Research on Recycling and Utilization of Shredded Waste Mask Fibers to Prepare Sustainable Engineered Cementitious Composites

Abstract

The widespread disposal of single-use masks has led to significant environmental concerns. This study investigated the effects of incorporating shredded waste mask fibers (SWMFs) on the compressive and flexural properties of concrete. The experimental design included four fiber volume fractions, i.e., 0%, 1%, 2%, and 3%, with three different sizes of mask fibers. The influences of these fibers on the load-bearing capacity, deformation behavior, and energy absorption of concrete under compression and flexure was examined. Scanning electron microscopy (SEM) was used to analyze the microstructure. The results show that the addition of 1% SWMFs enhances the mechanical performance, with the compressive and flexural strengths of 20.69 MPa and 6.95 MPa, respectively, for B-sized fibers. Furthermore, the incorporation of discarded mask fibers improved the toughness of the material. In the design with general strength requirements, a B-dimensional SWMFs of 1% volume can be incorporated, which can improve the bending toughness by 75% for the control group.

1. Introduction

In recent years, various viruses, including COVID-19, Ebola, and monkeypox, have emerged worldwide. Global development has faced significant obstacles due to the spread of these viruses, particularly presenting serious challenges to public health and economic growth [1]. To better prevent and control the transmission of the virus, the World Health Organization (WHO) strongly recommends wearing masks in crowded places. This measure not only protects healthy individuals but also helps slow the spread of the virus from those who are infected [2]. Research indicates that using face masks in social interactions can decrease virus transmission by more than 70% [3]. During the COVID-19 pandemic, the demand for such protective measures increased, prompting most countries and regions to mandate disposable masks in public spaces [4]. Even after the pandemic, mask wearing has become part of daily routines for many people, potentially minimizing the spread of other contagious diseases [5]. According to related reports, global mask consumption is projected to continue rising, with the market value expected to reach USD 8.88 billion by 2026 [6]. Considering the growing global population, which currently stands at 8 billion people, alongside the widespread transmission of viruses, the monthly consumption of masks is estimated to reach USD 129 billion [7]. However, the widespread use of disposable masks has led to improper disposal, with many finding their way into rivers, oceans, and other bodies of water, increasing plastic waste [8,9]. These microplastics infiltrate ecosystems through the water cycle and can lead to the death of animals such as birds, ultimately posing a serious threat to human health and ecological security [10].

Disposable masks generally feature a three-layer design comprising outer and inner layers of polypropylene or polyester nonwoven fabric. The most distinctive component is the middle filtering layer, which is composed of melt-blown ultrafine polypropylene fibers [11]. Polypropylene nonwoven fabric is a synthetic material known for its light weight, flexibility, and waterproof properties. Additionally, the cost-effectiveness and ease of use of polypropylene make disposable masks a widely adopted option [12]. In China, a variety of methods are used to handle waste masks, including landfilling, incineration for energy, mechanical processing, and chemical recycling [9]. Each method has advantages and disadvantages, necessitating the selection of the most appropriate disposal technique based on practical circumstances and environmental requirements. For example, burning masks for energy can release harmful gases, such as dioxins and furans, polluting the air and harming human living environments, which contradicts the principles of sustainable development [13,14]. Since polypropylene constitutes the main component of masks, a minimum of 25 years are needed for decomposition in landfills. The extensive presence of mask waste has led to increased marine pollution, endangering marine organisms and compromising ecosystem sustainability [15,16,17]. Given that masks are composed of polypropylene, which has excellent mechanical properties, they are suitable for recycling and reuse. The addition of polypropylene fibers to strengthen the mechanical properties of concrete has excellent potential in civil engineering [18]. The incorporation of disposable medical masks as reinforcing fibers not only improves the structural characteristics of concrete but also helps reduce the pandemic’s environmental impact. Research recommends adding shredded waste masks to recycled concrete aggregates as fillers in base or subbase layers, significantly improving the compressive strength, stiffness, plasticity, and flexibility of the composite materials [19]. One study [20] reported that concrete masonry is one of the most important building components used to absorb and recycle medical waste worldwide.

The incorporation of fibers is a globally recognized technique that effectively decreases the brittleness of concrete materials while improving their toughness. The integration of diverse fiber types and sizes effectively suppresses the formation and growth of cracks at various structural levels in concrete, thereby significantly improving its durability and resistance to fracture [21]. Polypropylene is the main component of disposable masks and features low density, a low melting point, and moderate stiffness and strength. This resin is among the most widely used materials and is also incorporated into some construction applications [22]. Koniorczyk et al. [23] proposed a recycling method in which used masks are subjected to heating at 190 °C and 130 bar, ensuring complete virus inactivation and conversion into uniform polypropylene chains. This suggests that the incorporation of processed mask waste, which contains polypropylene, into concrete is a viable approach. Additionally, Saberian et al. [24] examined the performance of pavement base layers with the addition of shredded mask waste. Their research revealed that the inclusion of varying amounts of recycled aggregate masks can improve the strength, stiffness, ductility, and flexibility of concrete. Recent research by Kilmartin-Lynch et al. [12] examined the potential for incorporating masks into concrete. The results revealed that the addition of a small percentage of masks increased both the compressive strength and tensile strength. Another study on masks in mortar indicated improvements in the mechanical properties and thermal resistance [25]. One study [26] mentioned the possibility of using face masks in concrete instead of PP fibers.

In the study by Idrees et al. [27], waste masks were processed through fiberization and square fragmentation. The concrete demonstrated enhanced compressive and tensile strengths with the addition of 1% waste mask fibers, but the strength decreased when the fiber content exceeded 1.5%. In the study by Koniorczyk et al. [23], the mask waste was transformed into 5 mm fibers under high temperature and pressure, which were then incorporated into the concrete mixture. After 100 freeze–thaw cycles, the compressive strength of the concrete slightly increased compared with that of the reference mixture, resulting in a 5% increase at 28 days. Ali et al. [28] determined that the fiber content of mask waste should remain below 1%, as this prevents entanglement and clustering, addressing uniformity concerns. As a result, the concrete demonstrated the increased tensile strength and crack resistance, which reduced the infiltration of harmful substances and prolonged its lifespan. Alrshoudi et al. [29] reported that concrete samples incorporating waste mask fibers presented a drying shrinkage rate that was 29.5% lower than that of standard mixes, effectively mitigating shrinkage-related cracking. The bridging effect of waste polypropylene fibers resulted in a considerable capacity to withstand impact loads, thus decreasing the brittleness of the concrete and preventing abrupt failure of the prefilled aggregate samples. Idress et al. [27] reported that crushed waste masks mixed with a 1% concrete volume presented low chloride permeability; that is, the crushed mask had high corrosion resistance. Moreover, the 0.5% crushed waste mask had better waterproof performance. Therefore, the durability of SWMFs can be improved.

In summary, our research provides a way to solve the problem of disposing of waste masks. The existing studies have focused mainly on mixing masks into concrete in the form of flocculent fibers, but few studies have focused on mixing recycled waste masks into concrete after shredding. In this study, the mechanical properties of the slurry were enhanced by using recycled mask waste cut into fragments as substitute fibers. The experiments were designed to use fresh water as mixing water, as well as four volume mixtures of 0%, 1%, 2%, and 3%, and three sizes of mask fibers. The effects of different sizes of waste mask fibers on concrete are discussed. Compressive tests, flexural tests, and SEM analysis were conducted to investigate the failure modes of the cement-based materials, assessing their compressive and flexural strengths, load-bearing capacity, deformation capability, and energy absorption potential. Through compressive and flexural tests, along with scanning electron microscopy (SEM) analysis, this study investigated the behavior of cement-based materials during compressive and flexural failures. It assesses compressive and flexural performances, with a specific focus on load-bearing capacity, deformability, and energy absorption capability. Additionally, research has examined how the microstructure and surface morphology of materials influence their mechanical properties. The use of shredded waste mask fibers in concrete is highly important to environmental protection and sustainable development of the construction industry while improving flexural performance.

2. Materials and Methods

2.1. Raw Material

The cement used in this experiment is Sea Conch-brand ordinary Portland cement manufactured in Yingde City, which is in Guangdong Province, China. This cement conforms to the Chinese national standard GB 175-2007 [30]. The chemical composition of the cement was determined via X-ray fluorescence (XRF) spectrometry, specifically the S4 PIONEER model manufactured by Bruker AXS in Germany. The detailed data can be found in

Table 1. Chemical contents of the cement.

%	SiO2	Al2O3	CaO	K2O	Fe2O3	MgO	SO3
Cement	20.31	5.62	61.78	1.55	3.54	2.11	2.47

. A D8 high-resolution X-ray diffractometer produced by Bruker in Germany was used to analyze the crystal phase in the cement, revealing the presence of alite (C₃S) and belite (C₂S). The particle size distribution of the cement was measured using a TRI-LASER particle size analyzer, and the results indicated that the particle size was predominantly 30 μm. Figure 1 shows the dimensions and parameters of the three types of shredded waste mask fibers (SWMFs), with the tensile strength obtained by averaging five samples measured in a tensile testing machine. Pure water was used for the experiment, which was in accordance with the Chinese standard JGJ 63-2006 [31].

2.2. Mixture Proportions and Specimen Preparation

The mix proportions used in this study are presented in

Table 2. Proportion composition of each group of samples.

	Mixture No.	w/b	Cement	SWMF (Vol%)
CG		0.4	1	0
A	A-1	0.4	1	1
	A-2	0.4	1	2
	A-3	0.4	1	3
B	B-1	0.4	1	1
	B-2	0.4	1	2
	B-3	0.4	1	3
C	C-1	0.4	1	1
	C-2	0.4	1	2
	C-3	0.4	1	3

. To investigate the effects of SWMFs, a control with 0% SWMFs was used. The number of samples in the control group (CG) is indicated. All the groups used purified water with a water-to-cement ratio of 0.4 and ordinary Portland cement. A related study [32] revealed that engineered cementitious composites (ECCs) have better mechanical properties when the fiber content is 2%. This experiment uses shredded waste mask fibers, with a 2% volume content. The influences of the 1% and 3% dosages were subsequently studied. Therefore, the selection of numbers 1, 2, and 3 indicates that the dosages are 1%, 2%, and 3%, respectively. Research [27] shows that there are various ways to incorporate waste mask fibers into concrete, but few studies have focused on ways to shred waste mask fibers; thus, three sizes, i.e., A, B, and C, are assumed to be common shredded sizes. Three different sizes of SWMFs were selected, denoted as A, B, and C in the specimen numbering.

The specimen preparation followed the Chinese standard GB/T 17671-1999 [33] with the dimensions of 40 mm × 40 mm × 160 mm. After casting, the samples were demolded after one day of rest. They were then placed in a constant-temperature and -humidity curing chamber, which was maintained at 20 ± 2 °C and over 95% humidity. Upon reaching 28 days of curing, the samples were removed for subsequent testing.

2.3. Experimental Methods and Analysis

2.3.1. Flexural and Compressive Tests

The tests for compressive and flexural strengths were conducted by following the Chinese standard GB/T 17671-1999 [33]. After 28 days of curing, the samples were removed and tested using a constant-loading cement flexural and compressive testing apparatus (model YZH-300.10) manufactured by Luda Machinery Co., Ltd (Ruian, China). The flexural test was performed first, with the loading rate set at 50 N/s. Following the flexural test, the fractured samples underwent compressive testing at a loading rate of 2.5 kN/s.

2.3.2. Mechanical Property Analysis

In accordance with the reference standard ASTM C1018-97 [34], the stress–strain curve’s initial point of deviation from linearity, where concrete first exhibits cracking, is referred to as the limit of proportionality (LOP). Concurrently, the point at which the maximum stress is attained subsequent to initial cracking is referred to as the modulus of rupture (MOR). In this study, the descriptions of LOP and MOR from reference [32] were adopted, and these parameters were used as key indicators to assess the compressive and flexural properties. These parameters were used to investigate the material’s mechanical behaviors at both the onset of initial cracking and the point of maximum stress, with a focus on factors such as strength (load-bearing capacity), ductility (deformability), and toughness (energy absorption capability). Moreover, LOP and MOR indicate the interactions between the fibers and the cement matrix during flexural tests. As shown in Figure 2a, the compressive stress–strain curves for both the CG and SWMFG groups exhibit similar patterns. Before reaching the LOP, the curves exhibit linear trends. However, between the LOP and MOR, the slopes of the curves gradually decrease. At the MOR, the curves reach horizontal slopes, and beyond this point, the curves continue to decline. As observed in Figure 2b, the flexural load–deflection curves for both the CG and SWMFG groups show a linear progression before reaching the LOP. However, after the LOP, the curve typically exhibits one of three behaviors. Ordinary cement materials are classified as brittle, fracturing directly upon reaching the LOP. In contrast, fiber-reinforced ECCs retain their load-bearing capability after fracture and exhibit softening or hardening behavior during bending. This is reflected in the curve where the load at the MOR may be either lower or higher than the load at the LOP. After reaching the MOR, the load tends to decrease. This behavior is attributed to the fibers in the ECCs, which increase the toughness of the matrix and allow the sample to retain some load-bearing capability post fracture.

To better characterize the toughness after LOP, this study references the d5, d10, and d20 points used in reference [35]. However, the results obtained from testing SWMFG in this study show improved ε_LOP and δ_LOP values compared with those reported in the literature. Therefore, the characteristic point lengths were adjusted, and d3, d5, and d10 were selected for representation, as shown in Figure 2a,b. Here, d3 corresponds to the point on the curve where the stress/deflection is twice the value at the LOP, whereas d5 and d10 represent the points corresponding to 3 times and 5.5 times the length, respectively.

To analyze the results of the flexural tests, a detailed assessment was conducted on five critical characteristic points (LOP, MOR, d3, d5, and d10) for all the test samples. This evaluation included the determination and analysis of parameters such as load, strength, deflection, and toughness. The analysis approach from the literature [32] was referenced, and specific symbols were adopted to represent the parameters corresponding to each characteristic point: the load is denoted by P, the strength by f, the deflection by δ, and the toughness by T. For example, the load, strength, deflection, and toughness at the MOR point are represented as P_MOR, f_MOR, δ_MOR, and T_MOR, respectively. The load P and deflection δ are derived directly from the flexural load–deflection curve. The stress of bending f is determined via the load using Equation (1). The toughness of bending T is determined by calculating the area under the flexural load–deflection curve from the origin O to the MOR point. In this study, f, δ, and T serve as indicators of the material’s flexural load-bearing capacity, ductility (plastic deformation ability under bending), and energy absorption capability under bending.

$$f=1.5{\displaystyle \frac{PS}{b{h}^2}}$$

(1)

2.3.3. Scanning Electron Microscopy

After a 28-day curing period, each set of samples was removed. To inhibit further hydration, the samples were subsequently immersed in anhydrous ethanol. Following this treatment, the samples were transferred to a drying oven maintained at 60 °C until they reached a constant weight. The microstructural features of the samples were examined via a Quanta 250 FEG environmental scanning electron microscope manufactured by FEI (Hillsboro, OR, USA). To increase the conductivity and obtain clearer, higher-quality images, the samples were gold-coated prior to the SEM analysis.

3. Results and Discussion

3.1. Mechanical Properties Under Compression

3.1.1. Compressive Stress–Strain Curves

Figure 3a–c show the compressive stress–strain curves for Groups A, B, and C, respectively. Both the compression strength and modulus of elasticity for Groups A and B decrease as the SWMF content increases. In contrast, Group C exhibited trends in which the compressive strength and elastic modulus initially increased before subsequently declining as the fiber content increased. However, due to certain discrepancies in strain and toughness, direct conclusions cannot be drawn from Figure 3. Additionally, Figure 3 shows that both the compression strength and modulus of elasticity in the CG are greater than those in the SWMFG. Notably, after sample failure, the stress of CG decreases rapidly to very low levels, whereas that of SWMFG gradually decreases, with its stress eventually surpassing that of CG. This suggests that while CG may absorb more energy before failure (reaching the MOR), SWMFG demonstrates a superior capacity for energy absorption after failure. To enhance the mechanical performance analysis during compression, key parameters derived from the compressive stress–strain curves of CG and SWMFG are summarized in

Table 3. Compressive characteristic parameters of CG and SWMFG.

		Unit	CG	A1	A2	A3	B1	B2	B3	C1	C2	C3
LOP	PLOP	kN	52.66	26.05	20.24	16.68	28.30	19.82	15.75	21.43	23.72	13.83
	σLOP	MPa	32.91	16.28	12.65	10.42	17.69	12.39	9.84	13.16	14.82	8.64
	εLOP	%	1.52	0.96	0.84	0.86	1.10	1.02	0.77	0.85	0.94	0.74
	TLOP	MPa	0.25	0.08	0.05	0.04	0.10	0.07	0.04	0.06	0.07	0.03
MOR	PMOR	kN	57.85	31.05	24.01	19.29	33.10	22.60	18.01	23.56	26.01	15.77
	σMOR	MPa	36.15	19.4	15.00	12.06	20.69	14.13	11.25	14.73	16.26	9.86
	εMOR	%	1.83	1.35	1.24	1.18	1.56	1.47	1.09	1.27	1.21	0.99
	TMOR	MPa	0.36	0.15	0.10	0.08	0.19	0.13	0.07	0.12	0.12	0.05
d3	Td3	MPa	0.66	0.25	0.17	0.14	0.31	0.22	0.12	0.18	0.21	0.10
d5	Td5	MPa	0.77	0.41	0.27	0.23	0.49	0.35	0.20	0.30	0.33	0.16
d10	Td10	MPa	0.84	0.68	0.48	0.42	0.76	0.59	0.37	0.53	0.58	0.30

. Each value is calculated as the average of six samples.

3.1.2. Compressive Strengths (Load-Bearing Capacity)

Figure 4 shows the compressive strengths of CG and SWMFG, with specific data available in Table 3. This section primarily discusses two strengths measured at LOP and MOR. Figure 4 shows that the σ_LOP and σ_MOR values for all SWMFGs are lower than those for the CG. This indicates that the inclusion of SWMFs negatively affects the compressive strength, potentially even leading to a reduction in strength. This phenomenon may be explained by related studies indicating that the use of fibers typically results in increased porosity [32].

With the increasing SWMFs, decreasing trends in the σ_LOP and σ_MOR values are observed for both Groups A and B. These trends are consistent with the findings by Al Swalqah et al. [36], who reported an inverse relationship between the strength of the mixture and the quantity of shredded masks added. In particular, the compressive strength decreases as the number of masks increases. Compared with the change in σ_MOR, that of Group A3 was 37.83% lower than that of Group A1, and that of Group B3 was 45.63% lower than that of Group B1. Similarly, reference [27] reported that the compressive strength typically decreases as the amount of added fibers increases. This phenomenon is attributed to the air trapped during the mechanical mixing of concrete, where an increase in fiber content leads to greater air content [37,38]. For Group C, σ_LOP and σ_MOR initially increase and then decrease with the increasing waste mask fiber content, and the σ_MOR of Group C3 decreases by 39.36% compared with that of Group C2. Reference [27] mentioned that the compressive strength diminishes after the fiber content reaches a certain level, as low fiber content does not lead to issues with fiber entanglement, clumping, or nonuniformity [39,40]. At lower fiber volumes, mixing occurs properly, minimizing air entrapment and clumping and thereby increasing strength. Throughout the 28-day curing process, B1 consistently maintains the highest σ_MOR among all SWMFGs, reaching 20.69 MPa, which is 1.29 MPa greater than that of A1. Compared with the compressive strength of 0.5% crushed mask fibers in related studies [27], the degree of reduction is 20%. This suggests that with the addition of SWMFs, the intensity reduction varies between groups and is not as large as that reported in existing studies.

3.1.3. Compressive Strains (Deformability)

Figure 5 shows the compressive strains of CG and SWMFG, with specific data available in Table 3. This section focuses on two types of strains measured at the LOP and MOR. Figure 5 clearly shows that the strains of the CG samples consistently exceeds that of the SWMFG samples, with Table 3 showing an ε_LOP of 1.52% and an ε_MOR of 1.83% for CG. In comparison, as the content of waste mask fibers increases in the other groups, the strain variation is minimal for Group A, whereas Groups B and C exhibit greater fluctuations. Compared with CG, SWMFG demonstrates a slight increase in deformability from ε_LOP to ε_MOR. This may be due to a more stable bond between the fibers and the matrix, resulting in a gradual decrease in compressive deformation. Compared with the other groups, Group B1 has higher ε_LOP and ε_MOR values, reaching 1.10% and 1.56%, respectively. This finding indicates that the degree of reduction in compressive strain varies among the groups with the addition of shredded waste mask fibers, among which Group B decreases the least, i.e., ε_LOP decreases by 27.63% and ε_MOR decreases by 14.75%.

3.1.4. Compressive Toughness (Energy Absorption Capability)

Figure 6 shows the compressive toughness of CG and SWMFG, with detailed data available in Table 3. This section focuses on five types of toughness measured at LOP, MOR, d3, ≥d5 and d10. Figure 6 shows that for SWMFG incorporating waste mask fibers of dimensions A and B, the toughness tends to decrease with the increasing fiber content. In contrast, the toughness of SWMFG with dimension C initially increases but then decreases as the fiber content increases. For T_d3, T_d5, and T_d10, B1 consistently maintains the highest values across all the groups, reaching 0.76 MPa, indicating that B1 retains considerable toughness even post-failure. However, both the toughness prior to failure (T_LOP and T_MOR) and the post-failure toughness (T_d3, T_d5 and T_d10) are significantly lower than those observed in the CG. Compared with Figure 5, the T_LOP, T_MOR, T_d3, T_d5, and T_d10 values of CG and SWMFG show diverging trends. The T_LOP value of SWMFG before failure is similar to that of CG, whereas the energies absorbed by SWMFG in T_d3, T_d5, and T_d10 after failure are notably greater than those of CG, indicating that the fibers significantly influence the toughness following crack propagation. The compressive toughness of Group B1 shows the largest improvement, being 285.7% greater than that of the CG.

3.2. Mechanical Properties in Flexure

3.2.1. Flexural Load–Deflection Curves

Figure 7a–c show the load–deflection curves during the flexural testing of each group. It is evident from all the graphs that as the SWMF content increases, the flexural strength decreases, whereas the deflection and toughness show discrepancies that cannot be directly inferred from Figure 7. After the matrix fails, the CG loses its load-bearing capacity immediately, whereas in SWMFG, the fibers carry the load independently until all fibers are either pulled apart or extracted. Therefore, before the samples fail (they reach the MOR), SWMFG typically absorbs more energy, and even after fracture, it continues to absorb energy, although the amount decreases as the fiber content increases. To improve the mechanical performance analysis during bending, key parameters derived from the flexural load–deflection curves of CG and SWMFG are summarized in

Table 4. Flexural performance parameters.

	Parameters	Unit	CG	A1	A2	A3	B1	B2	B3	C1	C2	C3
LOP	PLOP	kN	3.3	2.8	2.43	1.99	2.86	2.53	1.86	2.38	2.77	1.76
	fLOP	MPa	7.51	6.35	5.44	4.51	6.95	5.92	4.2	5.83	6.49	4.41
	δLOP	mm	0.29	0.29	0.27	0.30	0.27	0.26	0.24	0.35	0.22	0.2
	TLOP	N·m	0.44	0.39	0.32	0.30	0.39	0.34	0.22	0.36	0.3	0.19
d3	Td3	N·m	0.44	0.51	0.54	0.66	0.49	0.48	0.47	0.71	0.55	0.39
d5	Td5	N·m	0.44	0.62	0.74	0.98	0.58	0.64	0.71	0.98	0.77	0.51
d10	Td10	N·m	0.44	0.86	1.15	1.63	0.77	0.97	1.29	1.60	1.27	0.89

. The data of the MOR points are not used in the table because there is no tendency to peak again in the load–deflection curve after flexural fracture; that is, the MOR points do not appear. Therefore, LOP points can be considered MOR points. Each value was calculated as the average of three samples.

3.2.2. Flexural Strength (Load-Bearing Capacity)

Figure 8 presents the flexural strengths of CG and SWMFG, with specific data available in Table 4. This section primarily discusses the strength at LOP. Owing to the control group breaking directly and the absence of a subsequent rising phase in the load–deflection curve for SWMFG, only LOP is observed. Figure 8 shows that f_LOP decreases for all groups with the incorporation of SWMFs. However, for Group C, there was an initial increase in response to the increasing content of SWMFs, followed by a subsequent decrease. This may be due to the use of crushed masks rather than fibers, which could reduce strength. Notably, B1 has a greater f_LOP (=6.95 MPa) than the other groups, and at a fiber content of 3%, the flexural strengths of all the groups fall within a similar range. In related studies [27], the flexural strength of 0.5% crushed mask fibers was approximately 2.75 MPa, and the bending strength of B1 increased by 150%. The peak flexural strength of the blended fibers in the comparative study [18] is 4.44 MPa, and the flexural strength of B1 increases by 56.53%.

3.2.3. Flexural Deflections (Deformability)

Figure 9 presents the flexural deflections of the CG and SWMFG, with detailed data available in Table 4. This discussion focuses on the deflection at LOP. Figure 9 shows that Group A maintains a relatively stable deflection, measuring 0.29 mm, 0.27 mm and 0.30 mm. In contrast, Groups B and C exhibit a decrease in deflection as the amount of crushed waste mask fibers increases. These findings indicate that the dimensions of Groups A and B have minimal impacts on the flexural deflection of SWMFG. Notably, C1 has a larger δ_LOP (=0.35 mm) than the other groups.

3.2.4. Flexural Toughness (Energy Absorption Capability)

Figure 10 shows the flexural toughness of CG and SWMFG, with specific data available in Table 4. This section focuses on the four types of toughness exhibited by the samples upon reaching LOP: d3, d5, and d10. Figure 10 shows that the toughness increases with the addition of crushed waste mask fibers. The T_LOP values of Groups A and B decrease as the fiber content increases, whereas T_d3, T_d5, and T_d10 increase with the same addition. In contrast, the T_LOP, T_d3, T_d5, and T_d10 values of Group C tend to decrease with the increasing fiber content. After the samples failed, those using the A3 and C1 samples presented significantly better T_d3, T_d5, and T_d10 values. The C1 group presented higher T_d3 values (=0.71 N·m) and T_d5 values (=0.98 N·m), whereas the A3 group presented higher T_d5 values (=0.98 N·m) and T_d10 values (=1.63 N·m).

3.3. Microscopic Material and Morphology

The microscopic material and morphology of B1 at 28 days were observed, as shown in Figure 11. Figure 11 clearly shows that, after 28 days of curing, a substantial amount of hydration products accumulated on the matrix surface. The microscopic morphology revealed needle-like AFt, cotton-like C-S-H gel, and hexagonal prismatic CH. These hydration products interweave to create a compact, interconnected framework, thereby increasing the hardness, density, and overall structural integrity of the matrix. Moreover, the hydration products gradually fill cracks, contributing to improved mechanical performances [41]. This is consistent with the test results; the compressive strength of Group B1 is the highest among all the groups (20.69 MPa), and the folding strength is also the highest among all the groups (6.95 MPa).

The microscopic morphologies of the SWMFs and matrix after 28 days of curing are shown in Figure 12. As observed in Figure 12a, numerous air bubbles are present within the matrix of A3, likely introduced by the fibers, which act as initial defects and contribute to the reduction in the mechanical performances. This explains why the compressive strength and flexural strength of A3 are the smallest in size A, indicating that the mechanical properties of size A are reduced after the inclusion of more than 1% of the fibers. In addition, visible fiber scratches are observed on the matrix surface, with hydration products adhering to the surfaces of the extracted SWMFs. The ends of the SWMFs exhibit deformations due to the applied stresses. Although the sample fractured during loading, the matrix remained connected due to the fiber bridging effect, allowing it to continue bearing load even after cracks had propagated.

The failure modes of the fibers primarily include pull-out and fracture. Microscopic analysis revealed that SWMFs fail mainly through fiber fracture. This improvement is likely attributed to the sufficient hydration achieved during the 28-day curing period, which enhances the adhesion between the cementitious matrix and the fibers, thereby optimizing the mechanical performance of the material. Li et al. [42] indicated that fibers fracture when the friction stress surpasses its maximum threshold, whereas a lower friction stress promotes fiber pull-out. This implies that when the friction stress is maintained within an optimal range, it enhances the slip hardening behavior of fibers, leading to increased pull-out resistance. Accordingly, Ding et al. [43] suggest that an increase in the fiber pull-out efficiency and a decrease in the breakage rate contribute to the increases in the tensile strength and ductility of the material. These effects can be attributed to the effective energy absorption mechanism of the fibers, which helps delay material failure under external forces, thereby improving the overall performance. Consequently, the flexural toughness of the samples with SWMFs is greater than that of the CG samples, which indicates that SWMFG has excellent deformation capacity and toughness.

The microstructure of the concrete was thoroughly examined using a scanning electron microscope, and its mechanical properties were analyzed in detail. As shown in Figure 12a, the SWMF is coated with a large amount of hydration products, making it almost entirely invisible. The irregular rectangular morphology observed in the images reflects the overlap or wrapping of C-S-H gel on the fiber surfaces. As observed for Group C1 in Figure 12b, the C-S-H gel was also shown to cover the fiber surface, exhibiting a cotton-like appearance. Additionally, the hydration products had begun to extend along the longer fibers, indicating the significant role of the fibers in the hydration process. Since waste masks are made of spun-bonded polypropylene and melt-blown polypropylene layers, they may act as sites for different hydration reactions [28].

4. Conclusions

This study investigated the effects of incorporating SWMFs into concrete on its mechanical properties and microstructural characteristics. The following conclusions are drawn.

(1) Incorporating SWMFs reduces the compressive strength of all material groups. With the increasing fiber content, dimensions A and B tended to decrease. Compared with that of Group A1, the change in σ_MOR of Group A3 was a decrease of 37.83%, and that of Group B3 decreased by 45.63% compared with that of Group B1. Dimension C first increased and then decreased, and the size of Group C3 decreased by 39.36% compared with that of Group C2. However, after the addition of SWMFs, the compressive strain and toughness were improved, and the compressive toughness of Group B1 showed the largest improvement, being 285.7% greater than that of the CG. These findings indicate that SWMFs significantly improved the toughness after cracking.

(2) The use of SWMFs also reduced the flexural strength across all the groups. As the fiber content increases, dimensions A and B tend to decrease. Compared with the change in f_LOP, that of Group A3 was 28.97% lower than that of Group A1, and that of Group B3 was 39.57% lower than that of Group B1. The dimension C first increased and then decreased, and the size of Group C3 decreased by 32.05% compared with that of Group C2. When the fiber content reached 3%, the flexural strength across the groups stabilized within the same range. Despite the reduction in flexural strength, the addition of fibers enhanced the flexural toughness, suggesting an improvement in ductility and deformation capacity. Among them, T_d10 of A3 was the largest, reaching 1.63 N·m, and the T_d3 of C1 reached 0.71 N·m.

(3) SEM results revealed that the fibers introduced pores and voids into the concrete matrix. These hydration products formed a dense interlocking structure, increasing the hardness, compactness, and integrity of the material. The hydration products tended to extend along the longer fibers, indicating that the fibers significantly contributed to the hydration process.

(4) The experimental data show that the addition of B-dimensional SWMFs with a 1% volume can not only meet the strength requirements but also improve the energy absorption capacity of flexural toughness. However, it is not recommended for use in designs with high strength requirements.

(5) Further studies should investigate the effects of different dimensions and different SMF dosages on durability.

In summary, the incorporation of crushed waste mask fibers in concrete effectively improved the post-failure toughness and flexural performance of the material.