal recycling techniques have been developed to recycle FRPCs from retired wind turbine blaze materials. Thermal recycling is a process that converts waste materials into usable energy or materials through heat treatment methods. Pyrolysis, the fluidized bed process, and combustion are currently the most popular methods used in thermal recycling [19]. The pyrolysis technique, which extracts valuable GFs from RWTBs, is used to degrade organic parts of waste composites derived from the composite matrix into their molecular forms (gas or liquid), demonstrating considerable potential for resource utilization. What is specific about the fluidized bed process is that it can recover fibers as well as other by-products (liquid and gas) from EOL waste composites [36].

Fiber surfaces are often contaminated with solid char; thus, a subsequent post-pyrolysis treatment (oxidation using air) is required to burn the char to obtain clean fibers and fillers [20]. Therefore, the pyrolysis technique is an excellent method for producing clean fibers from a composite matrix. An inert atmosphere at a temperature ranging from 300 °C to 700 °C typically depends on the types of waste composites and reinforcement materials [36,37,38].

Carbon fiber-reinforced composites (CFRCs) have a great deal of applications in the aerospace, automotive, and wind turbine industries. The mechanical performance of recovered clean fiber, including carbon fiber and glass fiber, is ameliorated if the pyrolysis process is optimized. Existing research has indicated [39] that as the temperature increases from 350 °C to 700 °C, the tensile strength of recycled CFs improves from 2.34 GPa to 3.27 GPa (compared to 3.5 GPa for virgin CFs), reaching approximately 90% of the tensile strength of original virgin carbon fibers after pyrolysis at 500 °C. Yet it still faces environmental and energy consumption issues related to the high temperature. There have been some efforts to decrease the energy consumption. Microwave-assisted pyrolysis (MAP) processes and catalytic pyrolysis capitalized on public concern. Figure 4 shows the processes of MAP [40]. Waste composite panels with an epoxy resin matrix were subjected to microwave-assisted heating treatments to facilitate the volatilization of the polymer matrix, thereby yielding clean fibers qualified for potential reuse in high-performance structural and industrial applications. Compared to conventional pyrolysis, this method can efficiently reduce energy consumption. Current research has affirmed [40] that CFs were successfully recycled emerging from cured carbon fiber/epoxy (CF/EP) at temperatures of 450 °C, 550 °C, and 650 °C, followed by oxidation. The highest tensile strength of the CFs was achieved at 450 °C compared to other temperatures due to the char protective effect. Compared with original virgin fibers, the strength of recycled fibers slightly decreased, which could be due to the reduced temperature. However, carbon fiber derived from EoL waste boosted in the composite industry has often been omitted from research [6,41]. From an economic point of view, MAP has a significant time-efficient advantage in dealing with carbon fiber.

Fluidized bed technology has been established over the last two decades and is currently operating on a pilot scale, as illustrated in Figure 5. CFRP waste is generally aerated under pressure (10–25 kPa) using a heated gas (450–550 °C) within a silica sand bed. Fluidized bed reactors have the ability to treat mixed and contaminated materials [20]. The high temperature breaks down composites, such as retired wind turbine blades, while preserving the fibers. These fibers are subsequently oxidized using gases released by the matrix. However, this technique involves certain risks, including those related to the presence of organic solvents and the release of harmful gases. The major drawback of the fluidized bed process is the significant decrease in the mechanical properties of the fibers during recovery [40]. Recycled CFs and GFs [42] have tensile strengths that reach 75% and 50% of the strengths of virgin CFs and GFs, respectively. One typical study [43] investigated the plant capacity, feed rate, and air leakage of recycling plants, which are vital operating parameters. To achieve energy-efficient CF recycling, the feed rate per unit of bed area was identified as an especially crucial parameter. The additional enhancement of the fluidized bed recycling procedure should aim to balance maximizing the feed rate per unit of bed area to reduce process energy consumption and the potential effects on the mechanical properties of recycled carbon fiber (rCF).

2.4. Landfills

Landfills were a public option for disposing of decommissioned wind turbine blades in the early days. This method is the lowest on the waste disposal hierarchy due to the fact that it does not require energy recovery.

Little research has been conducted on the effect of landfills on environmental problems. Three possible ways of handling Irish blade waste were compared by [44], including co-processing in a cement kiln in Germany, co-processing in a cement kiln in Ireland, and landfill in Ireland. Based on the single-score output shown in Figure 6, scenario 2 is 1007% more advantageous than scenario 3 and 78% more advantageous than scenario 1. This suggests that co-processing should be developed in Ireland due to environmental benefits. Co-processing Irish blade waste at a 10% material substitution rate in the German cement kiln was found to be six times more environmentally beneficial than depositing waste in an Irish landfill. However, there are some limitations of the study that originate from uncertainty regarding the actual amount of raw materials replaced in the cement factory. Some research has indicated that the environmental issues associated with blades may be more serious due to the increase in the proportion of carbon fiber [45]. At the same time, several countries have inhibited disposal in landfills, as it would pollute the nearby soil.

2.5. Negative Effects of Current Treatment Strategies

Mechanical recycling has already achieved industrialized production. However, the product acquired using this method is only used as filler, and the method is also used to treat glass fiber-reinforced polymer. In addition, energy consumption, product damage, and environmental pollution are generally associated with this technology.

Chemical recycling is commonly used in laboratories. However, supercritical fluid is formed under conditions that typically involve high temperatures and high pressure. Therefore, the reaction facilities involved have performance requirements and operating costs that are naturally high. Furthermore, reclamation involves strong inorganic acid decomposition and organic solvent decomposition; the use of a great deal of solutions can lead to environmental pollution that is more severe than that caused by mechanical recycling. The industrialization implementation and widespread use of this technology are potentially affected by these negative factors.

Thermal recycling is advantageous because of its ability to break down heavy hydrocarbons into lighter, more valuable, and clean fibers. However high energy consumption and environmental impacts are the main drawbacks.

Landfill disposal is an easy way to handle all kinds of waste. However, improperly implemented landfills can lead to soil settlement, structural damage, and drainage problems.

3. Direct Reuse in Construction

3.1. Infrastructures

Given that a large number of wind turbine blades are approaching the end of their service life, the wind energy industry urgently needs to establish a comprehensive recycling system for retired blades. Research [45] forecasts indicate that by 2050, approximately 25,000 wind turbines will be decommissioned, highlighting the urgency of developing sustainable recycling solutions. In the coming years, managing and reusing decommissioned wind turbine blades will be necessary, but a clear recycling solution has not yet been found due to the blades’ complex composition [46].

To date, some researchers have attempted to introduce these crushed WTBs as fine or coarse aggregates. The research [7] has already been designed to determine whether the GFRPs generated by cutting pultruded profiles can be used as aggregates in concrete. In the study, different concrete mixes were produced with an increasing proportion (0–20%) of GFRP fine waste as a replacement for the natural aggregate. The study indicated that filler-sized waste produced during the cutting of GFRP elements is fit for concrete production if it is used at a low proportion. Additionally, the reuse of GFRP in concrete is technically appreciated for applications in which compressive strength is not the primary condition. As reported in the literature, study [46] has systematically assessed the applicability of mechanical recycling technology in the treatment of wind turbine blades by obtaining recycled wind turbine blade (RWTB) material, which can replace up to 10% of the volume of aggregates in the production of fiber-reinforced concrete (FRC). The experimental results show that although the elastic modulus of the material exhibits a gradual decreasing trend, its compressive strength remains above 40 MPa. These mechanical properties can be accurately predicted using a regression correction model based on hardened density. Additionally, the researchers [47] found that for each 1% increase in RWTB content, the splitting tensile strength of concrete can be improved by 0.03 MPa, while Poisson’s ratio decreases and the flexural strength remains stable. Despite these findings, ongoing studies are needed to explore how to maintain consistent long-term strength development and enhance durability. Moreover, the authors reported [48] that the waste glass fiber-reinforced polymer (GFRP) material extracted from WTB was processed into powder, aggregate, and fiber for use in concrete. The results showed that the powder leads to an increase in the setting time, owing to the wood content and its associated soluble sugars, as well as a pronounced drop in the compressive and flexural strengths; however, it must be noted that with the addition of 10% cement replacement, the compressive strength was comparable to that of the reference sample, which was free of GFRP, at 90 days. They also indicated that when GFRP was incorporated as fibers in concrete, the flexural capacity was enhanced by up to 15%, with no significant decrease in compressive strength. Consequently, incorporating GFRP into concrete is a viable approach for producing mixtures that exhibit comparable long-term compressive strength, enhanced flexural capacity, and a positive contribution to sustainable development. Therefore, maintaining or improving mechanical property reduction by incorporating GFRP is still a challenge.

Furthermore, in the field of 3D-printed concrete (3DCP), researchers have already made strides in enhancing tensile strength, toughness, and cracking resistance by incorporating fibers into 3DCP. It was observed [49] that adding 2.1 vol% of 6 mm steel fibers to 3DCP significantly enhanced its flexural strength, although the 3DCP still displayed noticeable strain-softening behavior. In addition, the use of carbon fiber and glass fiber [50] can also effectively boost the tensile proportional limit, fracture modulus, and fracture toughness of 3D-printed concrete. The printability and physical–mechanical properties of 3DCP incorporating RWTB fiber (3–7 vol%) were investigated by [51], and concrete containing 3–7% RWTB fiber exhibited good extrudability and buildability, although its fluidity was slightly reduced. The extrudability and buildability were visually demonstrated through the printed quality of filament and stacked layers. Figure 7 displays five printed concrete filaments containing 0%, 3%, 5%, 7%, and 9% of RWTB fiber. During the extrusion process, when the fiber content was less than 5%, there were no observations of blockages, interruptions, tears, or bleeding. When the fiber content was 7%, minor surface scratches emerged on the extruded filaments, which could be attributed to the pull-out of fibers. Once the fiber content reached 9%, the extruded filaments showed a decrease in width, accompanied by multiple defects, and obvious disruptions occurred. These findings imply that the mixture at this fiber concentration level became progressively more challenging to extrude continuously, probably because of the degraded flow consistency. Figure 8 shows photos of the printed single layers of concrete with 0–7% of RWTB fiber. It can be seen that as the RTWB fiber content increased, partial distortion occurred in the printed filament, as shown in Figure 8d. Additionally, the incorporation of 5 vol% of RWTB fibers in 3D-printed composites demonstrated significant mechanical enhancement, exhibiting 13.2–29.9% greater unconfined compressive strength and 147.4–465.4% higher bending toughness in the Y/Z loading directions compared to non-reinforced controls. However, the durability of GFRP in this field should be investigated to shed light on broad recommendations. These studies provide valuable insights into the potential reuse of recovered wind turbine blades in building applications.

3.2. Architecture

As the number of retired wind turbines continues to rise, the disposal of discarded turbine blades has gradually become an urgent environmental issue. Repurposing involves using sections of wind turbine blades for purposes other than their original function. These blade sections can effectively be utilized in the built environment or for structural applications. A notable illustration of this concept is the Wikado playground in Rotterdam (Figure 9), showing how repurposed wind turbine blades can be used to create innovative architecture [52]. In addition, the first footbridge for bicycles and pedestrians (Figure 10) constructed using beams made from recycled wind turbine blades has been installed over the Szprotawa River [53] by Anmet and the GP Renewables Group. Additionally, public seating, bus stop shelters, and bicycle sheds represent other potential uses for repurposed sections of turbine blades in urban settings. However, the current challenges of repurposing include the absence of standards and design and certification guidelines, the difficulty of large-scale implementation, and societal acceptance of the intended reuse application [13].

3.3. Coastal Protection

Fiber-reinforced polymer (FRP) has a high strength-to-weight ratio and is not affected by corrosive media such as chloride ions. Therefore, applying this material as a reinforcement material in coastal reinforced concrete (RC) structures has attracted widespread attention. FRP exhibits excellent corrosion resistance in marine environments, maintaining 75% of its tensile strength even after 100 years [7]. The failure prediction of the structure is imperative due to the harsh marine environment. Researchers [54] have designed tests on samples of epoxy/glass and polyester/glass with different fiber layout configurations by submerging them under the sea for periods of 6, 12, and 24 months to test the mechanical properties and predict the model of the FRP. They reported that all samples exhibited a notable increase in mass and decreased tensile strength due to seawater absorption. This outcome of the research provided a basic step for predicting lifetime structure. As reported by [54], recent research has highlighted the importance of developing a model for predicting structure service life again. The paper proposed [55] a model for predicting the fatigue life degradation of fiber-reinforced polymer (FRP) composite materials subjected to extended exposure in real marine environments. It utilizes data collected from earlier phases of a broader study on how marine environments affect the mechanical properties of FRP composites, assessing the impact of these observed changes on the endurance properties of the materials. The paper also considered the design and operational guidelines set by classification societies regarding this issue. Furthermore, it examined the need to modify the process for deriving S–N curves of composite materials to account for the effects of the marine environment. The experimental data were analyzed through regression analysis, leading to a mathematical model that effectively describes strength degradation over time and also has an acceptable correlation value. The S–N curves for E-glass/polyester composites, which feature three different fiber layout configurations, are evaluated and adjusted to reflect the findings of this research. Unfortunately, there is still a gap in applying it in real construction due to its short prediction period.

Building upon the investigation to include the direct reuse of blades, it is essential to consider their integration into cementitious materials to maximize their potential benefits. The physical characteristics of the blades, such as their tensile strength and durability, make them suitable as reinforcement components within cement-based matrices. Additionally, their chemical composition can influence the overall performance of the composite material, potentially improving resistance to environmental degradation. By applying blades in cementitious formulations, we can not only promote sustainable material usage but also enhance the mechanical properties of the final product. This transition from simple reuse to practical application underscores the multifaceted benefits of incorporating these materials into construction practices.

4. Application of Retired Wind Turbine Blades in the Production of Cementitious Materials

4.1. Micro- and Macro-Fibers, Flakes, and Powder

The resource utilization of wind turbine blades in service has shown significant potential in the field of gelling materials. The recycled materials can be divided into recycled macro-fiber, recycled micro-fiber, flakes, and powder according to the size of the specification. Regenerated macro-fibers with a fiber diameter larger than 0.3 mm are key reinforcement materials that serve as crack-reinforcing elements or structural rebar due to their high splitting tensile strength [56]. Regenerated fibers with a fiber diameter of less than 0.3 mm can be classified as regenerated micro-fibers that are typically used as plastic or dry shrinkage control elements, but the dosage needs to be controlled (usually ≤10 wt.%) to avoid reducing the compressive strength. In addition, retired wind turbine blades are mechanically cut fragments (large chunks of material) that can replace coarse aggregates or structural fillers, thereby reducing material density and improving flexural performance. The powder (fine particles or resin-based powder) obtained via the mechanical grinding of decommissioned wind power blades is used as an additive to improve material properties; for example, epoxy resin powder can optimize the fluidity and workability of cement-based materials, but the dosage needs to be controlled (usually ≤8 wt.%) to avoid delaying hydration reaction.

Several researchers have investigated the application of GFRP with different particle sizes in cementitious materials. The aim of one study [57] was to incorporate solid waste generated from the retired wind turbine blades into a Portland cement-based mortar (at a cement/aggregate ratio of 1:3). Part of the aggregate (sand) was replaced by solid waste, thereby reducing the use of natural resources and simultaneously providing a suitable way to dispose of and use wind blade waste. Solid waste is collected and converted into powder using a turning process that results in a particle size similar to that of sand. The results showed that the cementing composites produced by displacing up to 15% of the sand met the relevant technical standards, such as compressive strength (8 MPa). The unit mass of mortar containing waste is also lower, providing an advantage for civil construction applications. The results show that it is feasible to incorporate the waste into the cement matrix, providing advantages for civil construction applications. However, recycling GFRP has proven to be challenging due to its high mechanical properties and high resistance to harsh chemical and thermal conditions. Furthermore, past efforts to machine GFRP waste for use in concrete mixtures by cutting it into large (centimeter-scale) pieces, as opposed to crushing, were studied [58]. These fragments can be divided into two main categories: coarse aggregate and fibrous GFRP. The study affirmed that using GFRP as a coarse aggregate can significantly reduce both the compressive strength and tensile strength of concrete due to its elastic modulus mismatch. However, surprisingly, GFRP results in a significant increase in the energy absorption capacity of concrete. In addition, if the fibrous GFRP is aligned longitudinally within the concrete, the tensile strength of the concrete can be significantly increased. At the same time, it takes less energy and time to process GFRP waste into a fibrous form compared to GFRP coarse aggregate. In addition, fibrous GFRP is a high-strength composite compared to crushed GFRP waste consisting of separated glass fibers and resin particles and can be used as a low-quality filler. As a result, to obtain optimal performance with the incorporation of GFRP, choosing various particle sizes for different applications of cementitious materials can be the best way to use this recycled GFRP.

4.2. Reinforcement Mechanism

The addition of recycled glass fiber can significantly enhance the tensile strength of concrete. Its high elastic modulus (70–90 GPa) forms a composite reinforcement network with the concrete matrix, limiting crack propagation by bridging cracks and improving the fracture toughness and frost resistance of the cementitious material [56]. The dropped flow, decreased compressive strength, and enhanced tensile strength were attributed to the additional GFRP [47,59]. It is worth noting that compressive strength competes with tensile strength as the fiber content increases. The reason is that strength enhancement is closely related to dense pores or fewer microcracks. However, with the incorporation of fiber, the strength degradation seems to be a manifestation of rising numbers of pores or microcracks [60]. The incorporation of fiber can lead to insufficient compaction and extra microcracks within the matrix. These defects are influenced by fiber types and sizes. Similarly, study [61] is in line with the mentioned mechanism. The study investigated the use of three varying concentrations of macro synthetic fibers, corresponding to 0%, 0.5%, and 1% of the total volume of recycled aggregate concrete (RAC), testing their mechanical properties and explaining the related mechanisms. The study indicated that synthetic macro-fibers improved the tensile strength and spot peak behavior of fiber-reinforced concrete due to improved fracture energy. In addition, using GFRP can improve the performance of defective drywall [62]. The study conducted the single compression test, repeated loading test, and scanning electron microscope (SEM) test on defective gypsum board. The results exhibit that the compressive strength of defective gypsum board can be improved threefold compared to reference by incorporating GFRP. The primary mechanism for the reinforcement of GFRP is that the addition of GFRP can reduce stress concentration, enhance integrity, and inhibit brittle damage, especially at high stress levels. Furthermore, microstructure analysis affirmed that incorporating GFRP improved adhesion and prevented microcracks, thereby boosting the flexural strength of the sample under cyclic load. The research results offer a path for incorporating glass fiber into defective brittle materials. Overall, the enhanced tensile strength mechanism can be explained by the macro-fiber “bridging effect”, which enhances the load-bearing capacity of material, even after crack initiation. This mechanism serves to restrict the propagation of shrinkage cracks by functioning as a bridging element at crack interfaces during their opening. Additionally, it creates controlled weak interfaces between the fibrous components and the concrete matrix [61].

4.3. Chemical Reactions at the Interfacial Zone

Figure 11 shows the dissolution process of GFs derived from WTBW in cement. GF in WTBW consists of amorphous silicate–aluminate chain structures. The calcium ion or sodium ion is attached to the end of the silicate–aluminate chain. In an alkaline environment, a progressive dissolution of the silica network is induced as the hydroxyl group (hydroxide) reacts with the (≡Si-O-) bond. GFs, in a high-pH concrete environment, dissolve into Si-O tetrahedral or Al-O tetrahedral monomers. As shown in Figure 12, the primary step involves hydroxyl-induced epoxy ring-opening reactions, which both consume substantial hydroxide ions and generate chiral centers [56]. The pH reduction in the pore solution, which is associated with alkali consumption, demonstrates the retarding effect of epoxy resin on the cement hydration process. In addition, amine groups (usually in curing agents) and carbon groups (usually in uncured epoxies) can be stabilized under high-pH conditions [63]. They have no significant effect on the cement hydration process, while mercaptan and anhydride functional groups constitute minor components of the epoxy resin. The mercaptan group reacts with the hydroxyl ion to form the sulfate ion. Anhydride groups react with hydroxyl ions to form carboxylic acid ions and corresponding alcohols. As a result, these reactions may cause a reduction in the pH, which can result in a delaying effect.

4.4. Mechanical Properties and Durability

A series of experiments have been published in the existing literature to investigate the effects of recycled wind turbine blade fibers on the mechanical properties and durability of gelled materials. Experiments have shown that there is a certain optimal limit for the addition of recycled fibers to the cementing material, beyond which the increased amount of waste decreases the compressive strength [59]. This phenomenon arises from the cumulative effect of multiple competing mechanisms that govern material performance. The main benefits of using fiber components as reinforcing media in cementing materials are improved properties, such as tensile strength and durability [69]. Repurposing waste glass fiber-reinforced polymer (GFRP) materials from wind turbine blades (WWTB-GFRP) in concrete production was explored by [47]. Comprehensive material characterization was conducted to analyze how WWTB-GFRP in powdered, granular, or fibrous morphological forms influences concrete performance. Various cement replacement rates (10–30%) and coarse aggregate replacement levels (33–100%) were tested, followed by fiber addition rates of 11.75 vol.%. The study evaluated the compressive and flexural strength of the resulting concrete mixtures. The findings indicate that the form of WWTB-GFRP significantly influences concrete properties. While using WWTB-GFRP powder increases setting time and reduces strength, a 10% cement replacement showed no obvious reduction in compressive strength compared to the reference following a 90-day curing period after removing the wood content. Additionally, incorporating WWTB-GFRP as a fiber reinforcement enhanced the flexural capacity by up to 15% without compromising the compressive strength. As a result, valorizing WWTB-GFRP in concrete is viable, offering comparable long-term compressive strength and enhanced flexural capacity, thus supporting sustainable development efforts. However, the implementation of long-term tests could provide a practical basis.

The feasibility of enhancing select mortar durability by incorporating a low dosage (5–10 wt.%) of very fine glass fiber-reinforced plastic byproduct was investigated [70]. Although substituting 5–10% of sand volume with GFRP dust could prolong the cement slurry setting time, this effect can be mitigated through the application of preheat-treated GFRP dust. Meanwhile, the mean radius is small, and GFRP dust can improve the workability, autogenous shrinkage, deformation, and total porosity of mortar. When wet curing conditions are applied, it significantly reduces the mechanical properties of the mortar. However, in the presence of GFRP dust, constriction restriction and capillary water absorption lead to a lower risk of cracking. When GFRPs are used as reinforcement materials, this reduction may indicate the enhanced durability of GFRP mortar. As a consequence, the durability of incorporating FRP into the concrete industry is still a challenge to solve.

5. Application of Retired Wind Turbine Blades in the Production of Asphalt Materials

Currently, asphalt pavement is the predominant choice for road construction projects. The performance characteristics of asphalt mixtures are predominantly governed by two key factors: (1) aggregate gradation and (2) the rheological properties of the asphalt mastic, which is a composite binder system that consists of bitumen and mineral filler and provides interfacial adhesion [71]. Asphalt mixture, a typical multiscale material, comprises a chemically complex binder, size-graded aggregates with varied mineralogy, and interconnected/disconnected air voids [72]. The ratio of filler to asphalt, size distribution, and type of filler significantly affect the rheological property of asphalt mastic [73]. FRP, especially GFRP, has been widely utilized in construction, navigation, and chemical engineering due to its excellent physical and mechanical properties. A large proportion of GFRP from RTWBs was produced due to the Eol of RTB. Although studies have demonstrated that waste GFRP (as short chips/fibers) improves bitumen performance, this approach fails to enable the bulk recycling of GFRP waste. Therefore, to broaden the GFRP originating from RTWBs in the application of asphalt materials and achieve an economic cycle, the feasibility of utilizing crushed GFRP as a filler in asphalt mastic has been investigated.

5.1. Dispersion in Asphalt

Asphalt binder distributes around the filler with a complex structure at the mastic scale; the distribution of mastic among the fine aggregates seems random at the mortar scale; asphalt mortar also disperses randomly between coarse aggregates with different thicknesses at the mixture scale [71]. Extensive research has validated the technical viability of utilizing GFRP waste as a filler component in asphalt mastic. Waste GFRP powder, due to its excellent adhesion performance with bitumen, appears to be positively related to both the aging and moisture resistance of asphalt mastics. In study [74], mechanically recycled GFRP powder from epoxy-based composites served as an effective mineral filler in asphalt mastic, and its modified properties were thoroughly examined. The results exhibited that the improved rutting resistance and fatigue resistance in asphalt materials due to the anti-put-off effect generated regular cylindrical particles in GFRP powder. It also demonstrated that the presence of GFRP fillers in asphalt greatly enhances moisture resistance due to improved adhesive and cohesive capabilities. Additionally, GFRP fillers in asphalt samples had fewer shade-offs compared to limestone. Unfortunately, the incorporation of GFRP powder adversely affects the low-temperature performance of asphalt mastic due to its low density. Surprisingly, recent research [75] has indicated that modified GFRP in asphalt mastic can improve its low-temperature crack resistance. The study used a coupling agent to pretreat the GFRP to enhance its adhesion to asphalt mastic, both before and after modification, as illustrated in Figure 13. The surface of the original GF-WTB is smooth, with only some resin-curing agent residues scattered across it. After modification with UP152, the fiber surface is coated with a rough graft layer of the silane coupling agent, and the fiber diameter increases, suggesting that the graft layer of UP152 has a certain thickness. The study also indicated that the addition of GFRP with the optimal incorporation of 2 wt.% of asphalt material as filler can elevate the low-temperature crack resistance of asphalt, and the enhanced asphalt properties directly translate to improved pavement performance across all corresponding mixture formulations. Thus, the pavement mixture performance needs to be further explored. In addition, in a wheel track test performed by [76], it was observed that asphalt mixes with 0.3% fiber content demonstrated improved resistance to rutting. This enhancement is likely attributed to the randomly distributed fibers within the matrix, which help resist shear deformation. In summary, several lines of evidence suggest that to achieve optimal asphalt mastic characteristics, studies should carefully consider that factors such as the surface texture of the fiber, length of the fiber, or fiber mechanical properties affect glass fibers’ contribution to the performance enhancement of the asphalt.

5.2. Effect of Mixing Temperature

Recent studies have substantiated that fiber-reinforced polymer (FRP) can significantly enhance the high-temperature performance characteristics of asphalt mastic, including penetration resistance, softening point elevation, ductility improvement, mechanical strength, and durability [77,78]. Study [78] also unveiled that fiber-reinforced asphalt mastic exhibits temperature-dependent behavior, with both the cumulative deformation and strain variation rate escalating as temperatures climb. To date, to decrease energy consumption, numerous empirical studies have specifically focused on using warm mix asphalt (WMA) additive due to its capability of reducing greenhouses gas emissions and energy consumption, as well as improving the compact ability of fiber-reinforced asphalt mixtures. Recently, study [79] investigated how production temperatures, WMA additives, and fiber content influence the crack resistance of GFRP-reinforced asphalt mixtures. Fiber-reinforced hot mix asphalt (HMA) demonstrated equivalent low-temperature cracking resistance to warm mix asphalt (WMA) additive-modified mixtures, despite the WMA specimens being processed at 15 °C lower mixing and compaction temperatures. Furthermore, the results demonstrated that WMA additives enhanced workability during the compaction of fiber-reinforced mixtures without compromising their low-temperature performance, even when processed at substantially reduced temperatures. Similarly, this is consistent with the conclusion by [80]. While the aforementioned benefits demonstrate encouraging outcomes, there are concerns regarding the long-term performance of this technology due to its shorter lifespan compared to asphalt pavement, as argued by [81]. Moreover, the research investigated the rutting performance and moisture damage characteristics of RAP-containing WMA mixtures, both with and without GFRP modification, through comprehensive wheel track testing at KNTU. The results show that while the addition of glass fiber alone to warm mix asphalt (WMA) mixtures enhances the final loading cycles, as well as the C-12.5 value, and improves the resistance to rutting, it does not enhance the moisture susceptibility of warm mix asphalt due to the weak water effect on asphalt aggregate. Additionally, there is a gap to explore the reinforcement mechanism between GFRP from WTBs and asphalt under warm mix conditions.

5.3. Chemical Interactions Between the Asphalt and the Remaining Polymer

Recycled glass-reinforced polymer (GFRP) demonstrates significant potential in road engineering, particularly when applied to asphalt mixtures, as it can elevate asphalt performance. Therefore, it is vital to elucidate the effects of residual polymers, such as polyurethane (PU) and epoxy resin, on asphalt performance before the widespread application of GFRP in asphalt mixtures [82]. Waste polyurethane (WPU) from RWTBs has been investigated by [82]. The study explored the effect of WPU on the rheological properties and compatibility of SBS-modified asphalt, which is typically used in pavement engineering under low-temperature and high-temperature conditions. It also uncovered the adhesive mechanisms between WPU from RWTB and asphalt using molecular dynamics (MD) simulations. The models of virgin asphalt, SBS-modified asphalt, and WPU/SBS-modified asphalt are presented in Figure 14. In these models, the SBS content was set at 3 wt.%, and the WPU content was set at 4 wt.%. The models were constructed using Materials Studio 2020 software. The density values of virgin asphalt, SBS-modified asphalt, and WPU/SBS-modified asphalt are shown in Figure 14. The density of virgin asphalt stabilized at approximately 1.0 g/cm³ after 500 ps, exhibiting a minor deviation of 0.031 g/cm³ from the experimentally measured value of 1.031 g/cm³. Similarly, the densities of SBS-modified asphalt and WPU/SBS-modified asphalt also reached equilibrium, with the SBS-modified asphalt stabilizing in the range of 0.98–1.0 g/cm³, while the WPU/SBS-modified asphalt stabilized between 0.98 and 1.025 g/cm³. This figure also illustrates the microstructural configurations at two density levels, showing that WPU/SBS-modified asphalt has a more uniform and compact microstructure at higher densities. Therefore, the study concluded that the low-temperature crack resistance and overall compatibility of the asphalt were improved due to the significant synergistic effect between WPU and SBS, as well as the interaction between SBS and the asphalt components, which was boosted owing to the incorporation of WPU. Despite these findings, however, studies on asphalt modification remain narrow in focus by only dealing with the performance of the remaining polymer, thereby causing challenges that always exist in chemical interactions between asphalt and epoxy resin.

5.4. Aging and Degradation

Currently, significant efforts have been made to explore the main factors that contribute to the degradation of asphalt pavement, which is a key issue for its long service life. Asphalt aging is influenced by diverse factors, especially varying environments. Thus, it is essential to validate the field performance of fiber-reinforced asphalt mixtures and assess their long-term durability under actual service conditions [83]. Broad exploration has shown that the incorporation of FRP makes asphalt aging specimens less sensitive to low temperatures. However, as the degree of aging increases, the fiber may not provide significant reinforcement or crack resistance, although it remarkably enhances aging resistance [74,84]. The progressive deterioration of asphalt mixtures due to FRP’s adverse impact on the service life of the asphalt pavement is notable. The research conducted in [85] found that reinforcing asphalt mixtures with synthetic fibers resulted in a slight increase in bitumen content compared to unreinforced mixtures. In contrast, fiber-modified asphalt mixtures exhibited superior resistance to cracking when compared to other types of mixtures. Additionally, a performance evaluation of aged samples indicated that the incorporation of synthetic fibers into steel slag-aggregated asphalt mixtures significantly reduced long-term thermal oxidation aging (LTOA). This improved resistance to aging is primarily due to the enhanced elastic properties of the fiber-reinforced composite system. Similarly, in another study [86], RTWBs were crushed until they reached particle sizes between 0.075 mm and 9.5 mm. These were then used as filler and fine aggregate with various incorporations. The results revealed that the asphalt mixture with RWTBs could improve overall road performance, and optimal pavement performance, including low-temperature crack resistance, could be achieved in the face of construction degradation. They indicated that the reinforcement mechanism was significantly attributed to the compatibility between RTWBs and asphalt mixtures. However, the literature mentioned above failed to explore asphalt mixture durability after incorporation with RWTBs. If the large-scale application of RWTBs is implemented in the future, durability is incredibly urgent.

6. Conclusions, Challenges, and Future Perspectives

This review examines potential EoL strategies for managing the rising volume of composite waste generated from retired wind turbine blades in the expanding wind energy industry. Blades consist of reinforced thermoset polymer, predominantly GFRP composites, which pose challenges in recycling. On the other hand, CFRP has developed expectations in specific areas, especially in blades longer than 45 m, resulting in a considerable amount of CFRP produced from EoL blades. This study focuses on how to address the waste management issue associated with EoL blades, exploring the recycling processes available in the literature and indicating gaps that need to be addressed. In addition, it also concentrates on the potential applications of these recycled composites in construction, highlighting their current uses and some unresolved challenges in these areas.

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Current treatment strategies for recycled FRP materials

GFRP is typically recycled through mechanical recycling due to its low cost and environmental friendliness. Other recycling approaches are not cost-effective and, under certain conditions, lead to a reduction in the fibers’ mechanical properties. Chemical recycling, although expensive, shows promise in retrieving high-quality fibers and resin, making it a potentially profitable solution. Moreover, there is a challenge associated with decreasing the cost of chemical recycling to produce clean GFRP. In contrast, the expense associated with carbon fiber reinforcements makes recycling CFRP more cost-effective than producing virgin fibers. Pyrolysis recovers discontinuous, fluffy, unsized fibers with high mechanical properties that are suitable for manufacturing intermediate products. However, further research should be conducted to design more energy-consuming technologies.

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Direct reuse in construction

Retired wind turbine blades can be directly used in architecture to protect public safety. While this method offers significant environmental benefits by diverting blade waste from landfills, there are some limitations to their use due to safety requirements. New laws are required to improve this action process globally.

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Application of RTWBs in the production of cementitious materials

The optimal dosage of recycled WTB micro-fibers and powder should typically be limited to <10 wt.% and <8 wt.%, respectively, to prevent compressive strength reduction and hydration delay. Additionally, the incorporation of recycled glass fibers markedly improves concrete’s tensile strength, as their high elastic modulus (70–90 GPa) enables the formation of an effective reinforcement network within the concrete matrix. This network mechanism bridges microcracks, thereby restricting crack propagation while simultaneously enhancing the composite’s fracture toughness and frost resistance. However, the durability of incorporating FRP into the concrete industry is still a challenge to solve.

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Application of RTWBs in the production of asphalt matrix

Incorporating 2 wt.% GFRP as filler optimally enhances asphalt’s low-temperature crack resistance, with these improved binder properties directly contributing to superior pavement performance across all related mixture formulations. However, mechanical properties such as compressive strength and tensile strength may exhibit slight reductions compared to virgin fiber-reinforced composites. In addition, the addition of GFs from RWTBs alone to warm mix asphalt (WMA) mixtures enhances final loading cycles and improves resistance to rutting. Further research is needed to address trade-offs in mechanical performance and elucidate the underlying reinforcement mechanisms.