Green Recycling and Long-Term Immobilization of Disposable Medical Masks for Enhanced Mechanical Performance of Self-Compacting Recycled Concrete

Abstract

The global outbreak and prolonged presence of Coronavirus Disease 2019 (COVID-19) have resulted in a substantial accumulation of discarded masks, posing serious environmental challenges. This study proposes an eco-friendly and low-carbon strategy to repurpose discarded DMFM fibers as a key component in fiber-reinforced self-compacting recycled aggregate concrete (FRSCRAC). The mechanical and environmental performance of FRSCRAC was systematically evaluated by investigating the effects of recycled coarse aggregate (RCA) replacement ratios (0%, 50%, 100%), discarded DMFM fiber material (DMFM) contents (0%, 0.1%, 0.2%, 0.3%), and fiber lengths (2 cm, 3 cm, 4 cm) on axial compression failure mode and stress–strain behavior. The results demonstrated that DMFM fibers significantly enhanced concrete ductility and peak stress via the fiber-bridging effect. Based on fiber influence, modified stress–strain and shrinkage models for SCRAC were established. To further understand the fiber fixation mechanism, X-ray computed tomography (X-CT) and scanning electron microscopy (SEM) analyses were conducted. The findings revealed a stable random distribution of fibers and strong interfacial bonding between fibers. These improvements contributed to enhanced mechanical performance and the effective immobilization of polypropylene microfibers, preventing further microplastics release into the air. This innovative approach provides a sustainable solution for recycling and effectively immobilizing discarded DMFM fibers in concrete over long curing periods, while also enhancing its properties.

1. Introduction

The COVID-19 pandemic has led to a dramatic increase in the production and disposal of single-use medical masks (DMFMs). Statistics show that during the pandemic, global daily mask consumption reached 3.4 billion, resulting in an annual generation of 150,000 to 390,000 tons of discarded masks [1,2,3,4]. Among these, disposable masks made from polypropylene (PP) have been particularly popular due to their high filtration efficiency, comfort, and ease of use. However, current disposal methods, such as landfilling and incineration, pose significant environmental and public health concerns [5]. These masks often end up in landfills, oceans, and other natural environments, where they may take hundreds of years to degrade, releasing harmful pollutants and microplastics into the ecosystem [6], thereby posing serious risks to both humans and other organisms [7].

The potential recycling methods for disposable masks can be broadly categorized into three types: mechanical methods [1], thermochemical decomposition methods [8,9], and chemical methods [10]. As a thermoplastic polymer, PP masks can be melted and reused, making mechanical recycling the most common approach for recovering thermoplastic plastics. However, the long-term performance of recycled waste remains insufficiently understood, and the potential decline in material quality limits its use and acceptance across various application areas. Thermochemical decomposition methods require high energy input, while chemical methods necessitate careful selection and proper treatment processes; improper handling could release harmful substances into the environment. On the other hand, with the rapid advancement of global urbanization and infrastructure development, the use of concrete materials has become increasingly widespread. However, this growth has also led to a substantial rise in construction and demolition (C&D) waste, which accounts for approximately 30–35% of the total global solid waste [11]. Recycling and efficiently utilizing C&D waste are therefore crucial for the sustainable development of the construction industry. Converting waste concrete into recycled aggregates offers a viable solution by replacing natural aggregates in the production of recycled aggregate concrete (RAC) [12,13,14]. This approach not only addresses the scarcity of natural aggregate resources [15,16] but also protects the ecological environment in aggregate production areas. Furthermore, it reduces environmental pollution caused by open-air stockpiling, landfilling, and the occupation of agricultural land. However, due to the porous nature of recycled aggregates, their use in conventional concrete tends to degrade the material’s performance [17,18,19,20,21], particularly in properties closely related to aggregate water absorption, such as workability. This negative impact becomes more pronounced as the recycled aggregate content increases.

Self-compacting concrete (SCC) is renowned for its high fluidity and resistance to segregation, allowing it to flow into complex formworks without the need for vibration. When recycled coarse aggregates (RCAs) are incorporated into SCC, the resulting self-compacting recycled aggregate concrete (SCRAC) enhances construction performance by compensating for the poor flowability typically caused by the high water absorption of recycled aggregates. In general, RCA replacement has a limited impact on the workability and mechanical properties of SCC; however, the additional water and cementitious materials required for SCRAC exacerbate early shrinkage and cracking, while RCA further aggravates long-term shrinkage deformation in SCC [21,22,23]. To address shrinkage and cracking in SCC, the incorporation of fibers is considered an effective solution [24,25]. Fibers such as carbon and glass fibers [26] or polypropylene fibers [27] can significantly improve the mechanical properties and shrinkage performance of SCRAC when appropriately added. Nonetheless, the high cost of these fibers increases the overall material expense, potentially limiting their application in practical engineering projects. In addition, previous studies have also demonstrated the effectiveness of glass fiber reinforced polymers (GFRPs) in improving the strength and ductility of concrete beams through both experimental and numerical approaches [28].

Since disposable medical masks are primarily composed of PP fiber material, they can be repurposed as a reinforcement material in concrete [29]. Converting waste masks into fibers and incorporating them into SCC not only helps manage the vast amount of discarded masks but also enhances the performance of SCRAC, thus further advancing the development of SCRAC [30,31,32,33]. Saberian et al. [2] modified and compacted a mixture of recycled disposable masks and RCA for use in road subgrade applications and found that adding 1–2% chopped masks increased the strength and stiffness of the mixture while improving its ductility and toughness. Kilmartin Lynch et al. [34] identified 0.2% as the optimal DMFM fiber content for improving both compressive and tensile strength, resulting in a 17.06% increase in compressive strength. Ahmed et al. [35] used DMFMs and basalt fibers to prepare sustainable green concrete, demonstrating good compatibility between DMFMs, basalt fibers, and the concrete matrix. Zhang et al. [36] have conducted in-depth research on the working and mechanical properties of DMFMs and SCRAC doped in the early stage. These findings suggest that adding an appropriate amount of DMFMs can effectively enhance the properties of concrete.

The above review highlights that research on the performance of DMFM fiber-reinforced concrete has already been conducted. However, to achieve the dual goals of green recycling and immobilization of discarded masks, whether DMFM fibers can be effectively immobilized over extended curing periods is the key issue that must be addressed for the application of DMFM fibers in self-compacting recycled concrete.

To provide a systematic overview of the development of recycled concretes,

Table 1. Summary of different types of recycled concretes.

Reference	Research Object	Parameter Selection Range				Research Contents
		RCA	RFA	RP	FC
Arun et al. [16]	Ordinary concrete	0–100%	/	/	/	Workability, Compressive strength, Splitting tensile strength, Flexural strength, Acid resistance
Barhmaiah et al. [17]	Ordinary concrete	0–100%	/	Fly ash 0–35% to replace cement	/	Compressive strength, Flexural strength
Huang et al. [18]	Ordinary concrete	0–100%	/	/	/	Compressive strength, Elastic modulus, Poisson’s ratio, Stress–strain curve, Crack propagation, Failure modes
Grdic et al. [21]	Self-compacting concrete (SCC)	0–100%	/	/	/	Workability, Compressive strength, Splitting tensile strength, Flexural strength, Acid resistance.
Jeevetha et al. [26]	Self-compacting concrete (SCC)	/	/	Micro silica 5%, 10% to replace cement	0.2–0.8%	Compressive strength, Splitting tensile strength, Flexural strength
Altalabani et al. [27]	Self-compacting concrete (SCC)	/	/	/	0.22–0.66%	Compressive strength, Elastic modulus, Splitting tensile strength, Impact resistance, Flexural strength, Toughness index
Ahmed and Lim [35]	Ordinary concrete	50%	/	FA + GGBFS 20% to replace cement	0–0.5%	Compressive strength, Splitting tensile strength, Flexural strength, Density, Ultrasonic pulse velocity (UPV), Water absorption
Zhang et al. [36]	Self-compacting concrete (SCC)	0–100%	/	FA + SF to replace cement	0–0.3%	Workability, Compressive strength, Splitting tensile strength, Flexural strength, Elastic modulus, Pore structure (X-ray CT), Microstructure (SEM/EDS)
This study	Self-compacting concrete (SCC)	0–100%	/	FA + SF 20% to replace cement	0–0.3%	Compressive strength, Stress–strain curve, Failure mode, Shrinkage model, Pore structure (X-CT), Microstructure (SEM/EDS)

summarizes the main categories together with their key properties and applications. Following the approach of Chen et al. [37], Table 1 also aims to present a structured classification framework that not only reflects the progress achieved through experimental investigations but also underscores the potential of data-driven optimization in sustainable concrete design.

Therefore, this study focuses on the mechanical properties of self-compacting recycled concrete incorporating DMFM fibers. Specifically, the research investigates the effect of DMFM fibers on the stress–strain behavior of fiber-reinforced self-compacting recycled aggregate concrete (FRSCRAC) from both macro and micro perspectives. Uniaxial compression tests were conducted to evaluate the influence of the RCA replacement ratio, DMFM fiber content, and fiber size on the failure mode, axial compressive strength, and stress–strain relationship of SCRAC. Furthermore, existing theoretical models were modified to incorporate influence coefficients for these factors, resulting in the development of an empirical stress–strain model for FRSCRAC. This research provides valuable insights into the green and low-carbon recycling of waste masks and supports the potential application of FRSCRAC in future construction projects.

2. Experimental Preparation

2.1. Materials

2.1.1. Recycled Disposable Medical DMFM Fiber

The preparation process of recycled DMFM fibers was divided into three steps, as shown in Figure 1. The first step involves collecting and sterilizing waste masks (Figure 1a). To minimize variability caused by different DMFM types, a single brand of masks was uniformly distributed to laboratory personnel, who were instructed to dispose of them in designated collection boxes. According to established protocols [38,39], the collected masks were first disinfected by immersion in 75% ethanol for 30 min, which effectively inactivates a wide range of bacteria and viruses. Following this, the masks were subjected to ultraviolet germicidal irradiation (UVGI, 254 nm) for a continuous period of 24 h, ensuring a cumulative dose well above the 1 J/cm² threshold that has been demonstrated to achieve at least a 3-log (99.9%) reduction in viral activity. This combined chemical and physical sterilization strategy guarantees complete decontamination and compliance with biosafety requirements. After sterilization, the masks were stored in a clean, ventilated environment for 7 days to further eliminate any potential residual pathogens before subsequent processing. Subsequently, the metal nosepiece, ear straps, and hot melt adhesive were removed to ensure material uniformity, as shown in Figure 1b. The masks were then cut into uniform strips of 5 mm width and lengths of 2, 3, and 4 cm (Figure 1c).

2.1.2. Cementitious Materials and Aggregates

The RCA used in this study was sourced from recycled concrete with an original design strength grade of C30. The particle gradation curves of all aggregates, produced through manual crushing and screening (Figure 2), are presented in Figure 3, and comply with the ASTM C33 standard [40]. The water absorption values are 0.9% for NFA, 1.1% for NCA, and 7.3% for RCA. The loose bulk density is 1500 kg/m³ for NFA, 1450 kg/m³ for NCA, and 1288 kg/m³ for RCA, while the compacted bulk density is 2680 kg/m³, 2690 kg/m³, and 2586 kg/m³ for NFA, NCA, and RCA, respectively.

2.1.3. Specimen Design

In this study, ten concrete mix ratios were designed, as shown in

Table 2. Mix proportions design (units: kg/m³).

Specimen No.	W/B	Water	Additional Water	OPC	FA	SF	NCA	RCA	Sand	PCE (‰)	DMFM Fiber
											(V%)	Length/cm
FRSCRAC0-P0-a	0.43	214	0	321.3	148.3	24.7	800	0	792	0.3	0	/
FRSCRAC50-P0-a	0.43	214	23	321.3	148.3	24.7	400	400	792	0.3	0	/
FRSCRAC100-P0-a	0.43	214	47	321.3	148.3	24.7	0	800	792	0.3	0	/
FRSCRAC0-P0.2-a	0.43	214	0	321.3	148.3	24.7	800	0	792	0.3	0.2	2
FRSCRAC50-P0.2-a	0.43	214	23	321.3	148.3	24.7	400	400	792	0.3	0.2	2
FRSCRAC100-P0.2-a	0.43	214	47	321.3	148.3	24.7	0	800	792	0.3	0.2	2
FRSCRAC50-P0.1-a	0.43	214	23	321.3	148.3	24.7	400	400	792	0.3	0.1	2
FRSCRAC50-P0.3-a	0.43	214	23	321.3	148.3	24.7	400	400	792	0.3	0.3	2
FRSCRAC50-P0.2-b	0.43	214	23	321.3	148.3	24.7	400	400	792	0.3	0.2	3
FRSCRAC50-P0.2-c	0.43	214	23	321.3	148.3	24.7	400	400	792	0.3	0.2	4

FRSCRAC50-P0.2-a, where 50 indicates an RCA replacement ratio of 50%, P0.2 represents a DMFM content of 0.2%, and ‘a’, ‘b’, ‘c’ denote DMFM fiber lengths. ‘W/B’ stands for water to binder ratio, ‘OPC’ stands for ordinary Portland cement, ‘FA’ stands for fly ash, ‘SF’ stands for silica fume; ‘NCA’ and ‘RCA’ refer to natural coarse aggregate and recycled coarse aggregate, respectively.

. The water–binder ratio for all mixtures was fixed at 0.43, with three varying parameters: RCA replacement ratio (0%, 50%, and 100%), DMFM fiber volume content (0%, 0.1%, 0.2%, and 0.3%), and fiber length (2, 3, and 4 cm), as outlined in Table 2. Considering the water absorption of RCA, an additional amount of water was determined based on its measured absorption characteristics. This water was not directly included in the mixing water but was used to pre-saturate the RCA prior to mixing. During the mixing process, the aggregates and cementitious materials were first blended, followed by the addition of water and superplasticizer. The DMFM fibers were gradually dispersed into the mixture together with the aggregates to avoid clumping and to ensure uniform distribution. The mixtures exhibited sufficient workability to satisfy the requirements of SCC. The concrete specimens’ mechanical properties were evaluated after 28 days of standard curing.

2.2. Test Procedures

2.2.1. Mechanical Performance Test

Standard prismatic specimens with dimensions of 150 mm × 150 mm × 300 mm were designed and fabricated for uniaxial compressive performance testing. Each group consisted of three specimens, and the axial compressive strength result was taken as the average of the three. A 5000 kN servo-hydraulic testing machine was applied to the load. The uniaxial compression test setup is shown in Figure 4. To further ensure the reliability of the strain data, two additional electronic extensometers were installed on opposite sides of the specimen to measure the average axial strain.

2.2.2. X-CT, SEM Analysis

The effects of RCA and DMFM fibers on the FRSCRAC microstructure were analyzed using X-ray computed tomography (X-CT) and scanning electron microscopy (SEM). X-CT assessed the internal void system and fiber distribution, while SEM focused on the interfacial transition zones (ITZs) between old and new concrete, as well as the microstructure.

3. Short-Term Performance Results and Analysis

3.1. Fiber Effect on the Axial Compressive Strength

The 28-day axial compressive strength of concrete incorporating DMFM fibers and recycled aggregates is shown in Figure 5. The parameters include the RCA replacement ratio, fiber content, and fiber sizes. The results indicate that increasing the RCA replacement ratio evidently leads to a decrease in the axial compressive strength of SCRAC, both with and without the addition of fibers. In specimens without DMFM fibers, compressive strength dropped by 5.94% and 8.17% at 50% and 100% RCA replacement, respectively, compared to those without RCA. This reduction is more pronounced, with the reductions in the 0.2% DMFM fiber addition case, which can be attributed to the weaker ITZ between the old mortar and the RCA, as opposed to the ITZ in fresh mortar. Additionally, mechanical crushing during RCA production introduces damage, resulting in inferior properties compared to natural aggregates (NA). As for the fiber content effect, adding DMFM fibers generally increases axial compressive strength at the same RCA replacement ratio. With increasing RCA replacement ratios, the three groups of DMFM fiber-reinforced specimens exhibit a noticeable drop in axial compressive strength compared to those without fibers. This difference is attributed to the varying impact of DMFM fibers on compressive strength across different RCA replacement levels. At a fiber content of 0.2%, the compressive strength of specimens with 0%, 50%, and 100% RCA replacement increased by 6.72%, 8.85%, and 0.4%, respectively, compared to those without DMFM fibers. The greatest improvement in compressive strength occurred at a 50% RCA replacement level; however, the variation in DMFM fiber length had a relatively minor effect. A 100% RCA replacement results in a maximum decrease of 14.3% in axial compressive strength, while a 0.3% fiber content leads to a maximum increase of 14.3% compared to specimens without fiber incorporation. The change in DMFM fiber length affects the axial compressive strength by less than 5%.

In a constant RCA replacement ratio, the axial compressive strength increases as DMFM fiber content rises. With fiber dosages of 0.1%, 0.2%, and 0.3%, compressive strength improved by 0.09%, 8.85%, and 14.3%, respectively, compared to specimens without fiber reinforcement. This improvement is attributed to the formation of a three-dimensional fiber network during the compressive failure of the concrete. This network inhibits further crack development through a bridging effect, which transmits and disperses stress within the concrete [39]. Additionally, the fiber bridging effect constrains the lateral expansion of the concrete, thereby improving its overall performance and enhancing its axial compressive strength. In terms of fiber length effect, the compressive strength decreases with growing fiber length. Compared to specimens with two cm fibers, those with three cm and four cm fibers showed a reduction of 5.69% and 4.92%, respectively. This reduction occurs because, with a constant fiber volume, increasing fiber length leads to less uniform fiber distribution in the concrete. Additionally, longer fibers tend to aggregate, weakening their bond with the cement matrix. This reduces their ability to prevent local crack propagation, resulting in a decrease in axial compressive strength.

3.2. Fiber Effect on the Axial Failure Mode

The typical failure modes of the FRSCRAC specimens are shown in Figure 6a. The three specimens (FRSCRAC0-P0-a, FRSCRAC50-P0-a, and FRSCRAC100-P0-a) without DMFM fibers exhibited similar behavior. Initially, no visible damage was observed. As the load approached the peak, multiple small cracks gradually appeared at the edges and corners of the upper and lower ends. Upon reaching the peak load, these small cracks propagated rapidly, forming multiple discontinuous longitudinal cracks, and the number of cracks increased significantly. In the later stage of loading, cracks expanded and extended rapidly. The combined compressive and shear stresses caused the cracks to penetrate and form inclined macroscopic fractures. During failure, large fragments of concrete peeled off, accompanied by the audible sound of fragmentation. When the loading was finally stopped, an inclined main crack running diagonally across the specimen appeared, penetrating the entire specimen and compromising its structural integrity.

For the DMFM fiber groups, no visible damage appeared before the ultimate load; however, after the peak load, the failure modes significantly differed from the non-fiber specimens. As the load approached the peak, no visible cracks were observed at the edges and corners. Upon reaching the peak load, small cracks expanded slowly, forming multiple discontinuous cracks of smaller width. In the later stage of loading, the cracks further spread and concentrated diagonally, forming cracks that penetrated the specimen. However, only minor concrete debris fell, and the sound was low and muffled. There were no obvious brittle failure characteristics, and the overall structure of the specimen remained relatively intact. Moreover, the failure modes of FRSCRAC50-P0.1-a, FRSCRAC50-P0.2-a, and FRSCRAC50-P0.3-a were observed. With higher fiber content, the number and width of cracks near the diagonal main crack decreased. Additionally, varying fiber sizes (FRSCRAC50-P0.2-a, FRSCRAC50-P0.2-b, FRSCRAC50-P0.2-c) showed no significant effect on the failure mode when the fiber content remained constant.

Adding DMFM fibers greatly improves the structural integrity of the specimens and reduces both the amount and size of cracks during failure. This enhancement is attributed to the fiber bridging effect in the concrete, as illustrated in Figure 6b. As the load reaches peak stress, micro-cracks develop due to the stress overload. DMFM fibers strengthen the internal structure by delaying crack initiation through early stage fiber bridging, effectively slowing down crack growth [41].

As the loading process continues, the cracks propagate further, but the high toughness of the DMFM fiber allows them to effectively absorb and dissipate stress. The bond between the fibers and the cement matrix provides additional interfacial strength, reducing stress concentration within the concrete, thereby improving the crack resistance of the concrete and reducing the width and area of the crack [28]. Additionally, during crack propagation, some DMFM fibers are either pulled out or fractured, absorbing part of the fracture energy and further slowing the crack’s growth; therefore, incorporating DMFM fibers boosts the concrete’s stiffness and effectively prevents crack formation.

3.3. Axial Compressive Stress–Strain Results

Figure 7 presents the stress–strain curves of each group of specimens after averaging, and Figure 8 shows the dimensionless stress–strain curves after normalization based on the averaged data. In these figures, σ_cp represents the stress corresponding to the peak load of the specimen and ε_cp denotes the peak strain at the peak stress. As shown in the figure, all specimens exhibit a similar pattern: the initial phase of the curve corresponds to the linear elastic stage, where stress increases linearly with strain. In the mid-term loading phase, due to the propagation of micro-cracks within the concrete, the curve becomes markedly nonlinear. Upon reaching peak stress, the curve transitions into a descending phase, during which cracks rapidly propagate and coalesce within the specimen. With further strain, stress decreases sharply and eventually stabilizes, reflecting the typical characteristics of a brittle fracture. As observed in Figure 8, the curves of the specimens exhibit minimal variation in the rising segment but display significant differences in the descending segment.

To further clarify the role of different parameters, Figure 9 illustrates the normalized stress–strain curves classified by the RCA replacement ratio, fiber content, and fiber length. Figure 9 illustrates the changes in the stress–strain results of test specimens under different RCA replacement ratios, fiber content, and fiber size. From Figure 9a, it is evident that, compared to specimen FRSCRAC0-P0-a, the curvature of the stress–strain curves for specimens FRSCRAC50-P0-a and FRSCRAC100-P0-a decreases as the RCA replacement ratio increases, resulting in steeper curves. Significant differences can be observed in the descending segments of the three curves. This behavior is due to the old mortar attached to the RCA surface. After surpassing the peak load, this old mortar may fracture in certain areas, making the concrete more brittle during failure. In Figure 9b, the stress–strain results of the 0.2% DMFM fiber addition specimen exhibits smoother descending segments compared to the fiber-free specimens in Figure 9a. This is because when concrete begins to crack, the bridging effect of DMFM fibers slows the rapid propagation of cracks, thus delaying the failure process and improving ductility. The descending segments of the FRSCRAC50-P0.2-a and FRSCRAC100-P0.2-a specimens (1 ≤ ε_c/ε_cp < 1.5) nearly overlap in the early stage, suggesting that their failure modes are similar post-crack formation. In the middle section (1.5 ≤ ε_c/ε_cp < 2), the stress in the FRSCRAC50-P0.2-a specimen is slightly higher than in the other groups, and in the later section (ε_c/ε_cp >2), it is significantly higher. This may be due to the stronger interfacial bonding between DMFM fibers and the matrix at lower RCA replacement ratios, which enhances crack control.

From Figure 9c, it is observed that with a constant RCA replacement ratio and fiber size, increasing the fiber content from 0% to 0.3% flattens the descending segment of the curve. Higher DMFM fiber content more effectively controls crack propagation and reduces the brittleness of concrete. Compared to specimen FRSCRAC50-P0-a, 0.1% and 0.2% fiber addition specimens’ results show minimal differences in the initial descent phase, but in the later phase (ε_c/ε_cp > 1.5), the differences become more pronounced. The specimen with 0.3% fiber content demonstrates superior crack resistance and ductility throughout the entire descent phase (ε_c/ε_cp > 1), indicating better overall performance at failure. In Figure 9d, it is evident that when the RCA replacement ratio and fiber content remain constant, fiber size has a notable impact on the descending segment of the stress–strain curve. The initial sections of the curves for the two cm (size a) and four cm (size c) fibers show little difference, while the curve for the three cm (size b) fiber is significantly higher. In the later descent stage, the curve for the two cm fiber surpasses those of the other two sizes. This indicates that the three cm fiber length may form a more efficient stress transmission path during the early stages of crack development, delaying crack propagation. However, in the latter part of the descending segment (ε_c/ε_cp > 2), as fibers break or are pulled out, the shorter two cm fibers result in a more gradual descent. The shorter fibers may not fully engage in the early stages of crack propagation, but their presence in the later stages enhances the specimen’s ductility.

3.4. Stress–Strain Modeling

The normalized theoretical formula for the stress–strain curves of FRSCRAC can be utilized in the engineering design and analysis of FRSCRAC structures. In accordance with the empirical model outlined in the Chinese standard GB/T 50010 [42], a piecewise function is employed to characterize the stress–strain relationship, incorporating shape coefficients for both the ascending and descending segments of the curve, the results are shown in Figure 10. The function is expressed as follows:

$${\displaystyle \frac{\sigma }{f_c}}=\left\{\begin{array}{ll}{\displaystyle \frac{nx}{(n-1+x^n)}}& x\le 1\\ {\displaystyle \frac{x}{a_c(x-1{)}^2+x}}& x>1\end{array}\right.$$

(1)

$$n={\displaystyle \frac{E_{\mathrm{c}}·{\epsilon }_{\mathrm{cp}}}{E_{\mathrm{c}}·{\epsilon }_{\mathrm{cp}}-E_{\sec }·{\epsilon }_{\mathrm{cp}}}}={\displaystyle \frac{1}{1-E_{\sec }/E_{\mathrm{c}}}}$$

(2)

$$a_c=0.157f_{\mathrm{c}}^{0.785}-0.925$$

(3)

Among them, n and ac represent the shape coefficients corresponding to the ascending and descending branches of the curve, respectively, where x denotes the normalized strain ε_c/ε_cp, ε_cp is the strain at peak stress. E_c and E_sec represent the elastic modulus and secant modulus of the concrete, respectively. The elastic modulus is the slope of the straight line from the origin point to the peak point, while the secant modulus is the slope of the straight line from the origin to 40% of the peak stress [41]. Additionally, fc denotes the 28 d compressive strength of the concrete.

It is evident that the Chinese standard model effectively describes the relationship between ε_c/ε_cp and σ_c/σ_cp in the ascending section of the curve. However, there is a significant discrepancy between the empirical model and the experimental results in the descending section. The model fails to accurately capture the changes between ε_c/ε_cp and σ_c/σ_cp after the specimen surpasses peak load. Consequently, shape parameters n and α_c for the ascending and descending sections of the Chinese standard model were recalibrated by fitting them to the experimental data. The modified model, as shown in Figure 10, demonstrates that the theoretical values align well with the experimental results, providing an accurate description of the stress–strain relationship for FRSCRAC specimens throughout the entire compression process.

Regression analysis was employed to derive the expressions of the modified shape coefficients. Previous analysis demonstrated that the Chinese standard model provided good predictive accuracy for the shape coefficient n in the rising segment. Therefore, based on Formula (2), the form and parameters of the function to be fitted were determined. Regarding the shape coefficient ac for the falling segment, it was found that the addition of RCA and fibers significantly influenced the curve’s descending behavior. Consequently, both RCA and fiber content were considered as influencing parameters for the shape coefficient in the falling segment, and a polynomial function was chosen for the fitting process. The final expressions for calculating the shape coefficients in the modified model are provided in Formulas (4) and (5). The fitting results indicate that the modified model achieves good accuracy for the shape coefficients in both the rising and falling segments. The R2 values for the rising and falling segments are 0.8935 and 0.9522, respectively.

$$n={\displaystyle \frac{0.0097}{0.00799-0.0073\cdot E_{\sec }/E_{\mathrm{c}}}}$$

(4)

$$a_{\mathrm{c}}=15.94r^2-107.5r{f}_{\mathrm{b}}+3.994r+21.07f_{\mathrm{b}}+7.465$$

(5)

where n—ascending branch shape coefficient;

• a_c—descending branch shape coefficient;
• E_sec—secant modulus;
• E_c—elastic modulus;
• r—RCA replacement ratio (%);
• f_b—DMFM fiber volume fraction (%).

In the formula, r represents the RCA replacement ratio (0–100%), f_b represents the DMFM fiber content (0–0.3%), and Figure 11b illustrates the fitting result for the shape coefficient ac in the descending segment. A larger value of the shape coefficient ac corresponds to a steeper descending segment of the stress–strain curve, indicating a reduction in concrete ductility [43]. The contour map on the right shows the variation in the shape factor ac with changes in the RCA replacement ratio and DMFM fiber content. It was observed that the growing RCA replacement ratio correlates with a higher value of ac, while growing fiber content leads to a decrease in ac. This further indicates that the incorporation of DMFM fibers can enhance the ductility of concrete and suppress the increased brittleness of SCRAC caused by RCA substitution.

4. Microstructure Analysis and Fiber Immobilization Evaluation

X-ray computed tomography (CT) was employed to evaluate the morphological characteristics of discarded DMFM fiber material (DMFM) and the evolution of porosity in fiber-reinforced self-compacting recycled aggregate concrete (FRSCRAC) at different curing stages. Cross-sectional and 3D images of the specimens were obtained, allowing fiber identification based on differences in gray values and physical morphology. Previous studies have extensively examined the effects of fiber incorporation on pore distribution, demonstrating significant improvements in concrete pore structure. Notably, a fiber content of 0.2% has been reported to yield the most favorable pore structure [44]. To assess the overall porosity variation over time, measurements were conducted at 28 and 180 days of curing. Given the specimen dimensions (two cm cubes) and CT resolution, porosity values in Figure 12 were calculated based on a pore resolution threshold of 40 μm. The results indicate a consistent decrease in porosity with extended curing, with 180-day specimens exhibiting lower porosity than their 28-day counterparts. This reduction is attributed to the continued growth of hydration products over time. Additionally, the findings confirm that DMFM fibers remain effectively embedded in the concrete matrix without increasing porosity over time, indirectly demonstrating the strong fixation capacity of concrete for DMFM fibers.

On the other hand, X-ray CT analyses were utilized to reconstruct the interaction between DMFM fibers and the concrete matrix from both three-dimensional and two-dimensional perspectives. As shown in Figure 13, these images illustrate the fiber distribution and orientation in FRSCRAC50-P0.2-a and FRSCRAC50-P0.2-b, as well as the three-dimensional reconstruction of fiber distribution obtained by X-CT. The results reveal that DMFM fibers are randomly oriented and irregularly curved within the SCRAC matrix. Some fibers are embedded at the interfaces between recycled aggregates, while others are wrapped around the aggregates. Importantly, no fiber clustering or agglomeration was observed, indicating that the selected fiber size was appropriate for this study. Although the fiber distribution exhibited a certain degree of randomness, it remained relatively uniform throughout the concrete matrix. This uniform dispersion contributed to enhanced structural integrity, improving the ductility and reinforcing the overall mechanical performance of the concrete. These findings further confirm that DMFM fibers not only strengthen FRSCRAC but also effectively reduce its shrinkage rate, contributing to long-term stability and durability.

SEM was utilized to analyze the microstructure of FRSCRAC, with a focus on the interfaces between the aggregates, cement slurry, and fibers. The ITZ between the cement mortar and the RCA is referred to as the “new ITZ,” while the ITZ between the old RCA mortar and the original aggregate is termed the “old ITZ.” It was observed that the old RCA mortar contained numerous pores and cracks, which negatively impacted the axial compressive strength and shrinkage performance of FRSCRAC (as shown in Figure 14).

SEM images in Figure 15 illustrate the microstructure of DMFM fibers at different magnifications and curing stages. At 25× magnification, the elongated DMFM fibers are clearly visible, confirming the random fiber distribution observed in the CT scans at various curing times. The characteristic three-layer structure of the DMFM fibers is evident, consisting of an outer non-woven fabric, a PP fiber melt-blown fabric layer, and internally distributed PP microfibers. These microfibers were effectively embedded in the surrounding cement paste, and as hydration progressed, the reduction in porosity further reinforced the fixation of DMFM fibers within the concrete matrix. At higher magnification, closer examination of the fiber-mortar interface revealed small black spots-fiber fluff-scattered at the fiber ends. These fluffs were tightly bonded to the cement mortar, indicating a high level of compatibility between DMFM fibers and FRSCRAC. Additionally, SEM cross-sectional images demonstrated that the layered DMFM fiber structure became infiltrated by cement mortar during mixing, enhancing the fiber-matrix bond and contributing to improved structural integrity. Notably, a comparison between the 28-day and 180-day curing periods showed no significant change in the surrounding microfiber quantity, further verifying the strong fixation of DMFM fibers within the concrete. The stability of both the PP fiber fabric and non-woven fabric layers over time also confirms their long-term integration within the matrix. Previous studies [6] have suggested that medical masks may release microplastics into the environment, posing a potential threat to soil ecosystems. A single unused mask can release approximately 500,000 plastic particles, while UV-weathered masks may release three times as many particles. By incorporating DMFM fibers into concrete, the progressive reduction in porosity over time effectively immobilizes the fibers, mitigating the risk of microplastic release. This multi-phase structure differs from conventional fiber-cement interactions, as DMFM fibers create a bridging effect within the cement slurry, reducing crack formation, enhancing mechanical properties, and improving shrinkage performance.

Although the SEM images used in this study are limited in resolution, they are sufficient to identify the fiber–mortar interfacial characteristics and bonding condition of DMFM fibers, which, together with the complementary X-CT analyses, provide a reliable basis for the microstructural interpretation.

5. Conclusions

This study proposes an eco-friendly approach for recycling discarded masks by incorporating them as reinforcement materials into recycled self-compacting concrete (SCRAC). This method achieves the dual objectives of green recycling of waste masks and enhancing the performance of recycled concrete. We investigated the effects of various parameters, such as DMFM fiber replacement ratios, on the mechanical properties of concrete. The stability of the interaction between self-compacting concrete and waste masks over time was validated through porosity analysis and microscopic SEM tests.

The incorporation of DMFM fibers significantly enhances the crack resistance and fracture toughness of SCRAC. As the fiber content increases, the number and width of cracks around the main cracks are further reduced. Additionally, incorporating DMFM fibers significantly improves compressive strength, with a maximum increase of 14.3%. Using X-ray CT scanning and SEM analysis, the random distribution of DMFM fibers in concrete and their positive effect on improving the pore structure were confirmed. The tight interfacial bonding between the masks and the concrete was shown to remain stable over a long period, demonstrating the durable interaction between the waste DMFM fibers and the concrete. This validates the effectiveness of DMFM fibers in enhancing the performance of concrete and their stable immobilization within the concrete matrix during the studied curing periods. The ability to immobilize microplastics shows no significant change over time, indicating the effectiveness of the concrete in securely locking waste masks and preventing further degradation in the environment.

Nevertheless, future research should further evaluate the economic feasibility of this approach, particularly regarding the costs of collecting, sterilizing, and processing discarded masks compared with conventional fiber materials. Additionally, a more comprehensive sustainability assessment, including life cycle analysis (LCA) and carbon footprint estimation, is necessary to quantitatively confirm the environmental benefits. These aspects will be crucial to determine whether this method can be realistically applied on a large scale in the construction industry.

These aspects will be crucial to determine whether this method can be realistically applied on a large scale in the construction industry. In its current form, DMFM fiber-reinforced SCRAC is particularly suitable for non-load-bearing or secondary structural applications, such as partition walls, pavements, sub-bases, and precast blocks, where enhanced crack resistance and durability are required. This not only ensures practical feasibility but also highlights the potential for sustainable recycling in civil engineering practice.