Waste Carbon Fiber Reinforced Plastics Recycling and Reutilization

Version	Summary	Created by	Modification	Content Size	Created at	Operation
1		piyu Chen	--	8324	2023-09-25 09:55:36	\|
2	format change	Peter Tang	-3439 word(s)	4885	2023-09-25 10:21:33	\| \|
3	revised entry description	Peter Tang	-146 word(s)	4739	2023-10-04 01:48:54	\|

This entry is adapted from the peer-reviewed paper 10.3390/polym15173508

The extensive use of carbon fiber-reinforced plastics (CFRP) in aerospace, civil engineering, and other fields has resulted in a significant amount of waste, leading to serious environmental issues. Finding appropriate methods for recycling CFRP waste and effectively reusing recycled carbon fibers (rCFs) has become a challenging task.

carbon fiber reinforced plastics (CFRP) waste recycling methods reuse of recycled carbon fibers (rCFs) recycled carbon fiber reinforced plastics (rCFRP) recycled carbon fiber reinforced cementitious (rCFRC) composites

1. Introduction

Carbon fiber reinforced plastics (CFRP) is a composite engineering material produced through carbon fibers as reinforcements and resin as the matrix. CFRP exhibits numerous advantages, including high strength, high stiffness, low density, and corrosion resistance ^[1]^[2]. Due to these properties, CFRP finds extensive applications in industries such as aerospace, civil engineering, wind power generation, sports and leisure, rail transportation, and pressure vessels. Since the beginning of the 21st century, there has been a rapid increase in global demand for carbon fibers. According to statistics, the demand has grown from 43,500 tons in 2012 to 82,400 tons in 2017 and reached 112,000 tons in 2020 ^[3]. In China, the demand for carbon fibers has maintained an annual compound growth rate of over 15% in recent years, reaching 33,000 tons in 2020. As a product of carbon fibers, the global demand for CFRP reach 199,000 tons in 2022 ^[4]. The demand for carbon fibers and CFRP is illustrated in Figure 1.

Figure 1. (a) Demand for carbon fiber ^[3]; (b) Demand for CFRP ^[4].

However, the extensive consumption of CFRP and the expectation of continuous consumption growth have resulted in an enormous accumulation of CFRP waste. Pimenta and Pinho ^[5] revealed that by 2020, retired CFRP products would reach approximately 26,000 tons, with manufacturing process waste amounting to 36,000 tons. Carberry ^[6] predicted a surge in the retirement of commercial carbon fiber aircraft by 2025, with an estimated quantity of 8500 aircraft, providing over 12,000 tons of recyclable CFRP. By 2050, the global CFRP waste is projected to reach a staggering 500,000 tons ^[7]. The resin in CFRP forms a three-dimensional cross-linked network structure, rendering it insoluble and non-melting, while carbon fiber exhibits inert characteristics. Consequently, CFRP is highly resistant to natural degradation. Traditional disposal methods primarily involve landfilling and incineration. However, over a hundred years, CFRP degradation in landfills would only account for 1% of its original mass, and it would take 60,000 years to degrade by 26% ^[5]. Incinerating one ton of CFRP would produce 2011 kg of CO₂ ^[8]. Therefore, adopting the conventional strategies of landfilling and incineration not only consumes vast valuable land resources but also results in a CO₂ production of over 200,000 tons in 2022, causing significant environmental pollution and conflicting with the long-term vision of carbon emissions neutrality. Typically, carbon fibers in waste maintain their mechanical properties to a large extent before recycling, and the use of appropriate recycling methods can achieve performance similar to virgin carbon fibers (vCFs). In 2020, the total CFRP waste reached 62,000 tons ^[5]. Approximately 600 kg of carbon fiber can be recovered from one ton of CFRP waste. Based on this calculation, approximately 37,200 tons of carbon fiber could be recovered in 2020, representing around 33.2% of the carbon fiber demand for that year. Therefore, CFRP waste holds tremendous potential for utilization.

Faced with the serious issue of CFRP waste and recognizing its significant potential for utilization, governments worldwide, particularly those of developed nations, have successively implemented regulations and policies to guide and encourage proper waste management by businesses. In 1999, the European Union (EU) introduced a policy ^[9] that unequivocally assigned responsibility to CFRP manufacturers for handling their products and strictly limited the amount of CFRP waste sent to landfills. Furthermore, tax policies were devised to incentivize businesses to engage in CFRP composite material recycling ^[10]. Since the beginning of the 21st century, policies about CFRP waste management have further tightened. Both the EU Management Committee and the United States Environmental Protection Agency (EPA) began prohibiting the landfill disposal of CFRP materials in 2004, while the UK initiated the imposition of a landfill tax on CFRP waste in 2014. In recent years, China has experienced a sharp increase in the manufacturing and use of carbon fiber-reinforced composite materials, leading to a pronounced issue of CFRP waste. Consequently, the country has released a plethora of targeted policies and regulations. The Ministry of Industry and Information Technology has mandated the promotion of CFRP recycling and reuse ^[11]^[12]^[13], while ministries such as the State Council and the National Development and Reform Commission (NDRC) have called for active endeavors in recycling and utilizing novel types of waste, including carbon fiber composites, along with the establishment of application demonstrations to advance the development of the resource recycling industry system ^[14]^[15]^[16].

The issue of CFRP waste has been escalating, while it also holds tremendous value. Coupled with the pressure from policies, the recycling of CFRP waste has become an urgent matter. Researchers have conducted studies focusing on the performance of carbon fiber recycling in terms of energy consumption, environmental impact, and costs. Carbon fibers are manufactured from polyacrylonitrile through a series of high-temperature production processes, with an energy consumption of approximately 183–594 MJ/kg ^[17]^[18]. By combining a life cycle assessment model with experimental results, the energy consumption in the carbon fiber recycling process has been evaluated. Mechanical recycling methods have an energy consumption of about 0.27–2.03 MJ/kg ^[19], thermal decomposition recycling methods have an energy consumption of about 3–30 MJ/kg ^[20]^[21]^[22], and chemical solvent degradation methods have an energy consumption of about 19.2–91 MJ/kg ^[23]^[24]^[25]. The energy consumption in the carbon fiber recycling process does not exceed 20% of the total energy required for vCFs manufacturing, making the utilization of rCFs to replace vCFs an effective way to reduce energy consumption. Furthermore, Akbar et al. ^[26], through a life cycle impact assessment, compared the environmental impact of the production processes between vCFs and rCFs. They found that the environmental impact of rCFs on non-renewable energy, greenhouse gases, ozone layer depletion, and aquatic acidification is only 4%, 12%, 24%, and 12% of that caused by vCFs, respectively. In comparison to vCFs, rCFs have minimal harmful effects on the environment ^[27]. Additionally, the production cost of carbon fibers is approximately USD 33 per kilogram ^[6]^[28], and the recycling cost is usually around 60–70% of the production cost of vCFs ^[5], mainly attributed to solvent, electricity, and equipment depreciation expenses. ELG Carbon Fibre Ltd. has achieved a 40% reduction in carbon fiber recycling costs compared to the production cost of vCFs ^[28]. When the production capacity reaches 100,000 kg per year, the carbon fiber recycling cost further decreases to only USD 15 per kilogram. The rCFs hold significant advantages over vCFs in terms of energy consumption, environmental impact, and cost. Therefore, the recycling and reuse of CFRP waste are feasible and practical solutions.

2. Research Status and Analysis of Recycling of Carbon Fiber Reinforced Plastics (CFRP) Waste

Substantial utilization of carbon fiber and its composites began in the 1960s, primarily in the military sector, gradually extending to civilian applications during the 1970s. As the service life of CFRP is typically 15–20 years, the issue of CFRP waste emerged in the 1990s, prompting research into carbon fiber recycling. Researchers attempted mechanical recycling by collecting and crushing CFRP waste, using the resulting particles as fillers. This method is referred to as mechanical recycling. Subsequently, thermal decomposition recycling, represented by fluidized bed technology, emerged, achieving higher levels of carbon fiber and resin separation. With a deeper understanding of epoxy resin properties and technological advancements, high-temperature and high-pressure super/subcritical conditions, along with various chemical solvents, were applied to carbon fiber recycling, developing into the chemical solvent degradation recycling method. Today, a diverse range of recycling techniques has been developed, significantly improving recycling efficiency. These recycling methods can still be roughly categorized into three main types: mechanical recycling, thermal decomposition recycling, and chemical solvent degradation recycling, based on their characteristics. Their key technical indicators are presented in Table 1, which will be elaborated upon in this chapter.

Table 1. The key technical indicators of the different recycling methods.

	Recycling Method	Mechanical Recycling Method ^[8]^[29]^[30]	Thermal Decomposition Recycling Method		Chemical Solvent Degradation Method
	Recycling Method	Crush	Fluidized Bed ^[31]^[32]^[33]	Pyrolysis ^[34]^[35]^[36]^[37]^[38]^[39]	Super/Subcritical ^[40]^[41]^[42]^[43]^[44]^[45]	Atmospheric Pressure Solvent ^[46]^[47]^[48]^[49]^[50]	Electrochemical ^[3]^[51]	Organobase/Organosolvent ^[52]^[53]^[54]^[55]
Recycling conditions	Temperature (°C)	Room temperature	450–500	400–700	250–450	90–350	23–75	130–200
	Pressure (MPa)	Atmospheric pressure	0.1–0.25	Atmospheric pressure	5–35	Atmospheric pressure	Atmospheric pressure	Atmospheric pressure
	Toxicity	None	None	None	Yes	Yes	None	Yes
Recycling effect	Tensile strength (%)	50–65	50–75	50–85	85–98	85–98	−90	/
	Interfacial shear strength (%)	/	−80	/	88.6–99	/	−120	/
	Degradation rate (%)	/	/	−92.4	79.3–98.6	90–99	99–99.9	95–99.7
	rCFs size (mm)	<10	10–50	−500	10–50	10–50	−200	/
	Resin products	Resin dust	Carbon–oxygen and carbon–hydrogen gases	Carbon–oxygen and carbon–hydrogen gases	Small molecule compounds	Small molecule compounds	Small molecule compounds	Oligomers
Environmental impact	Environmental impact	Dust	CO₂, dust, and heat	CO₂, flotsam, and heat	Solvents such as alcohols, acids and bases, and heat	Solvents such as alcohols, acids, and bases	Trace amounts of Cl₂ and H₂	Organic bases and organic solvents

Note: “/” means that the item has not been reported in the literature.

3. Research and Analysis of Recycled Carbon Fiber Reuse

The research on the reutilization of rCFs began relatively late, starting in the first decade of the 21st century. Researchers in the materials-related field collected prepreg or discarded CFRP particles from the production process to use as reinforcing phases in the manufacturing of composites, but these composites often exhibited poor mechanical properties. With the development of recycling techniques, researchers gained access to rCFs obtained through pyrolysis or solution degradation methods. These rCFs had low resin content and strong adhesion to the resin, resulting in significantly improved mechanical properties of CFRP produced using them. Currently, using rCFs to manufacture resin-based composites remains mainstream, while some researchers have begun exploring the application of rCFs in the production of carbon fiber felts. In recent years, researchers in the civil engineering field have also started using rCFs to reinforce cementitious composites, studying the mechanical properties, electrical conductivity, and environmental impact of the composite materials. The rCFs primarily come from collected waste carbon fibers and mechanically recycled CFRP particles. This research provides an overview of the reutilization of rCFs in the field of fiber composites and cementitious composites.

3.1. Research on the Reuse of rCFs in Fiber Composites

Researchers have utilized collected carbon fiber waste, prepreg scraps, or rCFs to re-manufacture CFRP. The mechanical properties, such as tensile strength, flexural strength, and impact strength, of the re-manufactured CFRP have been investigated to explore the utilization of rCFs. Regarding the manufacturing of CFRP using waste carbon fibers and prepreg scraps, Aravindan et al. ^[56] employed high-performance discontinuous fiber technology to re-manufacture waste fiber bundles into highly aligned discontinuous fiber prepreg tapes, which were then used to produce unidirectional laminates. Compared to virgin carbon fiber reinforced plastics (vCFRP), the recycled carbon fiber reinforced plastics (rCFRP) showed reductions in tensile strength, stiffness, and failure strain by 85.9%, 72.4%, and 47.4%, respectively. Souza et al. ^[57] used uncured prepreg to produce different sizes and shapes of laminates. They found that the tensile strength, flexural strength, and compressive strength of rCFRP decreased by 13%, 56%, and 23% compared to vCFRP. Mohama et al. ^[58] utilized waste prepreg particle-reinforced polypropylene, where fully cured prepreg was first crushed into 5–10 mm fine particles, and then composite materials were prepared using 3–5% by mass of these particles. However, the adhesion between prepreg and polypropylene was poor, resulting in composite materials with tensile strength equivalent to pure PP. As seen from the results, due to discontinuity, waste carbon fibers, and CFRP often struggle to fully harness the mechanical advantages of carbon fibers when manufacturing composites. Additionally, the poor bonding between the cured resin on the surface of waste carbon fibers/CFRP and the new resin further contributes to the significant reduction in the mechanical properties of rCFRP.

Regarding the preparation of CFRP using rCFs, Schwarz et al. ^[34] conducted a 2 h pyrolysis of prepreg in a nitrogen atmosphere at 480–560 °C, resulting in a resin content of 22.6% in the rCFs at 560 °C. SEM analysis revealed an abundance of resin particles on the surface of the rCFs. The tensile strength and elastic modulus of rCFRP were both below 50% of vCFRP. Giorgini et al. ^[35] treated prepreg and CFRP waste in a nitrogen atmosphere at temperatures ranging from 450 to 600 °C but achieved poor resin removal. Short rCFs and vCFs were used to manufacture new composite materials, but the resin failed to fully impregnate the rCFs. Tensile tests indicated delamination between the rCFs and the resin, with rCFRP exhibiting fracture stress, Young’s modulus, and elongation at fracture of 33.3–81.6%, 68.4–94.7%, and 41.1–91.6% of vCFRP, respectively. Oliveux et al. ^[59] used solvent-recovered carbon fiber bundles and vCFs bundles to prepare carbon fiber sheets. Residue on the surface of the rCFs hindered the bonding between the carbon fibers and the resin, and the low alignment of the rCFs resulted in voids and resin-rich regions. The tensile strength and modulus of rCFRP were approximately 63.1–72.4% and 85–137.5% of vCFRP, respectively.

Regardless of pyrolysis recycling or solvent degradation, if the surface of the rCFs still contains a significant amount of resin, it will affect the bonding between the rCFs and the resin, ultimately leading to a decrease in mechanical properties such as tensile strength and flexural strength, as confirmed by previous studies ^[60]^[61]. When the resin on the surface of the rCFs is completely removed, and the surface becomes hydrophilic, it can enhance the bonding between the rCFs and the resin. Deng et al. ^[48] obtained rCFs with very little resin on the surface and increased contact angle under subcritical conditions. During the manufacture of CFRP, they found that the vCFs and the resin interface tend to form resin-rich regions and voids, resulting in stress concentration. Due to the good wetting properties between the rCFs and the resin, a stronger interface was formed, leading to an increase of 102.4% in the flexural strength of rCFRP compared to vCFRP. Furthermore, the dispersion of rCFs can also have adverse effects during the production of CFRP. Guo et al. ^[62]^[63] prepared CFRP using 10 mm long rCFs and vCFs separately and found that the clustered structure of rCF resulted in lower dispersion compared to vCF, leading to brittle fracture behavior in rCFRP and a decrease of 88.14% in flexural strength compared to vCFRP.

Additionally, the utilization of rCFs has been extended in recent research. Hu et al. ^[64] further ultrasonically dispersed the resin-free carbon fibers obtained from solvent recycling and then utilized a papermaking method to produce recycled carbon fiber felts decorated with cationic polyacrylamide (rCFF/CPAM). The rCFs exhibit conductivity and good dispersibility, resulting in rCFF/CPAM demonstrating significant conductivity of 140.06 S/m, highly efficient EMI shielding effectiveness of 66.15 dB, and a special SEA/SET ratio of 83.8% at 0.875 mm. Moreover, the recycled carbon fiber felt possesses advantages such as lightweight, flexibility, environmental friendliness, and cost-effectiveness.

In conclusion, in the field of fiber-reinforced composites, researchers have utilized rCFs for manufacturing CFRP and carbon fiber felts. Due to the discontinuity of the rCFs and their high resin content, rCFRP exhibits a significant decrease in mechanical properties such as tensile strength, flexural strength, and impact strength. In contrast, the recycled carbon fiber felt demonstrates excellent conductivity and efficient shielding effectiveness. The application of rCFs in the fiber-reinforced composites field is a promising utilization method that allows for the “rebirth” of discarded CFRP, enabling it to continue fulfilling its intended function.

3.2. Research on the Reuse of rCFs in Cementitious Composites

In recent years, rCFs have been utilized by some researchers in the field of cementitious composites. They have been employed to enhance the mechanical properties and conductivity of cementitious composites. Additionally, an assessment of the environmental impact of incorporating rCFs in cementitious composites has been conducted.

3.2.1. Research on Carbon Fiber Reinforced Cementitious (CFRC) Composites

Cementitious materials, composed of cement, aggregates, mineral admixtures, and water ^[65], possess advantages such as strong adhesion, good plasticity, high compressive strength, and excellent durability, making them fundamental materials in civil engineering. However, as brittle materials, cement-based materials have low flexural and tensile properties, making them prone to cracking. Moreover, conventional cementitious composites exhibit a resistivity range of 104 to 107 Ω m, placing them between insulators and semi-conductors, with nearly no conductivity under fully dried conditions ^[66]. By incorporating high-strength carbon fibers, which possess excellent thermal and electrical conductivity, into cementitious materials, it is possible to enhance their compressive ^[67]^[68]^[69], flexural ^[70]^[71]^[72], tensile ^[73]^[74], and crack resistance ^[75]^[76], as well as their fatigue properties ^[77]^[78]. Additionally, carbon fibers act as conductive elements, significantly reducing the resistivity of the composites. This expansion of conductivity ^[79]^[80], thermal conduction ^[81]^[82], sensor ^[83]^[84], and electromagnetic shielding ^[85]^[86] attributes in cementitious composites open up new possibilities for various functional applications.

However, the vCFs are inert materials, with a hydrophobic surface and low density, making it difficult to disperse them within the cement matrix ^[87]^[88]. Additionally, the weak chemical reactivity and smooth surface of the vCFs result in poor adhesion with cement hydration products, thereby reducing the interfacial bond strength with the cement matrix ^[89]^[90]^[91]. Consequently, carbon fibers tend to agglomerate in the cement matrix, leading to reduced flowability and workability of CFRC composites ^[80]^[92]. Moreover, the agglomeration introduces more air voids, causing the formation of pores in CFRC ^[93] and further weakening the bond between carbon fibers and the cement matrix. As a result, the enhancement of mechanical and electrical properties of CFRC is diminished ^[94]^[95]^[96], especially compressive strength ^[97]^[98]^[99]. This is because the clustering of carbon fibers results in the generation of numerous cracks and voids in the nearby matrix ^[87]^[100], and insufficient contact between clustered carbon fibers and the matrix leads to the failure of load transfer between them, impeding the crack-inhibiting effect of carbon fibers. In contrast, rCFs are often subjected to oxidation, resulting in a hydrophilic surface with abundant oxygen-containing functional groups, effectively improving their dispersion in the cement matrix ^[101]. Moreover, the wider and deeper grooves on the surface of rCFs provide effective nucleation sites for nearby hydration products, enhancing the mechanical anchoring effect between rCFs and the cement matrix ^[101]^[102].

Additionally, while carbon fibers can significantly enhance the mechanical and electrical properties of cementitious composites, their usage requires consideration of cost issues. Taking the application of carbon fibers in concrete as an example, the price of carbon fibers is approximately USD 33/kg ^[6]^[28], while the cost of C40 concrete in the Shenzhen area is about 700 CNY/m³. A total of 1 m³ of concrete contains approximately 500 kg of cement, and if the carbon fiber dosage is calculated as 1% of the cement mass, 1 m³ of concrete would require 5 kg of carbon fibers. The cost of carbon fibers would be approximately 1155 CNY, which already far exceeds the cost of the concrete itself. This significantly increases the cost of carbon fiber-reinforced concrete, hindering the application of carbon fibers in the field of civil engineering.

3.2.2. Research on Recycled Carbon Fiber Reinforced Cementitious (rCFRC) Composites

Faced with the drawbacks of vCFs, such as low surface chemical activity, hydrophobicity, and smoothness, leading to difficulties in dispersion within the cementitious matrix and weak bonding with cement, some researchers have attempted to utilize rCFs to reinforce cementitious composites. Currently, researchers have utilized collected CFRP waste or mechanical rCFs, further crushing them before adding them to cementitious composites. The focus of the research has mainly been on the mechanical properties of rCFRC composites, with only a limited amount of attention given to the composites’ electrical conductivity and environmental impact.

1. Research on mechanical properties

Currently, research on the mechanical properties of CFRC mainly includes compressive strength, flexural strength, and tensile splitting strength. Rangelov et al. ^[103] used CFRP particles to improve the properties of pervious concrete. It was found that the porosity of the composite material could be reduced, leading to an increase in permeability. Additionally, the compressive strength, tensile splitting strength, and elastic modulus increased by 4-11%, 11-46%, and 6-45%, respectively. Xiong et al. ^[104] investigated the influence of 0-1.5% CFRP sheets and rubber on the mechanical properties of concrete. They found that CFRP sheets reduced the slump of concrete but slightly increased the compressive strength (up to 5.5%) and flexural strength (up to 10%). Moreover, they significantly improved ductility, flexural toughness, and impact resistance. Mastali et al. ^[105] used 10-30mm length and 0.5-2% content of CFRP sheets to reinforce self-compacting concrete, resulting in a 50% increase in maximum compressive strength and a 60% increase in maximum flexural strength. However, the slump decreased by 15%, significantly reducing the workability of the concrete. SEM images indicated that the failure of the recycled CFRP sheets occurred mainly due to debonding. The reduced workability of the composite material was primarily attributed to the non-continuous state of the added CFRP waste, and its surface lacked chemical activity, preventing a strong bond with the cementitious slurry, thus impeding the flowability of the composite slurry.

The epoxy resin residue on the surface of CFRP waste is an organic substance that can weaken the interfacial bond between carbon fibers and the cementitious matrix, thus adversely affecting the mechanical properties of CFRC ^[106]^[107]^[108]^[109]. Therefore, researchers have attempted to remove the resin from the surface of CFRP waste. Wang et al. ^[101] treated waste CFRP particles with a NaOH solution and used them to reinforce cement mortar. They found that a 1 mol/L NaOH solution could partially remove the epoxy resin residue on the surface of rCFs, making the surface of the waste carbon fiber particles rougher and allowing for better bonding with hydration products. Compared to untreated CFRP particles, the compressive strength of cement mortar increased by approximately 6%. This indicates that removing the cured resin from the surface of CFRP waste is an effective method to enhance its bond with the cementitious matrix. Li et al. ^[110] removed the remaining resin and carbonaceous impurities on the surface of rCFs obtained through thermal decomposition using an electrochemical anodic oxidation method. This process enhanced the bond between rCFs and the fly ash-activated composite matrix, resulting in a 185% increase in single-fiber interfacial shear strength and a 25% and 19% increase in compressive and flexural strength of the fly ash-activated composite, respectively. Although waste carbon fibers' surfaces do not contain cured resin, surface sizing agents still negatively influence the interfacial bond strength ^[111]. Therefore, rCFs can effectively improve the macroscopic mechanical properties of cementitious composites, and rCFs without sizing agents or resin impurities on their surface exhibit better-reinforcing effects.

2. Research on electrical conductivity

Some researchers have investigated the influence of waste carbon fibers on the electrical conductivity of cementitious composites. Faneca et al. ^[112] reinforced high-strength concrete with waste carbon fiber bundles and CFRP sheets, and the results indicated that both carbon fibers and CFRP sheets reduced the workability of concrete and introduced more porosity. When the carbon fiber bundle content was in the range of 0.2% to 0.8%, the resistivity values ranged from 3 Ω.m to 0.6 Ω.m, and the electrical conductivity showed no significant difference compared to carbon fiber-reinforced concrete reported in other literature. Overall, the bundle-shaped carbon fibers exhibited slightly better enhancement of concrete's electrical conductivity than CFRP sheets. Belli et al. ^[113] used waste carbon fibers to enhance cement mortar, and the results showed that when the carbon fiber content was in the range of 0.1% to 0.2%, the resistivity of vCFRC decreased from 5491 Ω.m to 2070 Ω.m. Under the same conditions, the resistivity of rCFRC decreased from 1392 Ω.m to 355 Ω.m, indicating that waste carbon fibers were more effective in enhancing the mortar's electrical conductivity compared to vCFs. Therefore, rCFs not only significantly improve the electrical conductivity of cementitious composites but also outperform vCFs in this regard.

3. Research on environmental impact

Furthermore, the environmental impact of using waste carbon fiber to reinforce cementitious composites has been investigated by researchers through lifecycle assessment studies. It was found that using CFRP sheets to enhance concrete can effectively reduce CO₂ emissions ^[104]. Vitale et al. ^[114] utilized prepreg waste material to reinforce cementitious materials and observed not only an improvement in the mechanical performance of the composite but also reductions of approximately 12%, 11%, and 11% in carbon emissions, fossil energy consumption, and inorganic respiratory emissions, respectively. Akbar et al. ^[115], through a lifecycle assessment, suggested that by incorporating 1% rCFs while substituting 10% of cement with silica fume in cementitious composites, the overall global warming potential index for CO₂ emissions decreased by 13.69% compared to ordinary cementitious mortars. Additionally, replacing vCFs with rCFs resulted in energy and cost savings of 22% and 70%, respectively. Therefore, the use of rCFs to reinforce cementitious composites can reduce their environmental impact and lead to cost savings.

As reviewed, in the field of cementitious composites, researchers have utilized rCFs to reinforce cementitious composites and investigated the influence of different dosages and lengths on the mechanical and electrical properties of CFRC. The incorporation of rCFs decreases the workability of CFRC but effectively enhances its compressive, flexural, and tensile strengths, as well as its electrical conductivity. Overall, rCFRC slightly outperforms vCFRC, demonstrating the feasibility of using rCFs as a substitute for vCFs. Moreover, rCFs exhibit significant advantages over vCFs in terms of carbon emissions and other environmental impacts, energy consumption, and cost.

4. Conclusions and Prospects

Recycling and reusing CFRP waste are critical for the sustainable development of various industries, such as aerospace and civil engineering, as it has caused severe environmental issues. Over the past three decades, recycling CFRP waste has evolved into three main technological systems: mechanical recycling, thermal decomposition recycling, and chemical solvent degradation recycling. These methods effectively separate carbon fibers from the resins, mitigating environmental pollution. However, challenges remain in the recovery of intact carbon fibers and the recycling and utilization of resin degradation products. Currently, the reuse of rCFs is in its early stages, and more in-depth research is needed. The rCFs used often have higher resin content, weaker hydrophilicity on the surface, smaller dimensions, and a scattered morphology. Furthermore, the focus of research mainly concentrates on the macroscopic mechanical properties, with limited exploration of the types of rCFs reuse. Therefore, apart from advancing research in both recycling techniques and rCFs reuse individually, there is a need to strengthen collaborative research between these two areas. By exploring a closed-loop cycle for CFRP waste recycling and rCFs reuse, the fundamental problem of CFRP waste can be effectively addressed. In the field of fiber composites, there is a demand for large-scale resin-free rCFs to produce integrated high-performance CFRP products and enhance their application value. Similarly, the civil engineering domain requires functional and cost-effective rCFs. Research should focus on improving the mechanical properties of rCFs through recycling techniques while imparting more functionality to them. Simultaneously, investigating the reinforced mechanisms of rCFs in fiber composites and cementitious composites will provide valuable feedback to the recycling techniques. Ultimately, the joint development of recycling CFRP waste and reusing rCFs will contribute to solving the CFRP waste problem. Therefore, the following aspects warrant particular attention in future research:

(1) The efficiency of heat and solvent transfer is hindered by the dense structure of the resin, resulting in low resin degradation efficiency. Suitable auxiliary conditions can be investigated to facilitate rapid heat and solvent transfer into the resin. For instance, pre-treatment techniques such as microwave or resin expansion to enhance porosity, as well as highly penetrative reactive solvents, can be explored.

(2) Mechanism of performance evolution in rCFs. Previous studies have indicated that high temperatures, high pressures, corrosive chemical solvents, and electric currents can lead to the deterioration of rCFs performance. However, the mechanisms underlying the deterioration of rCFs remain unclear. Investigating the performance evolution mechanism of rCFs and using it to guide the design of recycling techniques can contribute to reducing or even avoiding the deterioration of rCFs' performance.

(3) Recovery and degradation mechanism of resin degradation products. At present, the majority of resin degradation products are by-products of rCFs, mainly consisting of various gases and small-molecule compounds, which have virtually no commercial value and require disposal as waste. Moreover, due to the complexity of small-molecule compounds or secondary products in resin degradation, the accurate analysis of resin cleavage sites becomes challenging, making it difficult to research the degradation mechanism of resins. High-molecular-weight resin degradation products, on the other hand, retain the main molecular backbone, enabling a more precise analysis of the resin degradation mechanism. Therefore, a comprehensive approach that considers both carbon fibers and resin recovery is needed. By focusing on the properties of the resin, the corresponding solvents can be designed, and the reaction temperature can be reduced to facilitate the degradation of the resin into high-molecular-weight oligomers, thereby laying the foundation for the recycling and degradation mechanism research of the resin.

(4) Carbon fiber lap splicing and alignment techniques. The small and disordered dimensions of rCFs result in reduced size and increased porosity of rCFRP, leading to diminished mechanical performance and limited practical value. Elongating and aligning the overlapping of rCFs can be beneficial in addressing these issues.

(5) Interface bonding mechanism between rCFs and cementitious matrix. Macroscopic mechanical tests such as flexural strength and tensile strength demonstrate the strong bonding performance of rCFs. However, the underlying mechanism responsible for the improvement in bonding performance remains unclear and requires quantitative analysis at the microscopic level of individual carbon fiber filaments.

(6) Expanding the utilization types of rCFs to enhance the functionality of composites. Current research has predominantly focused on mechanical and electrical properties. However, there is potential to explore functional composites, such as carbon fiber felts with electromagnetic shielding capabilities or thermal insulation boards with heat conduction properties.

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