Synergistic Effects of Hybrid Basalt Fibers on the Durability of Recycled Aggregate Concrete Under Freeze–Thaw and Chloride Conditions

Abstract

To address the poor resistance of recycled aggregate concrete (RAC) to chloride ion penetration and freeze–thaw deterioration in cold coastal regions, this study introduces basalt fibers (BFs) as a reinforcement to improve its durability and structural integrity. Rapid freeze–thaw and electric flux tests, combined with scanning electron microscopy (SEM), were employed to systematically evaluate the effects of fiber volume fraction and length configuration on the frost resistance and chloride impermeability of basalt fiber-reinforced RAC (BFRAC). The experimental results demonstrated that the incorporation of basalt fibers markedly enhanced the coupled durability of RAC, with the mixture containing 0.15% fiber volume and a balanced hybrid of short (12 mm) and long (18 mm) fibers achieving the most favorable performance. This mixture effectively reduced mass loss and strength degradation under repeated freeze–thaw cycles while substantially lowering chloride ion penetration compared with plain RAC. Microstructural observations revealed that the hybrid fiber system formed a multi-scale three-dimensional network, in which short fibers restrained microcrack initiation and long fibers bridged macrocracks, jointly refining the pore structure and improving the interfacial bonding between recycled aggregates and the cement matrix. This synergistic mechanism enhanced matrix compactness and obstructed chloride transport, leading to a more stable and durable composite. The findings not only establish an optimal basalt fiber design for improving RAC durability but also elucidate the fundamental mechanism underlying hybrid fiber synergy. These insights provide valuable theoretical guidance and practical strategies for developing sustainable, high-performance concrete suitable for long-term service in cold-region coastal infrastructures.

1. Introduction

With the rapid advancement of urbanization, large volumes of waste concrete are generated during building demolition, posing severe environmental concerns. Recycling waste concrete into recycled coarse aggregates (RCA) for the production of recycled aggregate concrete (RAC), as a partial replacement for natural coarse aggregates (NCA), not only conserves natural resources but also mitigates construction waste pollution, thereby promoting sustainable development in the construction industry [1,2,3,4]. However, due to the high porosity, strong water absorption, and weakened interfacial transition zone (ITZ) of recycled coarse aggregates (RCA) [5,6,7,8], the mechanical properties and durability of RAC are generally inferior to those of conventional concrete, which limits its application in harsh environments [9,10]. Particularly in cold marine climatic regions—such as Northern Europe, Canada, and the northern coastal areas of China—concrete structures are simultaneously subjected to the coupled effects of freeze–thaw cycles and chloride ion ingress, leading to rapid degradation of material properties and the corrosion of steel reinforcement [11,12]. Therefore, enhancing the durability of RAC under such complex environmental conditions is crucial for advancing its practical engineering applications.

Previous studies have primarily focused on improving the mechanical and durability properties of RAC by optimizing mix proportions and the types of supplementary materials used. For example, Merve Akbas [13] found that the elastic modulus of recycled aggregate concrete decreases significantly under freeze–thaw cycles. Liang G [14] reported that the dense and low-porosity structure of green high-performance geopolymer concrete is key to enhancing both mechanical performance and environmental benefits. Li et al. [15] demonstrated that a higher water–cement ratio markedly exacerbates chloride ion penetration and freeze–thaw damage in RAC, recommending that the ratio be controlled below 0.45. Sun et al. [16] investigated chloride ion concentrations at various depths in concrete specimens subjected to immersion at room temperature and after 100 freeze–thaw cycles, finding that chloride concentrations doubled after 100 cycles compared to untreated specimens. However, most existing studies have focused on optimizing material composition and replacement ratios, with limited attention given to the coupled effects of chloride ion attack and freeze–thaw cycles.

In recent years, to further enhance the durability of RAC, researchers have focused on two primary approaches: the incorporation of mineral admixtures and fiber reinforcement. Studies have shown that the incorporation of mineral admixtures such as fly ash, slag powder, and silica fume effectively refines the pore structure, increases compactness, and improves both chloride resistance and frost durability of RAC [17,18,19,20,21]. These modifications have been found to significantly enhance long-term durability, sometimes achieving or surpassing the performance of natural aggregate concrete. Further studies by other researchers have indicated that fiber reinforcement technology is widely applicable for enhancing the frost resistance and chloride ion resistance of RAC.

However, existing research rarely considers the influence of fiber length and mixed-length fiber systems on the coupled freeze–thaw and chloride penetration behavior of RAC. Moreover, the underlying synergistic mechanisms of long and short basalt fibers in improving the microstructure and transport properties of RAC remain insufficiently explored. Studies have demonstrated that hybrid fiber systems, such as those incorporating basalt fibers (BFs) and polyethylene fibers (PEFs), offer superior resistance to freeze–thaw deterioration [22,23]. BFs in particular exhibit high tensile strength, chemical stability, and excellent bonding with the cement matrix, effectively bridging microcracks and mitigating permeability. In chloride environments, Wei et al. [24] reported that BFs significantly improved RAC performance, with an optimal BF content of 0.2% and 50% RCA replacement.

Therefore, this study investigates the combined effects of freeze–thaw cycles and chloride ion ingress on basalt fiber-reinforced recycled concrete (BFRRC), focusing on both fiber length and dosage as key variables. The research aims to determine the optimal BF content and length combination that maximizes durability performance and elucidate the mechanistic role of “long–short hybrid” fiber systems in improving RAC’s resistance to coupled environmental actions. The outcomes of this work provide both theoretical and practical insights for the design of sustainable, high-durability concrete applicable to cold and marine infrastructures worldwide.

Based on this, this study focuses on basalt fiber-reinforced recycled concrete (BFRRC) and systematically investigates the chloride ion permeability and freeze–thaw damage characteristics of the material under different fiber lengths and volume fractions. The aim of this research is to determine the optimal combination of basalt fiber content and length to maximize durability performance and to reveal the synergistic enhancement mechanism of hybrid basalt fibers with different lengths on the durability of RAC. The results provide a theoretical foundation and practical guidance for promoting the application of recycled concrete in cold and marine environments.

2. Materials and Methods

2.1. Materials

In this study, P·O 42.5 grade ordinary Portland cement was employed, whose physical and mechanical properties complied with the specifications for the experiments. Class I fly ash was provided by Henan Borun Foundry Materials Co., Ltd. of China. Crushed stone with particle sizes ranging from 5 to 20 mm was used as the natural coarse aggregate. The recycled coarse aggregate, as detailed in

Table 1. The main performance index of recycled aggregate.

Particle Size (mm)	Apparent Density (g·cm−3)	Crushing Index (%)	Water Absorption Rate (%)	Bulk Density (g·cm−3)
5~20 mm	2.78	14	13	1.5

, was supplied by Beijing Jingyulu Construction Engineering Co., Ltd. of China It was produced from waste concrete specimens through mechanical crushing and manual sieving, yielding particles with a size range of 5–20 mm. The gradation curves of the two aggregates are shown in Figure 1.

Natural river sand was used as the fine aggregate. Its particle grading conformed to the relevant standards, and it was classified as Zone II medium sand, with a measured fineness modulus of 2.8. The chemical composition of the basalt fibers is shown in

Table 2. The main properties of basalt.

Fiber Diameter (mm)	Density (kg/m−3)	Elastic Strength (GPa)	Tensile Strength (GPa)	Bonding Temperature (°C)	Thermal Conductivity (W/(m·K)
7–15	2.63–2.65	991–110	1800–2200	1050	0.03–0.038

. A polycarboxylate-based high-performance water-reducing admixture was used, with a water-reduction efficiency of not less than 25%.

2.2. Mix Proportion Design

Based on previous studies and existing research gaps [24], basalt fiber volume fraction (0.1%, 0.15%, and 0.2%) and fiber length (12 mm, 18 mm, and a hybrid of both) were selected as the experimental variables. Based on extensive preliminary experiments, it was found that short fibers primarily enhance matrix densification, while long fibers improve crack-bridging behavior. The hybrid incorporation in this study aims to achieve a synergistic effect. The selected dosage range covers the typical interval in which basalt fiber-reinforced concrete exhibits optimal performance, avoiding the fiber agglomeration and workability reduction associated with excessive addition. A full factorial experimental design method was adopted to systematically investigate the main and interaction effects of fiber length and content on the durability performance of RAC, ensuring the systematicity and statistical reliability of the experimental results. In addition, one standard RAC group without fibers served as the control, resulting in a total of 10 groups.

For each group, three prismatic specimens (100 mm × 400 mm) were prepared to evaluate the relative dynamic elastic modulus and mass loss rate after freeze–thaw cycles; nine cubic specimens (100 mm × 100 mm × 100 mm) were cast to assess the compressive strength loss rate after freeze–thaw cycles; and nine cylindrical specimens (φ100 mm × 50 mm) were used for the electric flux test following freeze–thaw cycling. After casting, all specimens were left indoors for 24 h before demolding and then cured in a constant-temperature and constant-humidity chamber at 20 ± 2 °C and 95% relative humidity for 28 days. The specific coordination of this experiment is shown in

Table 3. Sample mix ratio design parameters.

Test Part Number	Water	Sand	Cement	Fly Ash	Regenerated Aggregate	Natural Aggregate	Basalt Fiber Content	Basalt Fiber Length	Water-Reducing Agent
RAC	192.0	649.0	297.5	127.5	602.5	602.5	0%	0	1.1 g
BR0.1-18	192.0	649.0	297.5	127.5	602.5	602.5	0.10%	18 mm	1.1 g
BR0.1-6	192.0	649.0	297.5	127.5	602.5	602.5	0.10%	6 mm	1.1 g
BR0.1-1:1	192.0	649.0	297.5	127.5	602.5	602.5	0.10%	6 mm + 18 mm	1.1 g
BR0.15-18	192.0	649.0	297.5	127.5	602.5	602.5	0.15%	18 mm	1.1 g
BR0.15-6	192.0	649.0	297.5	127.5	602.5	602.5	0.15%	6 mm	1.1 g
BR0.15-1:1	192.0	649.0	297.5	127.5	602.5	602.5	0.15%	6 mm + 18 mm	1.1 g
BR0.2-18	192.0	649.0	297.5	127.5	602.5	602.5	0.2%	18 mm	1.1 g
BR0.2-6	192.0	649.0	297.5	127.5	602.5	602.5	0.2%	6 mm	1.1 g
BR0.2-1:1	192.0	649.0	297.5	127.5	602.5	602.5	0.2%	6 mm + 18 mm	1.1 g

.

2.3. Test Methods

2.3.1. Freeze–Thaw Cycle Test

This study followed the Chinese National Standard GB/T 50082-2009 [25] Standard for Test Methods of Long-Term Performance and Durability of Ordinary Concrete, with necessary adjustments and improvements to reflect the salt–frost coupling conditions characteristic of concrete in cold coastal areas. The specific test process is shown in Figure 2. Specifically, the test apparatus used was a TDR-28 rapid freeze–thaw machine. The freezing temperature was set at −18 °C and the thawing temperature was maintained at 5 °C to enhance the thermal gradient intensity during the freeze–thaw cycles. A total of 100 cycles were conducted to evaluate the degradation characteristics of the RAC. To simulate a marine environment, the specimens were immersed in a 3.5% NaCl solution during the freeze–thaw process, instead of the freshwater medium specified in the standard.

In addition, to improve the resolution of the degradation data, the specimen mass and relative dynamic modulus were recorded every 25 freeze–thaw cycles. The electrical flux test was conducted in accordance with ASTM C1202. Prior to testing, the lateral surfaces of the specimens were sealed with epoxy resin, and a vacuum saturation process was applied, consisting of a 30 min vacuum followed by 24 h of water immersion, ensuring a saturation degree above 95%. The detailed test procedure was as follows:

The mass loss rate of the specimens was calculated according to Equation (1), and the average value of three parallel specimens was adopted for each group.

$$\Delta W_{ni}={\displaystyle \frac{W_{0i}-W_{ni}}{W_{oi}}}\times 100\%$$

(1)

where $\Delta W_{ni}$ is the mass loss rate (%) of the i-th concrete specimen after n freeze–thaw cycles; $W_{oi}$ is the mass (g) of the i-th specimen before the freeze–thaw cycles; $W_{oi}$ is the mass (g) of the i-th specimen after n freeze–thaw cycles.

The relative dynamic modulus of elasticity was calculated according to Equation (2), and the average value of three parallel specimens was adopted for each group.

$$En={\displaystyle \frac{{f_n}^2}{{f_0}^2}}\times 100\%$$

(2)

where $En$ is the relative dynamic modulus of elasticity (%) of the specimen after n freeze–thaw cycles;$f_n$ is the transverse fundamental frequency (Hz) of the specimen after n freeze–thaw cycles;$f_0$ is the initial transverse fundamental frequency (Hz) of the specimen before the freeze–thaw cycles.

The compressive strength was calculated according to Equation (3), and the average value of three parallel specimens was adopted for each group.

$$\Delta f_c={\displaystyle \frac{f_{c0}-f_{cn}}{f_{c0}}}\times 100\%$$

(3)

where $\Delta f_c$ is the compressive strength loss rate (%) of the specimen after n freeze–thaw cycles;$f_{c0}$ is the initial compressive strength (MPa) of the specimen before the freeze–thaw cycles;$f_{cn}$ is the compressive strength (MPa) of the specimen after n freeze–thaw cycles.

2.3.2. Rapid Chloride Permeability Test

To evaluate the chloride ion penetration resistance of recycled concrete, an NJ-AR multifunctional concrete durability tester was employed. After completion of the freeze–thaw cycles, the specimens were removed and dried. Their lateral surfaces were sealed with epoxy resin, followed by vacuum saturation using a vacuum water-saturation device. Upon completion of the saturation process, the electrical flux test was conducted in accordance with the Standard for Test Methods of Long-term Performance and Durability of Ordinary Concrete (GB/T 50082-2009) [25].

The electric flux of a single specimen was calculated according to Equation (4), and the average value of three parallel specimens was adopted for each group.

$$Q=900(I_0+2I_{30}+2I_{60}+...+2I_{300}+2I_{330}+2I_{360})$$

(4)

where $Q$ is the total electric flux (C) passing through the specimen; $I_0$ is the initial current (A) of the specimen; $I_t$ is the current (A) of the specimen at time t (min).

2.3.3. SEM Microscopic Testing

Scanning electron microscopy analysis was carried out using Hitachi SU3500 tungsten wire scanning electron microscope from Tokyo, Japan. After curing, the specimens were crushed, and cement paste fragments (approximately 5 mm × 5 mm × 5 mm) were selected from the interior. The samples were immersed in absolute ethanol for 24 h to terminate hydration and then dried in a vacuum oven at 60 °C for 48 h. To enhance conductivity, the sample surfaces were coated with a thin layer of gold before being mounted on the SEM sample holder. The fiber–matrix interface morphology and microcrack characteristics were observed under high-vacuum conditions at an accelerating voltage of 15 kV. The magnification ranged from approximately 50 to 10,000×, with a resolution up to 10 µm.

2.4. Error Analysis

For each mix proportion, no fewer than three parallel specimens were tested, and the arithmetic mean was taken as the representative result. If the difference between either the maximum or minimum value and the intermediate value exceeded 15%, both the maximum and minimum values were discarded, and the intermediate value was taken as the test result for that group. If both the maximum and minimum values differed from the intermediate value by more than 15%, the test results for that group were deemed invalid.

3. Results and Discussion

3.1. Apparent Damage

Chloride salt freeze–thaw tests were conducted on each group of specimens to obtain the apparent damage characteristics of recycled concrete under 0–100 freeze–thaw cycles, as shown in Figure 3. Freeze–thaw damage in concrete developed progressively from the surface toward the interior, and the degree of macroscopic deterioration increased with the number of cycles. For RAC and BR0.15-1:1 specimens, pore expansion and slight mortar spalling appeared on the surfaces after 25 and 50 cycles, with deterioration continuing to intensify during subsequent freeze–thaw cycles. When the number of freeze–thaw cycles reached 50, the RAC specimens exhibited leaching of cementitious materials and exposure of fine aggregates on the surface. After 75 cycles, the surface layer of RAC had completely detached, most fine aggregates were lost, and coarse aggregates began to be exposed; in contrast, the basalt fiber-reinforced concrete specimens still retained most of the mortar matrix. By the end of 100 cycles, extensive coarse aggregate exposure was observed in RAC specimens, indicating structural failure, whereas the basalt fiber-reinforced concrete specimens only began to show aggregate exposure while maintaining overall integrity.

3.2. Quality Loss Rate

As shown in Figure 4, the variation in mass loss rate of each RAC group during 0–100 chloride salt freeze–thaw cycles is presented. It can be observed that at 25 freeze–thaw cycles, all groups exhibited negative mass loss rates, with the lowest value being approximately –1%. This phenomenon is primarily attributed to the high water absorption and porosity of recycled coarse aggregate [26]. In the early stages of freeze–thaw cycles, water absorption and swelling led to an increase in specimen mass that exceeded the mass loss caused by freeze–thaw damage and chloride erosion.

As shown in Figure 4a, the mass loss rate exhibited an approximately linear increase during 25–75 cycles. A higher fiber content corresponded to a lower loss rate, with the optimal content of 0.2% reducing the rate by 1.97% compared with RAC, indicating the bridging effect of short fibers on early cracks. From 75 to 100 cycles, the loss rate increased sharply, accompanied by mortar spalling and even aggregate detachment; however, the high fiber content groups still maintained a relatively lower rate of increase.

As shown in Figure 4b, the inhibitory effect of 18 mm fibers on the mass loss rate of RAC is also evident, but the improvement trend with increasing fiber content follows a “first beneficial, then detrimental” pattern. The initial 25 cycles still correspond to the water absorption and weight gain stage, followed by the main degradation phase from 25 to 75 cycles, during which the differences among groups gradually widened. The optimal dosage was found at 0.15%, where the mass loss rate increased more slowly, reaching a reduction of 2.05% compared with the control after 100 freeze–thaw cycles. However, when the fiber dosage increased to 0.2%, the frost resistance did not continue to improve and instead showed a pronounced increase in mass loss. This phenomenon indicates that at appropriate dosages, long fibers are more effective than short fibers in dispersing stress, improving overall toughness, and enhancing crack resistance. Nevertheless, excessive fiber length and content may lead to uneven distribution and entanglement, reducing the effective bonding area with mortar. Furthermore, water absorption and expansion within fiber-induced pores may trigger new cracks, thereby damaging the internal structure of the concrete and weakening the toughening effect of the fibers.

As shown in Figure 4c, compared with the single-fiber groups, the hybrid fiber group consistently exhibited superior resistance to mass loss throughout the entire freeze–thaw cycle. During the 25–75 cycle stage, the increase in mass loss rate was relatively gradual, indicating that the hybrid fibers were more effective in delaying the progression of freeze–thaw damage in the mid-term. From 75 to 100 cycles, although the overall mass loss rate increased, the growth amplitude of the hybrid group remained significantly lower than that of the single-fiber groups, demonstrating good long-term durability. The optimal mixture was achieved at a 0.15% dosage, where the mass loss rate was reduced by 2.47% compared with RAC. This improvement can be attributed to the synergistic effect of combining short and long fibers, which enhanced the tensile strength and integrity of the concrete surface layer, thereby effectively restraining frost-induced spalling. However, excessive fiber content still led to agglomeration, weakening this beneficial effect.

Overall, basalt fibers (BFs) significantly reduced the mass loss of RAC under chloride freeze–thaw conditions, thereby enhancing its durability. Short fibers contributed to improving surface compactness but exhibited limited crack-bridging capacity. Long fibers were more effective in restraining the propagation of large cracks in the mid- to late stages; however, excessive dosage tended to cause fiber agglomeration. The hybrid incorporation of short and long fibers at a 0.15% dosage showed the best performance, demonstrating a nonlinear enhancement effect: appropriate fiber addition stabilized the internal structure, whereas excessive content increased porosity and agglomeration, ultimately compromising performance.

3.3. Relative Dynamic Elastic Modulus

To further clarify the influence of different basalt fiber incorporation strategies on the frost resistance of RAC, the evolution of relative dynamic elastic modulus (RDME) of each group under 0–100 freeze–thaw cycles in chloride solution was analyzed, as shown in Figure 5. In general, the RDME of all specimens exhibited a decreasing trend with the increase in freeze–thaw cycles, indicating continuous degradation of the internal elastic structure caused by frost damage. However, the rate of reduction was significantly alleviated after the addition of basalt fibers, demonstrating that fiber incorporation effectively delayed the deterioration of structural stiffness during freeze–thaw exposure.

As shown in Figure 5a, the specimens incorporating 6 mm short-cut fibers exhibited a significantly lower modulus degradation rate during the early freeze–thaw cycles compared with the plain RAC group, indicating superior control of initial damage. This improvement can be attributed to the more uniform distribution of short fibers, which effectively bridged microcracks and maintained continuous transmission paths for elastic waves, thereby enabling the material to retain a relatively high strain recovery capacity in the early stages of freeze–thaw cycling.

As shown in Figure 5b, the specimens with long fibers exhibited a slower reduction in dynamic elastic modulus during the mid-to-late stages of freeze–thaw cycling, demonstrating better retention capacity. This improvement was mainly attributed to the ability of long fibers to bridge wider cracks and provide superior stress dispersion, which enhanced local tensile resistance and effectively mitigated crack propagation at the macroscopic scale. However, when the fiber dosage reached 0.2%, the degradation rate of the elastic modulus accelerated. According to the theories of expansive and osmotic pressures, the repeated formation of ice and moisture migration within pores exerted internal stresses on the matrix. Fiber agglomeration at high dosages increased porosity and caused uneven distribution, leading to stress concentration zones that exacerbated crack development and compromised structural integrity.

As shown in Figure 5c, the modulus curve of the hybrid-fiber group remained relatively stable, indicating that the synergistic effect of short and long fibers effectively restricted crack propagation across multiple scales. Short fibers suppressed the initiation of microcracks, while long fibers delayed the penetration of macrocracks and enhanced stress dispersion. Together, they formed a complementary and synergistic mechanism that significantly improved structural stability. The optimal mixture was BF0.15-1:1, which achieved a relative dynamic elastic modulus of 79.8% after 100 freeze–thaw cycles—an increase of 23.8% compared with plain RAC—demonstrating that this dosage ensured uniform fiber distribution and yielded the most pronounced reinforcement effect.

3.4. Loss Rate of Compressive Strength

According to the results shown in Figure 6, the incorporation of basalt fibers (BFs) with varying dosages and lengths exerted a significant influence on the compressive strength evolution of BFRC during freeze–thaw cycles in NaCl solution. After 50 cycles, a general decline in compressive strength was observed across all groups. Specifically, the strength reductions for RAC, BR_0.1-18, BR_0.15-18, BR_0.1-6, BR_0.15-1:1, and BR_0.2-18 19.5%, 11.82%, 9.67%, 11.12%, 7.65%, and 10.71%, respectively. Following 100 cycles, the compressive strength of all BF-modified RAC specimens further decreased. Among them, BR_0.1-18 and RAC exhibited strength loss rates of 40.6% and 25.4%, respectively, both exceeding 25%, thereby meeting the failure criterion. In contrast, the hybrid-fiber groups BR_0.1-1:1, BR_0.15-1:1, and BR_0.2-1:1 showed lower strength losses of 21.81%,18.3%, and 20.7%, with BR_0.15-1:1 achieving the minimum reduction. Compared with RAC, the compressive strength loss of BR_0.15-1:1 was reduced by 22.4%. These findings demonstrate that the incorporation of BF effectively mitigates compressive strength degradation under salt–freeze coupling conditions, with performance improvements particularly notable within the 0–0.15% dosage range. At the optimal dosage of 0.15%, the post-freeze–thaw compressive strength reached the highest level.

The dominant mechanism by which basalt fibers (BFs) at dosages ≤0.15% improve the compressive strength retention lies in the densification of the interfacial transition zone (ITZ) between the recycled coarse aggregate (RCA) and the cement matrix. Due to the adhered old mortar, the ITZ of RCA is typically loose and porous, where microcracks readily initiate and interfacial debonding occurs under freeze–thaw cycles. Uniformly dispersed BFs penetrate and bridge nascent microcracks within the ITZ, reducing the stress intensity at crack tips and restraining crack propagation. Meanwhile, the fibers alter the local aggregate packing and the deposition of hydration products near the interface, hindering the continuity of capillary channels and promoting more uniform hydration. Consequently, a thinner and denser ITZ with fewer micropores is formed. Mechanically, the densified ITZ enhances the local stiffness and shear transfer capacity at the aggregate–matrix boundary, enabling external loads to be more evenly distributed rather than concentrated at weak points, thereby delaying interfacial debonding and instability. When the fiber content increases to 0.2%, fiber agglomeration introduces local voids and poor interfacial bonding, offsetting the improvement of the ITZ and explaining the observed decline in performance.

3.5. Electric Flux Test

Figure 7 presents the charge passed results of recycled aggregate concrete (RAC) specimens with different basalt fiber (BF) dosages and fiber length combinations after multiple freeze–thaw cycles. Compared with the reference RAC, the total charge passed generally decreased after the incorporation of BFs, and all groups exhibited a trend of initial reduction followed by a subsequent increase. This indicates that chloride ion transport resistance was enhanced at appropriate fiber contents, while excessive fiber addition led to performance degradation. Among all mixtures, the BR_0.15-1:1 group achieved the best performance, showing a 16.7% reduction in charge passed relative to the plain RAC.

From the perspective of transport mechanism, chloride ion migration is jointly governed by the concentration gradient and the material’s effective diffusion coefficient. The inclusion of fibers increased the tortuosity of ion diffusion paths and disrupted capillary continuity, thereby markedly reducing the apparent diffusion coefficient. This observation agrees with Fick’s second law of diffusion, which predicts that the effective diffusion coefficient decreases as pore connectivity diminishes. The hybrid combination of short and long fibers further refined the pore structure and enhanced path tortuosity, resulting in a more complex and time-consuming migration process for chloride ions, thus reducing the ion flux per unit time. The multi-scale network formed by the hybrid fibers achieved an optimal balance between matrix densification and diffusion path tortuosity, leading to the lowest charge passed value.

When the fiber content exceeded 0.15%, fiber agglomeration disrupted the matrix continuity and introduced local weak zones, facilitating the formation of continuous chloride transport channels and causing a slight increase in charge passed. Therefore, the “decrease–increase” trend in charge passed essentially reflects the competing effects between matrix densification and fiber agglomeration on the effective diffusion coefficient.

3.6. SEM Microscopic Analysis

Figure 8 displays the microstructural features of recycled aggregate concrete observed through SEM, which were analyzed to elucidate how basalt fibers enhance resistance to freeze–thaw degradation and chloride ion penetration. Fragments taken from the core of cubic specimens after compressive strength testing were selected as samples for microscopic examination.

Figure 8a,b present the internal microstructures of BFRAC before and after chloride freeze–thaw cycles. Prior to deterioration, the BFRAC matrix exhibits a relatively compact structure with only a few minor initial microcracks. However, after freeze–thaw exposure, the BFRAC shows a significant increase in both the number and width of pores and cracks, indicating progressive internal damage. As shown in Figure 8c, basalt fibers bond well with the cementitious matrix before freeze–thaw cycling, effectively restraining the initiation and propagation of microcracks. After cycling, Figure 8d clearly reveals that basalt fibers bridge across cracks, preventing further propagation and thus enhancing structural integrity. Moreover, Figure 8e shows that fibers aligned with the principal stress direction tend to be pulled out, while transverse fibers remain embedded, forming cross-arrangements that further restrict crack widening and provide multidirectional reinforcement. Figure 8f shows the formation of a three-dimensional disordered arrangement of long and short BF. By combining SEM with a large amount of experimental data for mutual verification and cross-comparison, it can be clearly observed that basalt fiber plays the following roles in concrete:

(1)

The SEM observations further corroborate the macroscopic findings that basalt fiber (BF) incorporation significantly enhances the mechanical and durability performance of RAC. The hygroscopic nature of both BF and RCA promotes uniform moisture distribution during mixing, thereby facilitating early hydration and refining the pore structure. As shown in Figure 8c, the BF is well bonded with the cementitious matrix, and the interfacial transition zone (ITZ) appears dense and continuous. This microstructural feature aligns with the microscopic results, where the relative dynamic modulus increased by 23.8% and the compressive strength loss decreased by 22.4%, indicating a substantial improvement in matrix compactness.

(2)

The incorporation of BF and RCA facilitates a uniform distribution of BFs and promotes the formation of an effective overlapping layer of cement mortar adhered to the RCA surfaces, thereby increasing their roughness. As BFs are flexible fibers, their simultaneous addition to the concrete allows them to be evenly dispersed throughout the matrix, forming a connected BF network that helps bear loads. Consequently, BFRRC exhibits enhanced toughness. Thereby achieving a higher compressive strength retention rate.

(3)

As illustrated in Figure 8f, the random distribution of long and short fibers forms a three-dimensional interwoven network that enables multi-scale crack control and stress dispersion at the microscale. Meanwhile, the fiber network increases the tortuosity of chloride ion transport paths, effectively delaying ion migration per unit time. These microstructural observations are consistent with the 16.7% reduction in electric flux measured in the experiments, jointly confirming the beneficial role of hybrid basalt fibers in enhancing both the mechanical integrity and chloride resistance of RAC.

4. Conclusions

(1)

Through systematic experimental validation, it was demonstrated that basalt fibers can effectively improve the resistance of recycled aggregate concrete (RAC) to both chloride ingress and freeze–thaw deterioration. The hybrid fiber system with a 0.15% volume fraction exhibited the most balanced performance, achieving superior durability and mechanical stability. This finding provides a reliable design reference for the incorporation of fiber reinforcement in sustainable concrete applications under coupled environmental stresses.

(2)

The enhancement mechanism arises primarily from the formation of a multi-scale fiber network that refines the pore structure and densifies the interfacial transition zone (ITZ). The hybrid distribution of long and short fibers increases the tortuosity of chloride diffusion paths and enhances crack resistance across different scales. This mechanism aligns with Fick’s second law, where reduced pore connectivity and increased path complexity effectively decrease the material’s effective diffusion coefficient. Thus, the study offers new insight into the physicochemical coupling between microstructural evolution and macroscopic durability improvement in fiber-reinforced RAC.

(3)

The reinforcing effect of basalt fibers exhibits a “first increase, then decrease” trend. When the fiber content exceeds approximately 0.15%, fiber dispersion deteriorates and agglomeration occurs, leading to an increased number of interfacial defects that weaken the densification effect. This threshold behavior reflects the competitive mechanism between matrix densification and fiber-induced porosity. From an engineering perspective, optimizing fiber dispersion and interfacial bonding quality is essential to achieve synergistic improvements in the durability and structural reliability of recycled concrete, thereby providing both theoretical and practical foundations for the design of low-carbon recycled concrete systems.

5. Future Work

This study investigated the strengthening mechanisms of basalt fibers in enhancing the freeze–thaw resistance and chloride ion impermeability of recycled aggregate concrete (RAC). Future research could focus on the following aspects: (1) The current work evaluated durability enhancement within 100 freeze–thaw cycles; however, the long-term performance under more complex exposure conditions remains unclear. Future studies should extend the exposure duration and incorporate multiple coupled factors such as chloride, sulfate, and wet–dry cycles. Based on the observed diffusion and strength retention trends, long-term datasets could be used to calibrate time-dependent deterioration models and establish more accurate service-life prediction frameworks for fiber-reinforced RAC. (2) SEM observations in this study revealed localized fiber agglomeration and partial debonding at higher contents (0.2%), which weakened the interfacial integrity and offset the benefits of fiber reinforcement. Therefore, future work should focus on fiber surface modification and interfacial tailoring to enhance chemical bonding and dispersion uniformity. Improved fiber–matrix adhesion could further reduce porosity and lower the effective diffusion coefficient, thereby surpassing the 18.9% reduction in chloride permeability achieved in this study. (3) Integrating numerical simulations with large-scale engineering experiments to evaluate the long-term applicability and cost-effectiveness of fiber-reinforced RAC in bridges, ports, and cold-region concrete structures provides both theoretical guidance and practical evidence for its large-scale implementation.