A multi-criteria evaluation and optimization of sustainable fiber-reinforced concrete developed with nylon waste fibers and micro-silica

Nylon waste fibers similar to new nylon fibers possess high tensile strength and toughness; hence, they can be used as an eco-friendly discrete reinforcement in high-strength concrete. This study aimed to analyze the mechanical and permeability characteristics and life cycle impact of high-strength concrete with varying amounts of nylon waste fiber and micro-silica. The results proved that nylon waste fiber was highly beneficial to the tensile and flexural strength of concrete. The incorporation of a 1% volume of nylon waste fiber caused net improvements of 50% in the flexural strength of concrete. At the combined addition of 0.5% volume fraction of nylon fiber and 7.5% micro-silica, splitting tensile and flexural strength of high-strength concrete experienced net improvements of 49% and 55%, respectively. Nylon fiber-reinforced concrete exhibited a ductile response and high flexural toughness and residual strength compared to plain concrete. A low volume fraction of waste fibers was beneficial to the permeability resistance of high-strength concrete against water absorption and chloride permeability, while a high volume (1% by volume fraction) of fiber was harmful to the permeability-resistance of concrete. For the best mechanical performance of high-strength concrete, 0.5% nylon waste fiber can be used with 7.5% micro-silica. The use of micro-silica minimized the negative effect of the high volume of fibers on the permeability resistance of high-strength concrete. The addition of nylon waste fibers (at 0.25% and 0.5% volume) and micro-silica also reduced carbon emissions per unit strength of concrete.


Introduction
High-strength concrete (HSC) and ultra-high-performance concrete (UHPC) families have attracted substantial attention in the construction sector owing to their superior mechanical and durability properties (Afroughsabet et al. 2016). Due to their compact microstructure, HSC and UHPC families have lower crack resistance and are vulnerable to explosive spalling at elevated temperatures (Abid et al. 2017;Xie et al. 2018b). To enhance the post-cracking toughness and overcome the brittleness and spalling issue of concrete in fire, the use of fiber reinforcements like steel, glass, and polypropylene is highly recommended (Afroughsabet and Ozbakkaloglu 2015;Raza et al. 2020;Zhao et al. 2022).
In the 1990s, the introduction of macro-synthetic fibers (MSF) into the construction industry has seen vast research into the likelihood of using discrete fibers to substitute light gauge steel grids and steel fibers in rigid pavements, shotcretes, and structural concrete (Oh et al. 2005;Suji et al. 2007;Buratti et al. 2011). Generally, synthetic fibers have an elastic modulus usually in the range of 3-7 GPa, considerably lower compared to engineered steel fibers with a modulus of elasticity of about 210 GPa; studies have shown that depending on the concentration and geometry, they substantially advance the toughness, ductility, spalling resistance, and durability of plain concrete (Oh et al. 2005(Oh et al. , 2007Kazmi et al. 2018). Contrary to steel fibers, MSF offers numerous advantages; they are lightweight, insulating, corrosion-resistant, and economical.
Steel fiber is the most commonly used MSF to enhance the tensile and flexural strength of concrete (Kachouh et al. 2021). The axial compression toughness, peak compressive strength, and crack resistance of concrete are also increased due to the incorporation of steel fiber (Afroughsabet et al. 2017;Zhang et al. 2022). The inherent tensile strength deficiency and brittleness of plain concrete can be effectively supplemented using steel fibers (Ali 2022). The degree of change in mechanical behavior due to fiber addition is affected by the change in type, shape, aspect ratio, and volume of fibers and the characteristics of the binder matrix. MSF like polypropylene, nylon, and polyvinyl alcohol fibers due to their low modulus of elasticity is used to advance the impact resistance and fracture toughness of plain concrete (Khan and Ali 2016;Sanchayan and Foster 2016;Das et al. 2018;He et al. 2020). Owing to their low melting points, MSF is useful in upgrading the elevated temperature performance of UHPC families (Sanchayan and Foster 2016;Hiremath and Yaragal 2018). Thus, the role of MSF is crucial in attributing the ductility characteristics to plain concrete. Chemically, MSF is inert and possesses high corrosion resistance. The use of MSF improves the crack resistance, tensile, and bending behavior of cementitious materials (Kazmi et al. 2018). Munir et al. (2021) noticed an increase in the energy absorption capacity, peak strength, and peak strains of plain concrete samples under axial compression load with the rise in macro-polypropylene fibers. An increase in the tensile and flexural strength of concrete has also been observed with the use of MSF (Das et al. 2018;Kazmi et al. 2018;Ali et al. 2020b).
One of the main drawbacks of the FRC is its high initial cost due to the requirement of cost-intensive materials, i.e., high contents of cementitious materials, engineered fibers, and water-reducing admixtures. According to Ali et al. (2020b), the incorporation of a 1% volume of steel fiber and polypropylene fiber can increase the initial cost of concrete by 85% and 18%, respectively. Similarly, the CO 2 footprint of FRC is considerably higher compared to that of normal concrete. Thus, the modern research interest is driven toward the investigation of eco-friendly substitutes for fiber reinforcements. Various metallic and polymer wastes are being converted into fiber reinforcement. Several studies (Figueiredo et al. 2019;Wang et al. 2019;Liew and Akbar 2020;Zhong and Zhang 2020) are available considering recycled tire steel fibers and recycled tire polymer fibers as alternatives to virgin steel fiber and synthetic fibers, respectively. Several types of synthetic plastic wastes have also been refined and studied as fiber, i.e., plastic sacks, carpet fibers, PET bottle wastes, fishnets, and paintbrushes (Boiny et al. 2017;Alrshoudi et al. 2020;Mendes et al. 2021;Farooq et al. 2022;Lamba et al. 2022;Tran et al. 2022). These plastic fiber types provide two functions, firstly as an inexpensive alternative to virgin fibers, and secondly, they serve as an eco-friendly consumption of non-biodegrading wastes. In terms of engineering performance, normally, recycled fibers are not as superior to virgin fibers, but some bargains can be made to conserve the quality of the environment. Studies (Domski et al. 2017;Ali et al. 2022b) reported that recycled tire steel fiber showed comparable or higher toughness and tensile performance compared to virgin steel fiber, as steel wires of tires are manufactured from high-grade steel. Baričević et al. (2018) showed that polymer fiber-recycled tire wastes showed performance comparable to new polypropylene fibers in enhancing the freeze-thaw resistance and permeability of concrete. Natural fibers are also being studied as fiber reinforcement in concrete, considering the benefits of them being environment-friendly and cheap. Coconut waste fibers, jute fiber, sisal fiber, and banana fiber have been proven beneficial to the tensile and flexural strength of concrete (Islam and Ahmed 2018;Khan and Ali 2019;Ali et al. 2022a;Ren et al. 2022;Shah et al. 2022). Such positive developments about the behavior of concrete with recycled fibers have accelerated the research interest in the exploration of new eco-friendly fibers.
Many types of plastic waste can be recycled into fibers for application in FRC. For example, nylon filaments are used in paintbrushes, hairbrushes, toothbrushes, and fishnets. Nylon waste fibers (NWF) derived from plastic waste possess high residual tensile strength and toughness; thus, they can be used as a replacement for virgin fibers. Among all countries, China is the largest producer of nylon waste comprising post-consumer brushes, and it generates about 80 million tons of nylon waste annually (Yin and Wu 2018). Similar scenarios exist all around the world, where recycling of nylon brushes is a challenging task. If nylon waste is left unrecycled, non-degradable solid wastes would accumulate and create serious water and land pollution (Wang et al. 2017). When compared to common polymer fibers, nylon fibers possess high tensile strength and heat resistance. Virgin nylon filaments yield tensile strength above 550 MPa under direct tension (MatWeb 2021). Thus, virgin nylon fibers are also used widely to improve the crack resistance mechanism of plain concrete (Khan and Ali 2016;Hedjazi and Castillo 2020). NWF has been reported to possess a tensile strength of over 400 MPa (Orasutthikul et al. 2017). NWF investigated in the present research was recycled from the filaments of scrap paintbrushes. A lot of paintbrushes are generated as wastes, after the finishing or refurbishment of the concrete and metallic structures. This typical NWF can be used as the MSF in HSC.
Based on the above premise, and to endorse the application of NWF in FRC manufacture, this manuscript is devoted to experimentally evaluating the performance of HSC with varying proportions of NWF, i.e., 0%, 0.1%, 0.25%, 0.5%, and 1%, by volume fraction. The addition of eco-friendly micro-silica (MS) has shown positive effects on the compressive and tensile strength of concrete (Kou et al. 2011). It has also shown a beneficial effect on the impact strength and toughness of fiber-reinforced concrete (Nili and Afroughsabet 2010;Xie et al. 2018c). Thus, it is promising to enhance the performance of recycled fiber-reinforced concrete by adding MS through the substitution of cement. In this study, the effect of MS as 0%, 7.5%, and 12.5% cement replacement was also studied on the behavior of NWF-reinforced HSC. To the best knowledge of the authors, this is the first time, the concurrent effect of MS and NWF is being studied on the mechanical and permeability properties of HSC. The examined parameters include compressive strength, splitting tensile strength, load vs. deflection behavior, flexural strength, flexural toughness, water absorption, and chloride penetration depth. SEM technology was also adopted to observe the microstructure of the NWF-reinforced matrix. Besides the evaluation of engineering characteristics, a life cycle analysis (LCA) of studied samples was also conducted to analyze the environmental impact (EI) to performance ratio of mixes. The results of the present study would be beneficial to encourage the application of NWF as fiber reinforcement in concrete manufacture.

Research significance
To achieve the sustainable development goals of the United Nations' 2030 agenda, the exploitation of raw materials for the construction industry must be minimized. For this purpose, the sustainable development of structural concrete and recycling of solid wastes into concrete manufacture is more essential than ever. Until now, various types of recycled materials have been found as suitable substitutes for aggregates, cement replacement materials, and fibers in concrete (Kou et al. 2011;Kurda et al. 2018a, b;Liew and Akbar 2020). Among all materials, fiber reinforcements are highly expensive and energy-intensive materials, which drives the modern research interest to explore more eco-friendly substitutes for fibers. This study investigates a prospective fiber derived from paintbrushes, hereafter called NWF. Until now, very few studies (Orasutthikul et al. 2017;Farooq et al. 2022) are available about the mechanical performance of NWF-reinforced concrete. The concurrent effects of NWF and MS on the mechanical and durability behaviors of HSC have never been explored. In addition, the post-peak behavior of NWF-reinforced concrete has never been studied before. Therefore, an extensive research effort is required to assess the mechanical performance, post-peak behavior, and permeability-related durability of NWF-reinforced concrete. The outcomes of this study will fill the research gap on the ductility and durability behavior of NWF-reinforced HSC and contribute to the sustainable development of FRC.

Binders
Portland cement of ASTM 53 Grade was used as the main binder, which complies with ASTM C150 (2018). As a cement replacement material, MS was used. Particle size gradation curves of MS and cement are shown in Fig. 1. Chemically, MS consisted of 98.5% silica or quartz. As shown by the X-ray diffraction analysis in Fig. 2, the main peak of quartz lies between 22° and 23°. Due to extremely fine particles, the specific surface area of MS is approx. 27,000 m 2 /kg. SEM image of MS is shown in Fig. 3. It can be observed that each particle of MS has a diameter smaller than 1 µm and spherical shapes; thus, it qualifies as a micromineral admixture.

Aggregates
Coarse aggregates used in this research were crushed dolomitic aggregates acquired from the Kirana-Hill quarry in Sargodha, Pakistan. The maximum aggregate size for coarse aggregate was 12.5 mm. For fine aggregate, silicabased quarry sand from Lawrencepur was acquired. This sand had a fineness of modulus around 2.89. Both of these aggregates have been widely investigated and found suitable for HSC development in Pakistan. The physical properties of aggregates are shown in Table 1. Gradation curves of coarse aggregates and fine aggregates are shown in Fig. 4, respectively. The gradation of both aggregates lies within the upper and lower bounds of ASTM C33 (2018).

Fibers
Expired paintbrushes were shredded to produce NWF. Brush scrap was acquired from a refurbished campus building. Filaments of brushes were separated from the ferrule and handle parts, and they were manually shredded into lengths varying between 25 and 40 mm. The diameters of NWF ranged between 0.25 and 0.5 mm. The density of the NWF filament is 1.1 g/cm 3 , which is almost similar to that of the new nylon fiber. Waste nylon filaments have a residual tensile strength of about 350 MPa (Yin and Wu 2018). The overview of NWF is shown in Fig. 5.

Superplasticizer and tap water
To achieve the desired workability of HSC at a low waterbinder ratio, the high-range water-reducing admixture "Viscocrete 3110" was used. The negative effect of fibers on the  workability of HSC was also controlled by using the Viscocrete chemical. Moreover, for the preparation and curing of all batches, tap water was used.

Design and preparation of concrete mixes
Three families of concrete were produced using 0%, 7.5%, and 12.5% of MS by volume replacement of cement. Each family consists of five different batches containing 0%, 0.15%, 0.25%, 0.5%, and 1% NWF by volume fraction. Thus, a total of 15 concrete batches were investigated in this study. Complete details about the proportioning of concrete batches are provided in Table 2. The first mix without MS and NWF content is referred to as the control mix, hereafter called CM. The mix proportions of CM were designed for HSC strength class after conducting various trials to attain compressive strength of 65 MPa at 28 days. The workability of CM was also attained in the range of 150-230 mm.
In each mix, the weight of NWF (W) for a given volume fraction (V) was evaluated using Eq. (1), where the density of NWF (D), i.e., is approx. 1100 kg/m 3 . The concentration of superplasticizer "Viscocrete 3110" was adjusted to attain the desired workability of FRC mixes at a low watercement ratio. As MS content increased and NWF dosage was enhanced from 0 to 1%, the workability of fresh concrete started to fall; thus, it was vital to adjust or increase the concentration of plasticizer to attain the slump value within 150-230 mm. As the density of HSC is key to the development of compressive strength, it is vital to ensure good workability at a low water-binder ratio. The high workability of FRC minimizes the negative effects of high-fiber volumes on porosity and compressive strength. All mixes were produced in a mechanical mixer as discussed in a companion study (Farooq et al. 2022).

Testing methods and setups
All batches given in Table 2 were studied for major mechanical and permeability-related durability properties. To determine each parameter, three replicate samples of each batch were tested and their average value with standard deviation Four-point bending test was performed on six samples containing 0, 0.5%, and 1% NWF with and without 7.5% MS, having batch IDs NWF0, NWF0.5, NWF1, MS7.5/ NWF0, MS7.5/NWF0.5, and MS7.5/NWF1. Load-deflection behavior and flexural strength of these six batches were determined by conducting a four-point bending test on 100 mm × 100 mm × 450 mm prismatic samples according to ASTM C1609 (2019). The typical test setup is shown in Fig. 6. A typical load-deflection curve is shown in Fig. 7. The load vs. deflection behavior of all samples was recorded till 2 mm deflection.
For the indirect assessment of the durability of all mixes, water absorption and chloride ion penetration depth tests were performed. Water absorption testing was performed on 50 mm × 100 ɸ mm size specimens at the age of 28 days, and it was done as per ASTM C948 (2016). Chloride permeability was assessed by the immersion method as discussed by Ali and Qureshi (2019). It was executed on 100 mm cubic samples, and penetration of Cl − was measured after exposing samples to 10% NaCl solution for 90 days. 0.1 normality silver nitrate (AgNO 3 ) solution was utilized to precipitate chloride ions (Cl − ) in saline-conditioned samples, and then, the chloride penetration depth was noted and reported in this manuscript.

Unit function
The environmental impact (EI) of studied mixes was assessed in terms of their climate change effect or global warming potential (GWP) due to CO 2 emissions. For the EI assessment, 1 (m 3 of concrete is taken as a functional unit. The EI of 1 m 3 of concrete was analyzed and compared with various percentages of NWF (0%, 0.15%, 0.25%, 0.5%, and 1%) and MS (0%, 7.5%, and 12.5%).

Boundaries (B1-B3)
The EI of the constituent materials of concrete mixes was considered from cradle to gate. Similar to the constituent materials, the EI for concrete was calculated from cradle to gate (B1-B3). B1 corresponds to the EI due to the mining and processing of raw materials; B2 represents the EI of the transportation of refined materials to the concrete plant; B3 is related to the EI of concrete production.

Dataset for LCA
For the EI assessment due to the processing of raw materials (B1), the database on the GWP (kg-CO 2 /kg) potential of cement, fine aggregate, coarse aggregate, superplasticizer, and water was taken from a study by Kurda et al. (2018b). Due to the absence of a reliable framework or information in Pakistan on the EI of these raw materials, the inventory was built with a European study (Kurda et al. 2018b). The EI of MS was taken from another European study (Hájek et al. 2011). MS has a negligible GWP compared to cement since it is the by-product of ferrosilicon alloys and involves no processing. Its EI is largely dependent on its packaging and transportation impact. The dataset for the EI of constituent materials (B1) is given in Table 3. The EI of NWF was for the first time assessed in this research. The processes involved in the preparation of NWF are illustrated in Fig. 8. The CO 2 emissions due to the NWF production were assumed to be primarily dependent on the shredding of nylon waste (while other processes such as a collection of expired brushes and separation of ferrules could not be quantified in terms of CO 2 footprint). A modern plastic shredder requires 2.5 kW power, and it approximately shreds 40 kg of plastic in one hour (GIT 2022). The input electricity was considered from a coal power plant, which generates 1.05 kg-CO 2 (2.2 lbs-CO 2 /kg) per kWh according to US Energy Information Administration (EIA 2022). This information leads to the EI estimation of NWF to be around 0.066 kg-CO 2 /kg.
The transportation EI of processed materials (B2) from the source to the concrete plant corresponds to the 2 nd largest contribution to the total EI of concrete production. The processed materials were supposed to be conveyed by a 17.3-ton lorry. The EI of transportation via lorry was considered as 6.57 × 10 −5 kg-CO 2 /kg·km (Kurda et al. 2018b) for all types of processed materials. A round trip of a lorry produces around 1.85 times of EI of a one-way trip. Thus, the EI of an empty lorry returning to the source was also included in the LCA study. The distances between the concrete plant and the sources of raw materials are given in Table 4. These distances were used to evaluate the transportation EI of the constituent materials. Subsequently, the EI of the concrete production (or stage B3) per cubic meter was considered as 4.65 kg-CO 2 . This EI was assumed same for all types of mixes.

Total environmental impact of concrete mixes
The EI of each concrete mix was calculated using Eq. (2), where EI C is the total the cradle-to-gate EI of a concrete mix at the end of the concrete plant, EI m is the impact due to refinement of constituent materials (B1), T m is the EI of mth material due to the round transportation of lorry from the source to concrete plant and then back to the source (B2), and EI P is the EI related to the production of concrete mix (B3). The seven raw materials from m = 1 to 7 are cement, MS, fine aggregate, coarse aggregate, NWF, SP, and water.
The total EI of all mixes was calculated in the MS Excel software by combining the EI of all three stages (B1, B2, and B3).

Compressive strength
The effect of varying NWF doses on the compressive strength of HSC with various levels of MS is illustrated in Fig. 9. As can be seen from the figure, variation in NWF content showed nominal effects on the compressive strength. Depending upon the fiber dose, both positive and negative effects of NWF on the compressive strength were observed. A nominal increment of 5% in the compressive strength was observed at 0.25% NWF content. This positive effect of low NWF volumes of fibers on compressive strength can be credited to the improvement in the confinement of plain concrete matrix. Crack-bridging capacity improves the stiffness of concrete. However, at higher fiber doses, the negative effect of the low density of NWF filaments starts dominating over its positive effect; therefore, a 6% decrease in compressive strength was noticed at 1% NWF incorporation. A similar phenomenon has been observed with the inclusion of synthetic polypropylene fibers (Das et al. 2018). High-fiber volume could also have increased the porosity, negatively affecting the compression stiffness and density of HSC.  The incorporation of 7.5% MS improved the compressive strength of all mixes with and without NWF. This is because MS consumes residual portlandite and produces more calcium silicate hydrate (CSH) gel. Moreover, its filler effect is beneficial for the density and hardness of the microstructure of HSC. A high percentage of MS showed a negative effect on compressive strength. This is because at 12.5% MS, lower calcium oxide in cementitious matrix produces a low quantity of portlandite, and consequently, the production of CSH gel is affected negatively. Due to continuing and slower pozzolanic reactions, MS-containing mixes gain higher net strength between 28 and 90 days compared to non-MS-containing mixes.
The combined incorporation of 7.5% MS and 0.25% NWF produces the best results among all mixes at the age of 28 and 90 days. A notable increment of 16.6% in compressive strength of HSC was observed at a combined 7.5% MS and 0.25% NWF incorporation. NWF0.15/MS7.5 exhibited about 14% higher compressive strength than CM. These results showed that the efficiency of NWF was improved due to the improvement in the bond strength of fibers. As 7.5% MS improves the strength and density of microstructure owing to its highly fine reactive particles, the interface between NWF filaments and binder matrix is strengthened. This is a well-understood phenomenon in literature (Wu et al. 2016;Qureshi et al. 2020). Due to the negative effects of both 1% NWF and 12.5% MS on compressive strength, their combined incorporation also caused drastic reductions in compressive strength. The incorporation of 0.15-0.25% NWF can control the negative effects of a high volume of MS on the compressive strength. Moreover, at the age of 90 days, mixes containing 12.5% MS and 0.15-0.5% NWF achieved higher compressive strength than that of the CM. In MS-containing mixes, the contribution of NWF toward the compressive strength further improved with age. As the microstructure of concrete reaches full maturity, it further strengthens the bond between fibers and the concrete matrix.

Splitting tensile strength (STS)
The effect of varying NWF contents on the STS of HSC with different levels of MS is shown in Fig. 10. STS was measured at 28 days. Compared to the compression testing results, STS experienced high net improvements due to NWF addition. In non-MS-containing mixes, STS was increased by 15%, 23%, and 13%, respectively, at the introduction of 0.25%, 0.5%, and 1% NWF. This is mainly credited to the improvement in the crack resistance of HSC, which delays the splitting tensile failure of the sample. NWF filaments provide a bridging effect over the cracks in the plain matrix. Furthermore, fibers are highly active under tensile loading before peak loading, unlike their performance under compression. The optimum dosage of NWF to obtain maximum STS is 0.5% by volume. The efficiency of NWF at high-fiber volumes decreases because of the negative effects of their low density and lack of dispersion. Similar variation in STS has been observed with the glass polymer fibers (Ali and Qureshi 2019) and synthetic polypropylene (Das et al. 2018).
STS was improved by a margin of 6% due to the incorporation of 7.5% MS. The mix containing NWF with 7.5% MS showed higher STS values than those of the non-MScontaining mix. Among all studied mixes, the maximum STS, about 38% higher than that of the CM, was attained by NWF0.5/MS7.5. The net gains in STS of mixes made with 7.5% MS were 24% and 31%, respectively, for 0.25% and 0.5% NWF. These results suggest an improvement in the efficiency of NWF when it was utilized with 7.5% MS. As already explained in the "Compressive strength" section, 7.5% MS strengthens the binder matrix owing to (1) pozzolanic reaction between silica and calcium hydroxide and (2) improvement in the density of cementitious matrix due to the "filler effect" of MS particles. This strengthening of the matrix enhances the bond strength of NWF, consequently enhancing the net improvement in STS. The STS of HSC was decreased with the incorporation of 12.5% MS. This is mainly because of the reduction in the portlandite content of the binder. The reduction in the matrix strength also showed a negative influence on the efficiency of NWF.

Modulus of rupture (MOR)
The effect of 0.5% and 1% volume of NWF on the MOR of HSC with and without MS is shown in Fig. 11. Contrary to compressive strength and STS, MOR is highly sensitive to NWF incorporation. Net improvements of 50% and 63% were obtained in MOR owing to the addition of 0.5% and 1% NWF, respectively. The main reason for these improvements is the contribution of fibers toward the crack-bridging capacity and tensile strength of HSC. The tensile stresses  Fig. 10 Effect of NWF and MS contents on the STS of HSC are transferred from the plain concrete to fibers, which supplement the MOR value. NWF filaments are strong under tension; therefore, they provide effective control over the early rupture of the plain matrix of the concrete. Even after the failure of plain concrete, fibers control the progress of rupture. Previous studies with glass fibers (Ali et al. 2020a) and steel fibers (Afroughsabet and Ozbakkaloglu 2015) have agreed that fiber reinforcement is more beneficial to MOR and STS due to the direct resistance of filaments under the tensile action of fibers. Unlike micro-synthetic polypropylene fibers (Afroughsabet and Ozbakkaloglu 2015;Das et al. 2018), NWF exhibited more net improvement in tensile strength and MOR. This could be because macro-filaments of NWF offer good bond strength in the matrix owing to their thicker filaments and rougher texture.
Compared to CM, a slight improvement in MOR was observed with the addition of 7.5% MS. Due to the incorporation of 7.5% MS, the efficiency of NWF improved notably. For example, the introduction of a 0.5% addition of NWF in the binder matrix showed a net improvement of 57%, which is about 7% greater than that observed in non-MS-containing mixes. These results of mechanical properties proved that both MS and NWF can be used for synergistic results. The interactive incorporation of 1% NWF and 7.5% MS results in an improvement of 75% in MOR.

Load versus deflection curves
The influence of NWF (0%, 0.5%, and 1%) and MS (0% and 7.5%) on the load-mid span deflection behavior of HSC mix is illustrated in Fig. 12 and Fig. 13. All mixes irrespective of the NWF content experienced similar first crack loads. The rising limbs for all mixes had almost the same slopes till the first crack load. CM showed insignificant capacity after the first crack load, and it experienced failure immediately after the peak load. The incorporation of MS showed a nominal increase in the peak load or first crack load of plain concrete. Plain concretes containing 0% and 7.5% MS had peak load values almost similar to their corresponding first crack load. Plain concretes exhibited brief descending load-deflection curves following the peak load. Both plain concrete mixes (CM and 7.5 MS) failed before experiencing a deflection of 0.1 mm. These results showed the high brittleness of plain concretes.
The 1 st crack load indicates the initiation of cracking in the plain concrete matrix. The incorporation or variation of NWF had no significant effect on the first crack load capacity of HSC. The 1 st crack load for fiber-reinforced mixes was almost similar to the peak load experienced by plain concretes (CM and NWF0/MS7.5). Mixes containing fibers experienced loading higher than the first crack load and exhibited a gradual decrease in the slope of ascending load-deflection limb until the peak load. Fiber-reinforced mixes experienced prominent 2 nd and 3 rd crack loads as well as between 1 st crack load and peak load. After experiencing the peak load, NWF-reinforced mixes did not undergo a rupture/failure like plain concretes, rather they experienced a strain-softening response with increasing deflection. Mixes containing 1% NWF showed higher peak loads than mixes with 0.5% NWF. NWF mixes showed sufficient residual strength indicating their high ductility compared to plain concretes. The residual strength of fiber-reinforced concrete was increased with the rise in fiber content. This is because, at a higher volume fraction of NWF, a high number of fiber filaments are present to contain the macro-cracks or fractures. Moreover, due to the presence of a high number of tension-strong filaments in the matrix, NWF-reinforced mixes retained notable residual capacities till 2 mm deflection. NWF-reinforced mixes showed a strain-hardening response after the 1 st crack load. Gradually cracking of plain concrete and crack-bridging action of fiber filaments play a crucial role in this stage. Immediately, after the peak load, a strain-softening response was observed for all types of fiberreinforced mixes. The low volume of NWF showed a more prominent strain-softening response compared to a high volume of NWF, especially between 0.2 to 0.5 mm deflections. The residual capacity of mixes with 0.5% NWF declined gradually after the peak load. The increase in NWF content flattened the curve after the 0.5 mm deflection. High-fiber content is useful for the strain-hardening behavior after the peak load. Similar behavior was observed with macro-synthetic fibers (Kazmi et al. 2018).

Flexural toughness
Flexural toughness was assessed by calculating the area under the load-deflection curve till the deflection of 2 mm. The results of flexural toughness are presented in Fig. 14. NWF incorporation caused outstanding improvements in the flexural toughness of plain concrete. The role of the singular incorporation of MS is insignificant compared to the role of the combined incorporation of NWF and MS. As shown in Fig. 14, the flexural toughness of HSC was improved by more than 24 and 35 times at the incorporation of 0.5% and 1% NWF, respectively. Fiber addition plays a crucial role in delaying the failure in the plain concrete matrix, which also results in a high energy absorption capacity. The flexural toughness till the 1 st crack load was almost similar for all mixes with and without fibers. However, in the case of fiberreinforced mixes, (1) strain-hardening response between 1 st crack load and peak load and (2) strain-softening responses after the peak load increase the length of the load-deflection curve. This consequently leads to an increase in flexural toughness.
The combination of MS and NWF caused a notable synergistic and coupling effect on the flexural toughness of HSC. As shown in Fig. 14, the flexural toughness of NWF1/ MS7.5 (i.e., 40 J) is higher than the sum of the flexural toughness of mixes with singular incorporations of 1% NWF and 7.5% MS. This can be credited to the increase in the bond strength of NWF, which caused an increase in the peak loading capacity of HSC. The pozzolanic and micro-filler  (Xie et al. 2015;Wu et al. 2016). Besides the synergistic effects of MS and NWF on the overall compressive and tensile performance of HSC, it is environmentalfriendly to utilize waste mineral admixtures like MS as the cement replacement. Thus, the incorporation of MS in HSC has performance and sustainability benefits.

Water absorption (WA)
WA test results give an estimated quantitative evaluation of the porosity and durability of concrete. As the ingress of fluids into the microstructure of concrete affects its durability, it assists in a reliable and quicker assessment of the durability of concrete. The permeable volume of concrete is affected by the size of voids and the interconnectivity of the pore network. In this study, the WA test was conducted on all samples after the age of 28 days. The effects of NWF and MS incorporation on the WA capacity of HSC are illustrated in Fig. 15. The results showed that NWF incorporation at 0.15-0.25% showed a nominal positive influence on the imperviousness of concrete. This is caused by the improvement in the cracking resistance of the plain matrix. Since fibers provide control over the cracking produced during the drying shrinkage stage (Afroughsabet et al. 2018), the number and sizes of micro-cracks in the cementitious matrix are restricted due to the presence of NWF filaments. The inclusion of a low volume of NWF filaments can also reduce the connectivity of the porous system. On contrary, at 1% NWF, the WA capacity of HSC was reduced by 16% compared to that of the CM. This drastic increase in WA can cause a reduction in the durability of concrete. This behavior can be blamed on increased air voids in a highly reinforced mix. The lack of dispersion of fibers and the tangling effect at high doses can create a connected porous network. This is a well-known phenomenon in literature (Simões et al. 2018;Ali and Qureshi 2019). Furthermore, the incorporation of high concentrations of hydrophobic fibers can lead to heterogeneity in the internal structure of the sample, causing the creation of capillary pores (Yuan and Jia 2021).
The incorporation of 7.5% and 12.5% MS resulted in a 25-30% decrease in WA capacity. This is credited to the pozzolanic reactions and micro-filler effect caused by the MS particles (Kou et al. 2011). As particles of MS occupy spaces between cement particles, the curving effect is induced on the connectivity network of pores. Besides that, the empty/ capillary pores can be effectively filled by micro-particles of MS, reducing the rate of water absorption (Kou et al. 2011;Ahmad et al. 2022). Thus, MS can control the negative effect of 1% NWF on the imperviousness and durability of HSC. Although 12.5% MS did not provide high strength results, it still contributes to the durability owing to its positive effect on the pore refinement.

Chloride penetration depth (CPD)
The effect of NWF and MS incorporation on the CPD of HSC is shown in Fig. 16. Prominent reductions of 9-18% were observed in the CPD values at the incorporation of 0.15% and 0.25% NWF. This is the result of a decrease in the WA capacity of concrete at a low volume of fibers. Reduction in shrinkage cracking yields better resistance against the ingress of detrimental chemicals inside the matrix of the concrete. This implies that the incorporation of 0.15-0.25% NWF resulted in an improvement in the corrosion resistance of HSC. Afroughsabet et al.(2017) also claimed the reduction in WA capacity to an increase in crack resistance of concrete due to fiber addition. However, the increase in NWF content to 1% volume resulted in a clear increase in the CPD values. At 1% NWF, CPD in HSC was increased by 23% compared to that of the CM. This can be blamed on the increase in the permeable porosity of concrete due to the dispersion issue of the high  Fig. 16 Effect of varying contents of NWF and MS on the CPD of mixes volume of fibers (Xie et al. 2018a). The "tangling effect" of NWF filaments at high volumes can lead to scattered concentrations of filaments with entrapped air voids. The role of MS is significant in minimizing CPD. The incorporation of 7.5% and 12.5% MS leads to 34% and 28% reductions in the CPD values compared to CM, respectively. The incorporation of 12.5% showed a negative effect on the mechanical performance, but it caused an improvement in the corrosion resistance of concrete. Micro-particles reduce the pore size in the cementitious matrix and increase the meandering of micro-channels leading to more CSH production. This ultimately reduces the rate of chloride ingress into the HSC matrix. In addition to the micro-filler effect of MS, pozzolanic reactions reduce pore connectivity (Çelik et al. 2022). The heat of hydration is a major concern in concretes having large amounts of Portland cement, for example, HSC and UHPC mixes. Uncontrolled heat of hydration leads to the formation of porous microstructure and inherent cracking. The incorporation of mineral admixtures, i.e., MS, reduces the heat of hydration of cement which controls the pore size and contains inherent cracking . MS incorporation also controls the negative effect of a high volume of fiber on the permeability-related durability performance of HSC. Both CPD and WA are interlinked with each other as both of these parameters showed a similar trend with the variations of NWF and MS contents. Thus, both of these permeability-related properties are predictable from each other (see Fig. 17). As CPD measurement is more complex, WA test results can be used to predict the corrosion risk potential of HSC.

SEM image analysis
SEM observation of binder paste made without MS is shown in Fig. 18. Since this paste only had Portland cement as the binder, a lot of quantity of calcium hydroxide (CH) or portlandite was produced along with bushy calcium silicate hydrate (CSH) crystals. In the figure, a significant number of platy or flaky crystals of CH could be observed at 5 µm, whereas the presence of CSH gel is marked by bushy or sponge-shaped crystals. These crystals can be observed scattered at various locations along with CH and other crystals. Due to the needle and bush-like structure, CSH crystals are the main contributing crystals to the strength and imperviousness of concrete. The more the number of CSH crystals in the matrix, the higher the strength of a material.
The incorporation of a highly reactive mineral admixture such as MS is necessary when a high amount of Portland cement is used to manufacture HSC; otherwise, a large amount of CH is produced as a residual component. Microparticles of MS consume the portlandite and contribute to the formation of more CSH gel. These irregular, bushy, and needle-like crystals form a strong connection or bonding between the ingredients of concrete to contribute to the high strength of HSC. The effect of MS incorporation on the morphology of binder paste at 5 µm resolution is shown in Fig. 19. In this figure, the number of platelike crystals of CH almost diminished (within the scope of the investigated spot). More growth of CSH gel was present in MS-containing mix. These images provide evidence for the improvement in the mechanical performance and imperviousness of HSC due to the MS addition.  Figure 20 shows the morphology of the sample containing NWF pulled out of the binder matrix. The main purpose of this figure is to illustrate the interaction between the binder matrix and NWF filament. It can be noticed that when NWF was pulled from the matrix, it did not rupture directly at the interaction with the binder paste; this indicates the high tensile strength of NWF filament compared to that of the paste. However, the shape of NWF was disturbed while pulling the fiber filament out of the matrix of cement paste. The cementitious products could be observed on the surface of NWF filaments. These fragments of cementing compounds could have attached to the rough surface of NWF filaments during the pulling action. Since NWF is non-reactive, it cannot chemically interact with cement and MS. NWF experiences splitting within its filament distorting its shape similar to macro-synthetic polypropylene fibers (Kazmi et al. 2018). This image proved that NWF filaments are capable of withstanding high pulling stress, which leads to the improvement in the tensile and flexural strength of HSC.

Volume-related GWP
According to the constituents of concrete mixes in Table 2, the life cycle dataset, and the calculation method shown in Eq. (1), the GWP of 1 m 3 of each mix is as shown in Table 5. It can be noticed that Portland cement consists of 84% of the total GWP of concrete mix, whereas both fine and coarse aggregates contribute 14% to the total GWP. The replacement of Portland cement with an eco-friendly admixture like MS reduces the GWP of concrete. The incorporation of 7.5% and 12.5% MS caused around 6% and 10% reductions in the GWP of concrete. Since MS is a by-product and already in its refined form, it possesses a negligible EI compared to cement. The use of mineral admixtures is proven to be effective in minimizing the EI of concrete production (Hájek et al. 2011).
Unlike MS addition, NWF showed an inconsequential decrease in the GWP of concrete. NWF replaces the volume of aggregates, and it can marginally minimize the EI contributed by the aggregates (see Table 5). At the same time, it also increased the demand for SP to compensate for workability. The increase in the EI due to increasing SP dosage reversed the positive effect of NWF on the GWP of concrete. Artificial fiber reinforcements like polypropylene fibers are known to drastically change the GWP of concrete. For instance, a 1% volume of polypropylene fibers increased the GWP of concrete by 16.6 kg-CO 2 (Ali et al. 2020b), while NWF at 1% volume caused a 0.75 kg-CO 2 increase in the total GWP, which is negligible compared to that of the artificial fiber. It can be concluded that the use of recycled fibers does not change the GWP of concrete.

Performance-related GWP
Due to the notable effects of MS and RNA on the mechanical and durability properties of concrete, it is essential to evaluate the GWP per unit performance of concrete. The mechanical performance-related CO 2 emissions were analyzed as GWP per unit CS and GWP per unit STS. It enabled an understanding of the EI of concrete to produce unit strength, whereas, for the assessment of the durabilityrelated emissions, GWP per unit chloride ion penetration resistance (CIPR) and GWP per unit water absorption resistance (WAR) can be analyzed. Figure 21 shows the performance-related emissions for all mixes. For unit CS, the minimum GWP of concrete was obtained when 0.1-0.5% NWF is used with 7.5% MS. For a constant level of MS, 1% NWF showed a negative effect on the CS and GWP; thus, the use of a high volume of NWF led to a higher GWP/CS value. The incorporation of 7.5% MS played a useful role in reducing the GWP, and it improved the CS, and thus, it yielded minimum values of GWP/CS. The incorporation of 12.5% MS showed no nominal effect on the GWP/CS. Despite phenomenal reductions in the volume-related emissions, its reducing effect on the CS led to an increased value of GWP/CS. It can be noticed that the sole incorporation of NWF showed a reducing effect on the GWP/STS. Fiber reinforcement exhibited a net high improvement in the STS, and it showed a insignificant effect on the volume-related emissions of concrete. Therefore, the performance-related emissions of NWF-reinforced mixes were significantly lower than their volume-related emissions. Minimum GWP/STS corresponded to the mixes made with the combined addition of 7.5% MS and 0.15-0.25% RNF. GWP/STS of the mix incorporating 0.5% NWF and 7.5% MS was 32% lower compared to that of the CM (see Fig. 21 b).
For unit WAR and CIPR, the GWP of concrete decreased drastically with the increase in MS content. MS was highly beneficial to the penetration resistance of concrete; it caused impressive reductions in the GWP/WAR and GWP/ CIPR values. The increase in permeability resistance and a decrease in the GWP owing to MS addition led to mixes yielding the minimum values of durability-related emissions, whereas NWF showed insignificant and mixed effects on the GWP/WAR and GWP/CIPR values. The mix with 12.5% MS and 0.25% NWF showed 60% lower GWP/CIPR than that of the CM.

Conclusions
In this study, the effect of varying contents of nylon waste fiber (NWF) was studied on the properties of high-strength concrete (HSC) with and without micro-silica (MS). Basic mechanical properties, load-deflection behavior, permeability-related properties, and morphology of mixes were evaluated and compared. The following are the key findings of this research: • The addition of low volumes (0.15-0.25%) of NWF showed nominal improvements in compressive strength.
Conjunctive use of 0.25-0.5% volume of NWF and MS as 7.5% substitution of cement resulted in the maximum compressive strength. • The use of 0.25-0.5% volume of NWF produced substantial improvements in the STS. Singular incorporation of 0.5% NWF improved the STS by 23% with reference to the control mix (CM). Synergistic effects were caused by the combined use of NWF and 7.5% MS. With conjunc- tive use of 0.5% NWF and 7.5% MS, a net improvement of 38% was observed in the STS of HSC. • Among all mechanical properties, modulus of rupture (MOR) highly benefited from NWF addition. With the addition of 1% volume of NWF, 63% net improvement was experienced by the MOR. The synergistic effect was also observed due to the combined use of 7.5% MS and NWF. With 1% NWF and 7.5% MS addition in HSC, a total net improvement of 75% was experienced in MOR. • The incorporation of 7.5% MS improved the net gain in the STS and MOR due to NWF addition. This was attributed to the strengthening of the binder matrix which results in an improvement in the bond strength of fibers. However, the efficiency of fiber reinforcement is reduced at the high-level incorporation of MS. • The incorporation of NWF improved the flexural ductility of HSC. The use of 1% NWF increased the flexural toughness of HSC by 35 times. The residual strength of HSC after the peak load substantially increased due to NWF incorporation. Mixes made with 0.5% NWF and 1% NWF showed prominent strain-hardening and strain-softening responses before and after the peak load, respectively.
• Water absorption (WA) and chloride penetration depth (CPD) were marginally minimized by the inclusion of 0.15-0.25% volume of NWF. However, higher volumes of fibers (0.5% and 1%) showed harmful effects on the imperviousness and durability of concrete. • Unlike NWF, MS incorporation at 7.5% and 12.5% showed drastic reductions in the CPD and WA values. The negative effects of a high-fiber volume on the imperviousness of HSC can be controlled with MS incorporation. • It was noted that the incorporation of 7.5% and 12.5% MS, respectively, showed 6% and 10% reductions in the volume-related global warming potential (GWP) of concrete, while NWF showed a minor effect in the GWP reduction of concrete. • The incorporation of NWF had a declining effect on the strength-related CO 2 emissions, whereas MS at 7.5% caused a declining effect on both strength and durability-related emissions. This study recommends the use of 0.25-0.5% NWF with 7.5% MS for minimum performance-related emissions.
Experimental findings of this research indicate that the interactive use of fiber and supplementary cementitious Net change in CIPR/CS GWP/CIPR (kg-CO 2 /mm -1 ) GWP/CIPR Net change in GWP/CIPR material can help in effectively advancing the tensile properties and durability of concrete. The use of 0.5% volume of NWF is suited for maximum overall mechanical strength.
The flexural ductility of plain HSC can also be advanced by using 0.5% and 1% volume of NWF. The use of 7.5% MS (as a partial replacement of cement) in fibrous mixes would benefit not only the environment but also the overall mechanical and durability performance of HSC. Future research efforts are required for the assessment of NWFreinforced concrete under harsh environmental conditions. NWF is plastic fiber and has a low melting point; it can be suitable for the spalling resistance of concrete in a fire. Hybrid steel-NWF combinations in concrete can also be investigated for enhancing the elevated temperature performance of concrete.