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Article

Effect of Basalt Fibers on the Performance of CO2-Cured Recycled Aggregate Concrete Composite Slab–Column Assemblies with Bolted Connections Under NaCl Erosion

by
Di Wang
1,
Yuanfeng Wu
2,
Zhiqiang Xu
1,*,
Na Xu
1,
Chuanqi Li
1,
Xu Tian
1,
Feiting Shi
3 and
Hui Wang
2,*
1
School of Chemical Engineering and Machinery, Liaodong University, Dandong 118000, China
2
School of Civil Engineering and Geographical Environment, Ningbo University, Ningbo 315000, China
3
School of Civil Engineering, Yancheng Institute of Technology, Yancheng 224051, China
*
Authors to whom correspondence should be addressed.
Coatings 2025, 15(9), 1053; https://doi.org/10.3390/coatings15091053
Submission received: 30 July 2025 / Revised: 23 August 2025 / Accepted: 6 September 2025 / Published: 8 September 2025
(This article belongs to the Special Issue Surface Treatment and Mechanical Properties of Sustainable Pavement Materials)
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Abstract

Basalt fibers possess high tensile strength and excellent corrosion resistance, properties that may enhance the chloride resistance of recycled aggregate concrete (RAC) structures. Nevertheless, the effects of basalt fibers on RAC structures under chloride attack remain poorly understood. This study investigates mass loss and the deterioration of key mechanical properties in basalt fiber-reinforced RAC composite slab–column assemblies (RAC composite assemblies) subjected to NaCl freeze–thaw cycles (F-Cs) and dry–wet alternations (D-As) and further explores the damage mechanisms of the concrete matrix through microscopic characterization. The results show that, compared with NaCl F-Cs, NaCl D-As have a more pronounced impact on the performance degradation of RAC composite slab–column assemblies. Moreover, basalt fibers effectively mitigate the deterioration of RAC composite assemblies in chloride-rich environments, particularly under NaCl D-As, where their protective effect is more evident. At 2.5 vol% fiber content, impact toughness peaked at an 83.7% improvement after 30 D-As, while flexural toughness showed a maximum enhancement of 773.6% after 100 F-Cs. Scanning electron microscopy energy-dispersive spectroscopy (SEM-EDS) analysis revealed a marked increase in Cl content within RAC, with NaCl D-As causing more severe erosion than NaCl F-Cs. Additionally, basalt fibers significantly inhibited chloride ion penetration and associated erosion in RAC. These findings provide valuable insights into utilizing basalt fibers to enhance the durability of RAC in coastal infrastructure exposed to chloride attacks. Further research on long-term performance and fiber parameter optimization is needed to support practical implementation.

1. Introduction

Exterior walls play a crucial role in building structures, as they serve as the primary barrier against environmental elements. Therefore, selecting the appropriate materials for these walls is essential for minimizing resource consumption and reducing environmental impact. In recent years, researchers around the world have extensively explored various exterior wall systems, including fly ash bricks, lightweight foam concrete hollow blocks, and glass fiber-reinforced cement porous panels [1,2,3,4]. However, many of these systems rely heavily on mineral resources like cement and natural aggregates. The extraction and processing of these resources place significant environmental and resource pressures on rapidly developing nations such as China [5]. Moreover, manufacturing these panels typically consumes large amounts of energy and produces substantial carbon emissions, leading to less-than-ideal energy-saving and environmental performance [6,7]. Consequently, developing sustainable and low-carbon wall material solutions has become critically important and urgent.
In response to the dual challenges of resource consumption and environmental pressure posed by the above exterior wall materials, the resource recovery of construction waste offers a highly promising alternative path. For example, recycled aggregate is a favorable option [8]. Rapid urbanization has led to a marked increase in construction waste, creating enormous stockpiling pressure on the environment [9,10]. Recycled aggregates produced by crushing, washing, and grading waste concrete can effectively reduce the demand for natural aggregates and the landfill volume of construction waste, which aligns with the concept of sustainable development [11,12,13]. However, conventional recycled aggregate concrete often requires higher water content and typically exhibits lower compressive strength and elastic modulus compared to natural aggregate concrete [14]. These performance disadvantages mainly originate from the porous residual mortar adhered to the surface of recycled aggregates, which weakens the interfacial transition zone between aggregate and cement paste and results in the overall degradation of durability indicators, such as impermeability and frost resistance [15,16,17,18]. Therefore, recycled aggregates are currently mainly limited to the production of low-grade concrete [19].
To improve the performance of recycled aggregate concrete, researchers have developed various strengthening techniques [20]. CO2 curing not only sequesters carbon dioxide but also reacts with the hydration products in the residual mortar on the surface of recycled aggregates to form calcium carbonate, significantly enhancing the compactness and mechanical properties of the aggregates themselves [21]. Using such strengthened recycled aggregates to produce exterior wall panels opens a new route for the high-value utilization of construction waste [22]. Nevertheless, even when using strengthened recycled aggregates, the brittleness and crack resistance of the resulting concrete materials still require further improvement. For this reason, fiber reinforcement has become an effective means of enhancement. Commonly used fiber reinforcement materials include steel fibers and polypropylene fibers [23,24]. However, steel fibers are prone to corrosion in chloride-rich environments, whereas polypropylene fibers generally exhibit low interfacial bond strength with the cement-based matrix [25,26]. By contrast, basalt fibers show superior overall performance, possessing excellent mechanical properties, high alkali resistance, and outstanding resistance to salt spray corrosion [27,28]. These characteristics render basalt fibers highly promising for applications in civil engineering materials, particularly in coastal and near-shore engineering [29,30]. Nevertheless, studies on the performance of basalt fiber-reinforced recycled aggregate concrete under chloride erosion environments remain insufficient. In particular, research on the long-term performance evolution of wall–column components made from such materials under the coupled action of mechanical loading and harsh environmental exposure in practical engineering applications is still lacking and urgently requires systematic, in-depth investigation.
Therefore, this study systematically investigates the performance evolution of basalt fiber-reinforced, CO2-cured recycled aggregate concrete composite wall–column assemblies under chloride erosion environments by combining experimental and microstructural analyses. The experiments used concrete prism specimens to simulate columns and thin slab specimens to simulate wall panels. These specimens were reinforced with basalt fibers and prepared from CO2-cured recycled aggregates, which were then bolted together to form an integral load-bearing system. The study focused on the performance responses of the composite slab–column assemblies after exposure to 100–300 F-Cs and 10–30 D-As in a 3% NaCl solution. Measured parameters included axial compressive strength, mass loss rate, relative dynamic modulus of elasticity, flexural toughness, and impact toughness. Additionally, SEM-EDS analysis was employed to characterize the microstructural morphology evolution and elemental distribution at the matrix interface following chloride erosion. This study provides important experimental evidence and theoretical support for the durability design and application of basalt fiber-reinforced recycled aggregate concrete in coastal engineering structures.

2. Experimental

2.1. Raw Materials

The cement used in this study was ordinary Portland cement (OPC), with its chemical composition and relevant performance indicators presented in Table 1 and Table 2. The coarse aggregate used was recycled aggregate, which underwent two key pretreatment processes. Carbonation was first conducted under 8% CO2 at 40 °C, which densified the microstructure through pore-filling precipitation of calcium carbonate (CaCO3) to reduce water absorption and strengthen interfacial transition zones. Subsequently, mechanical shaping partially removed the residual surface mortar, while improving particle geometry, enhancing the physical properties of the aggregate. The processed recycled aggregate was sieved to obtain a continuously graded, carbonation-enhanced recycled aggregate with a particle size range of 9.5–26.5 mm. The fine aggregate used was river sand, with a fineness modulus of 2.87. The water-reducing agent was a high-performance polycarboxylate-based superplasticizer, providing a water reduction rate of up to 40%. The fibers used were short-cut basalt fibers (Type XK-DQXWY), with a length of 3 cm, a diameter of 6 µm, and a density of 2.635 g/cm3.

2.2. Specimen Preparation

The weighed aggregates were added to an HJW-60 mixer, followed by the addition of cement and a premixed solution of water and superplasticizer. The mixture was homogenized at 45 rpm for 240 s. During mixing, basalt fibers were manually and uniformly dispersed. The superplasticizer dosage was adjusted to achieve self-compacting concrete, with a T50 cm flow time of 10 to 13 s [31]. After mixing, the concrete was poured into molds in a single lift. This casting procedure produced prism specimens (100 mm × 100 mm × 400 mm) and thin slab specimens (100 mm × 40 mm × 400 mm). The dimensions of the specimens were determined by proportionally scaling down the actual sizes of the external wall panel and column connection components in real structures. The filled molds were vibrated on a shaking table until the surface exhibited cement paste with no visible large air bubbles [32]. All specimens were covered with plastic film and cured at ambient temperature for 24 h before demolding. Finally, the demolded specimens were transferred to a standard curing room for curing [33]. Table 3 details mix proportions utilizing carbonation-modified and shaped recycled aggregate, with basalt fiber content expressed by volumetric fraction.
The water–cement ratio was consistently maintained at 0.30 across all groups to eliminate interference from this variable, thereby isolating the effects of basalt fibers. Previous research has established that a higher water–cement ratio reduces strength, while a lower ratio increases strength [34]. Consequently, this study employed a single water–cement ratio to evaluate the effects of basalt fiber incorporation exclusively.
As the fiber content increased from 0 to 2.5 vol%, the dosage of the water-reducing agent was progressively increased from 0.75 kg/m3 to 2.00 kg/m3 (Table 3, Column 7) to counteract the reduction in workability. This adjustment ensured consistent self-compacting properties (T50 = 10–13 s), allowing for a focused evaluation of fiber–matrix interactions, while controlling fresh-state variability. Increasing fiber volume is generally expected to enhance tensile strength, flexural toughness, and crack resistance but may also influence compactability and fiber dispersion.

2.3. Freeze–Thaw Cycles and Dry–Wet Alternations Test Procedure

After 24 days of curing in a standard curing room, the selected specimens were immersed in a 3% NaCl solution for 4 days. Subsequently, they were transferred to a fully automatic freeze–thaw testing chamber to undergo accelerated freeze–thaw cycles (F-Cs). During the F-Cs, the specimens were placed in rubber tubes filled with a 3% NaCl solution, with the temperature cycling between −18 °C and 8 °C. For the dry–wet alternation (D-A) test, the specimens were cured for 26 days in a standard curing room, then dried for 2 days, cooled to room temperature, and transferred to a cyclic corrosion tester for salt spray exposure. The solution used in the D-A test was a 3% NaCl solution. Each dry–wet alternation lasted 24 h, comprising 6 h of drying at 80 °C, 2 h of cooling to 25–30 °C, and 15 ± 0.5 h of salt spraying, with the solution temperature maintained at 25–30 °C. Both F-C and D-A tests were conducted in compliance with the GB/T 50082-2009 standard [35]. The total numbers of F-Cs and D-As were 300 and 30, respectively. During testing, the parameters of the specimens were evaluated after every 100 F-Cs and 10 D-As.

2.4. Measurement Methods

The test specimens were recycled aggregate concrete composite slab–column assemblies with bolted connections. The measurements focused specifically on the connection region between the slab and the column, where the strength was potentially weakest. The influence of bolted connections has been investigated in prior research [36,37]. This specific connection method was selected and held constant throughout the study. The primary objective focused on investigating the effect of incorporating basalt fibers into recycled aggregate concrete composite slab–column assemblies.

2.4.1. Dynamic Modulus of Elasticity Test

The probes of the HC-U91 ultrasonic detector were aligned with the central axis on both sides of the specimens. Measurement points were marked at 0 mm, 100 mm, and 200 mm from the center along the central axis, and ultrasonic velocity was measured [38]. The relative dynamic modulus of elasticity (RDME) of the specimens was calculated using Equation (1) [39].
R D M E = ( v t v 1 ) 2
where vt denotes ultrasonic velocity after NaCl solution exposure followed by F-Cs and D-As (m/s), and v1 represents the initial ultrasonic velocity prior to NaCl solution exposure (m/s).

2.4.2. Axial Compressive Strength

An electro-hydraulic servo testing machine with a load capacity of 0 to 100 tons was employed to determine the axial compressive strength of specimens through graded loading. The axial load was directly measured and recorded by the calibrated load cell of the testing machine throughout the test. Testing commenced with graded force-controlled loading at 5 kN/s, where each loading stage represented 1/10 of the ultimate load and was maintained for 2 min [40]. When the load reached 80% of the ultimate load, displacement-controlled loading was applied at a rate of 0.2 mm/min until specimen failure occurred [41]. Failure was defined as the point of maximum sustained load, which is the peak axial load (Fmax) reached during the test. The axial compressive strength values were calculated as the ratio of the measured peak load (Fmax) to the original cross-sectional area perpendicular to the loading axis.

2.4.3. Flexural and Impact Toughness

Figure 1a illustrates the displacement-controlled loading test procedure. As shown, the recycled aggregate concrete composite slab–column assemblies with bolted connections were subjected to four-point bending using a 1000 kN press. The pure bending segment under loading measured 120 mm with a support span of 360 mm. Displacement-controlled loading at a rate of 0.5 mm/min was maintained until specimen failure occurred [42].
Figure 1b depicts the impact test procedure and loading method. The impact toughness of recycled aggregate concrete composite slab–column assemblies with bolted connections was determined using the drop-weight test method. This procedure was conducted by the standard proposed by the American Concrete Institute (ACI) Committee 544.2R-89 [43,44]. The composite slab–column assemblies were subjected to impact loading in a four-point loading mode. The drop hammer and drop rod masses were 2.5 kg and 5 kg, respectively. Three specimens per group were tested, with impact toughness calculated using Equation (2) [45].
W = ( M + m ) g H N
where m is drop weight mass (kg); N is total number of impact failures; M is drop rod mass (kg); H is drop height, taken as 400 mm; g is gravitational acceleration, taken as 9.8 m/s2.
During impact testing, a 7.5 kg impact mass (2.5 kg hammer integrated with 5 kg rod) was released from a height of 400 mm onto one side of the specimen. The surface crack propagation width was monitored using a crack width detector, and the number of impacts until initial crack formation was recorded. The test was repeated until specimen failure occurred. Three repetitions were performed per test group.
The impact toughness index β for the drop-weight test is defined as the ratio of (N2 − N1) to N1, where N1 is the impact count at initial crack formation, and N2 is the impact count at failure. This index quantifies material toughness after crack initiation. The experimental procedure consisted of three sequential steps. Firstly, the bottom surface of the specimen was cleaned. Then, impact counts were recorded until a crack width detector identified the first crack with a width of 0.05 mm. Finally, the impact count at specimen failure was documented. The impact toughness index β was calculated using Equation (3) [46].
β = ( N 2 − N 1 ) N 1

2.4.4. Scanning Electron Microscopy Energy-Dispersive Spectroscopy

An SU3800 scanning electron microscope (SEM) was used to obtain micrographs of the concrete matrix within specimens and perform energy-dispersive spectroscopy (EDS) analysis. The sample preparation procedure consisted of the following steps. Firstly, a core sample was extracted from the central region of the specimen and trimmed into spherical particles 1–3 mm in diameter. These particles were then polished to a smooth finish and cleaned to remove surface dust. Subsequently, the samples were dried at 105 °C for two days in an oven. After being gold-sputtered under vacuum, the samples were analyzed using the SU3800 SEM.
The SEM characterized chloride-induced microstructural damage, including crack propagation, pore distribution, and surface morphology. The EDS analysis specifically targeted the quantitative determination of chlorine (Cl) content to evaluate the penetration depth of NaCl and the severity of corrosion. Supplementary quantitative analysis of silicon (Si) content was also performed at specific microstructural features (aggregate-paste interfaces) to assess the associated evolution.

3. Results and Discussion

3.1. NaCl Freeze–Thaw Cycles

3.1.1. Axial Compressive Strength

Figure 2 presents the axial compressive strength of recycled aggregate concrete composite slab–column assemblies with bolted connections after different numbers of NaCl F-Cs and the corresponding strength growth rate with the incorporation of basalt fibers. As shown, the axial compressive strength of the composite slab–column assemblies (control group with 0 vol% fibers) decreased by 4.4%–30.2% after exposure to 100–300 NaCl F-Cs (specifically, 4.4% at 100 F-Cs, 15.9% at 200 F-Cs, 30.2% at 300 F-Cs). This degradation can be attributed to two main factors. Firstly, fatigue damage occurs in the concrete matrix due to frost–heave stress and capillary pressure within the composite assemblies during cycling, leading to progressive deterioration [47]. Secondly, NaCl F-Cs accelerate concrete deterioration, as chloride ingress through cracks generates expansive crystalline phases that propagate cracks and reduce strength [48]. Furthermore, incorporating 0.5–2.5 vol% basalt fibers increased the axial compressive strength of the composite slab–column assemblies by 11.0%–87.1%. At 2.5 vol% fiber content, the maximum strength improvement (87.1%) occurred after 300 F-Cs, as shown in Figure 2. This improvement is primarily attributed to the crack bridging effect of the fibers during loading, which enhances concrete strength and mitigates strength loss induced by F-Cs [49]. Crucially, the degree of strength improvement and resistance to NaCl F-Cs degradation exhibited a strong positive correlation with the basalt fiber content. Higher fiber volumes (up to 2.5 vol%) consistently yielded significantly greater enhancements in both strength and freeze–thaw resistance. Consequently, the incorporation of basalt fibers enhances the NaCl freeze–thaw resistance of recycled aggregate concrete composite slab–column assemblies.

3.1.2. Mass Loss Rate and Relative Dynamic Modulus of Elasticity

Figure 3a presents the mass loss rate (MLR) of recycled aggregate concrete composite slab–column assemblies with bolted connections during F-Cs in NaCl solution. As shown, the MLR of recycled aggregate concrete composite slab–column assemblies reached 0.26%–1.13% after 100–300 NaCl F-Cs. This rise in MLR is primarily attributed to frost–heave stress and crystallization stress induced in the concrete matrix by NaCl F-Cs, which lead to crack initiation, propagation, and widening in the composite slab–column assemblies [50]. Moreover, the cumulative nature of F-Cs exacerbates fatigue damage in recycled aggregate concrete composite slab–column assemblies, accelerating crack propagation and interfacial spalling, thereby elevating mass loss [51]. Furthermore, incorporating 0.5–2.5 vol% basalt fibers reduced the MLR of recycled aggregate concrete composite slab–column assemblies to 0.08%–0.21%. The minimum MLR of 0.08% was recorded at 2.5 vol% fibers after 100 F-Cs. This reduction is predominantly driven by the crack bridging effect of basalt fibers, which retard crack progression within the composite slab–column assemblies matrix, thereby mitigating mass loss [52]. These results demonstrate that the effectiveness of mass loss suppression increases significantly with increasing basalt fiber content, with higher volumes providing substantially enhanced resistance to freeze–thaw-induced deterioration.
Figure 3b displays the RDME of recycled aggregate concrete composite slab–column assemblies with bolted connections during NaCl F-Cs. The data indicate that the RDME decreased from 100% to 73.4% after 100–300 NaCl F-Cs. This phenomenon similarly stems from frost–heave stress and crystallization stress induced in the concrete matrix by NaCl F-Cs, which increase the number and width of cracks in the composite slab–column assemblies. Consequently, ultrasonic pulse propagation during RDME measurement is impeded by freeze–thaw-induced cracks, which reduce ultrasonic velocity and thereby decrease RDME [53]. As shown in the figure, incorporating 0.5–2.5 vol% basalt fibers increased the RDME of the composite slab–column assemblies by 2.6%–19.9%. The maximum improvement of 19.9% was achieved at 2.5 vol% fibers after 300 F-Cs. This enhancement primarily results from the ability of basalt fibers to delay crack development, thereby reducing the degradation of dynamic modulus. Furthermore, the magnitude of improvement exhibited a strong positive correlation with basalt fiber content, where higher volumes consistently yielded greater retention of the relative dynamic modulus.

3.1.3. Flexural Toughness Behavior Analysis

Figure 4a illustrates the flexural load-deflection curves of recycled aggregate concrete composite slab–column assemblies reinforced with basalt fibers after 100–300 NaCl F-Cs. The peak flexural load and maximum deflection decrease after these cycles. This reduction is primarily attributed to frost–heave stress and crystallization stress generated during NaCl F-Cs, which accelerate crack formation in the recycled aggregate concrete composite slab–column assemblies. As these cracks increase, they directly impair the ultimate flexural capacity and deformation tolerance of the composite slab–column assemblies [54].
Figure 4b presents flexural toughness values calculated from the area enclosed by the flexural load-deflection curves of recycled aggregate concrete composite slab–column assemblies. The flexural toughness of recycled aggregate concrete composite slab–column assemblies (control group with 0 vol% fibers) decreased by 33.9%–47.5% after 100–300 NaCl F-Cs. Specifically, the reductions of 47.5% and 33.9% were recorded after 100 F-Cs and 300 F-Cs, respectively. This decrease primarily stems from the erosive effect of NaCl F-Cs, which exacerbate interfacial transition zone damage and crack propagation within the concrete matrix, thereby degrading flexural toughness [55]. Additionally, incorporating 0.5–2.5 vol% basalt fibers significantly enhanced the flexural toughness of recycled aggregate concrete slab stacked with concrete column. The maximum improvement in flexural toughness, reaching 773.6%, was achieved at 2.5 vol% fibers after 100 F-Cs. This improvement stems from energy dissipation mechanisms during flexural loading, primarily the pull-out of fibers and bridging of cracks, thereby increasing its flexural toughness [56]. Overall, the improvement in flexural toughness was significantly dependent on the fiber volume fraction (vol%). Generally, higher fiber content led to substantially greater improvements in flexural toughness compared to lower fiber contents.

3.1.4. Impact Toughness Analysis

Figure 5 and Figure 6, respectively, depict the impact toughness of recycled aggregate concrete composite slab–column assemblies subjected to NaCl solution F-Cs, along with the number of impacts and the impact toughness index. After 100–300 NaCl F-Cs, specimens showed reduced impact toughness. Notably, the number of impacts to initial cracking, impacts to failure, and the difference in impact counts between failure and initial cracking all decreased. The impact toughness degradation rate ranged from 3.2% to 19.4% (specifically, 3.2% at 100 F-Cs, 19.4% at 300 F-Cs for the control group). In contrast, the toughness index (β, defined by Equation (3) in Section 2.4.2 as β = (N2 − N1)/N1; measured using the drop-weight impact test) increased with the number of F-Cs.
Additionally, incorporating 0.5–2.5 vol% basalt fibers enhanced the impact toughness of recycled aggregate concrete composite slab–column assemblies by 6.5%–62%. The maximum improvement of 62% was achieved at 2.5 vol% fibers after 300 F-Cs. This improvement is attributed to basalt fibers effectively inhibiting crack propagation and restraining crack width development in the recycled aggregate concrete composite slab–column assemblies when subjected to impact loading, enhancing overall toughness [57]. Collectively, these findings demonstrate that the magnitude of impact toughness enhancement is strongly governed by basalt fiber content (vol%), with higher volumes yielding progressively greater improvements. Correspondingly, the number of impacts sustained by recycled aggregate concrete composite slab–column assemblies at both initial cracking and failure stages increased with a higher content of basalt fibers. This phenomenon is likely due to basalt fibers amplifying the disparity between the number of impacts required to cause failure and initial cracking. However, with further increases in the content of basalt fibers, the impact toughness index showed a downward trend. The incorporation of basalt fibers into recycled aggregate concrete composite slab–column assemblies increases the number of impacts required for initial cracking, thereby elevating the denominator in the toughness index formula and reducing its value.

3.1.5. Microscopic Analysis

Figure 7a presents an SEM micrograph and EDS spectra of the concrete matrix in recycled aggregate concrete composite slab–column assembly specimens without fibers before NaCl exposure. Microstructural analysis revealed that the pre-exposure specimens exhibited a comparatively smooth and dense microstructure, with minimal micro-voids predominantly localized at the aggregate interfaces. EDS confirmed the presence of C, O, Mg, Al, Si, Ca, Mn, and Fe. Figure 7b shows EDS analysis of the recycled aggregate concrete composite slab–column assemblies without fibers after NaCl F-Cs. Compared to Figure 7a, the concrete matrix developed a roughened surface with enlarged pores after NaCl F-Cs. Additionally, the Si content (wt.%) decreased, while the Cl content increased significantly after F-Cs. This phenomenon can be explained from two perspectives. On the one hand, during chloride salt erosion, the NaCl solution permeates the concrete pores, inducing osmotic pressure and chemical corrosion within the matrix. These effects jointly modify the pore structure [58]. On the other hand, NaCl F-Cs promote the deeper penetration of free chloride ions and elevate the chloride concentration in pore solutions, resulting in a significantly increased Cl content in concrete [59].
The microstructural characterization in Figure 7a,b serves the following defined purpose: to establish baseline deterioration mechanisms in fiber-free control specimens under NaCl F-C exposure. Key observations, specifically surface roughening, pore enlargement, and accelerated chloride ingress, provide essential reference metrics for evaluating fiber-reinforced systems in subsequent analyses.

3.2. NaCl Dry–Wet Alternations

3.2.1. Axial Compressive Strength

Figure 8 demonstrates the axial compressive strength of recycled aggregate concrete composite slab–column assemblies with bolted connections after different numbers of NaCl D-As and the corresponding strength growth rate with basalt fiber incorporation. As shown, the axial compressive strength of the composite slab–column assemblies decreased by 8.7%–32.1% after 10–30 NaCl D-As (specifically, 8.7% at 10 D-As, 17.9% at 20 D-As, and 32.1% at 30 D-As for the control group). This degradation is primarily attributed to the cyclic precipitation and dissolution of NaCl crystals during D-As, which induces expansion and crack formation within the concrete matrix, thereby reducing the axial compressive strength [60]. Moreover, the cumulative damage from these crystallization–dissolution cycles further compromises structural strength. Conversely, the incorporation of 0.5–2.5 vol% basalt fibers resulted in a 12.4%–91.7% increase in the axial compressive strength of recycled aggregate concrete composite slab–column assemblies. The maximum improvement of 91.7% was achieved at 2.5 vol% fibers after 30 D-As, as shown in Figure 8. A similar strength growth trend was observed in other concrete systems, where increasing basalt fiber content from 0% to 3% in sulphoaluminate cement-based reactive powder concrete induced a rising trend in compressive strength [61]. This enhancement stems from the crack bridging effect of basalt fibers, which effectively restrain microcrack propagation under load, thereby improving concrete strength and mitigating degradation induced by D-As. Importantly, the degree of strength improvement was driven by basalt fiber content, exhibiting a strong positive correlation where higher volumes (up to 2.5 vol%) yielded significantly greater enhancements. Comparative data from Figure 2 and Figure 8 demonstrate that the axial compressive strength of basalt fiber-reinforced recycled aggregate concrete composite slab–column assemblies after NaCl D-As is marginally lower than after NaCl F-C exposure.

3.2.2. Mass Loss Rate and Relative Dynamic Modulus of Elasticity

Figure 9a presents the MLR of recycled aggregate concrete composite slab–column assemblies with bolted connections during NaCl D-As. The MLR of recycled aggregate concrete composite slab–column assemblies reached 0.33%–1.19% after 10–30 NaCl D-As. This rise in MLR is primarily attributed to capillary pressure developing within the concrete matrix during NaCl D-As and fatigue effects accelerated by cyclic salt crystallization [62]. These mechanisms collectively induce progressive cracking and spalling, resulting in increased mass loss. However, incorporating 0.5–2.5 vol% basalt fibers reduced the MLR of recycled aggregate concrete composite slab–column assemblies to 0.09%–0.23%. The minimum MLR of 0.09% occurred at 2.5 vol% fibers after 10 D-As. This reduction occurs as basalt fibers restrict the development of cracks within the recycled aggregate concrete composite slab–column assemblies matrix, thereby mitigating mass loss. The effectiveness of mass loss reduction exhibited a strong dependence on basalt fiber content, with higher volumes (up to 2.5 vol%) providing progressively greater mass loss resistance. Comparative analysis of the data from Figure 3a and Figure 9a demonstrates that NaCl D-As results in marginally higher MLR in recycled aggregate concrete composite slab–column assemblies than F-Cs.
Figure 9b depicts the RDME of recycled aggregate concrete composite slab–column assemblies with bolted connections during NaCl D-As. The RDME degraded from 100% to 72.7% after 10–30 D-As. This trend is attributed to capillary pressure induced in the concrete matrix of recycled aggregate concrete composite slab–column assemblies by NaCl D-As, which promotes crack initiation and propagation [63]. Consequently, ultrasonic wave propagation speed is reduced during RDME measurement due to D-A-induced cracks, reducing the RDME. Additionally, incorporating 0.5–2.5 vol% basalt fibers increased the RDME of recycled aggregate concrete composite slab–column assemblies by 3.5%–20%. The maximum improvement of 20% was achieved at 2.5 vol% fibers after 30 D-As, as shown by the data. This enhancement primarily stems from the crack bridging effect of basalt fibers, which delays crack propagation in the concrete and thereby reduces the degradation of the dynamic modulus. Furthermore, the magnitude of RDME improvement correlated directly with basalt fiber content, where increased volumes consistently yielded higher dynamic modulus retention. A data comparison between Figure 9b and Figure 3b reveals marginally greater RDME reduction under NaCl D-As than F-Cs.

3.2.3. Flexural Toughness Behavior Analysis

Figure 10a presents the flexural load-deflection curves of recycled aggregate concrete composite slab–column assemblies reinforced with basalt fibers after NaCl D-As. Both the peak flexural load and maximum deflection decreased after 10–30 D-As. This reduction is primarily attributed to the fact that D-As accelerate crack formation within the recycled aggregate concrete composite slab–column assemblies matrix, consequently diminishing both flexural capacity and deflection. Moreover, comparative analysis of Figure 4a confirms that basalt fibers increase both the ultimate flexural load and maximum deflection of recycled aggregate concrete composite slab–column assemblies under F-Cs and D-As. This phenomenon is primarily attributed to the crack restricting effect of basalt fibers. The fiber reinforcement inhibits crack propagation during flexural loading, and increased fiber content further enhances this mechanism, thereby improving crack resistance, toughness, ultimate flexural load, and corresponding deflection values [64]. Collectively, these results demonstrate that incorporating basalt fibers effectively enhances the flexural toughness of recycled aggregate concrete composite slab–column assemblies.
Figure 10b displays the flexural toughness values obtained by integrating mid-span flexural load-deflection curves of recycled aggregate concrete composite slab–column assemblies with bolted connections. The figure shows that the flexural toughness of recycled aggregate concrete slab–column assemblies decreased by up to 59.7% after 10–30 NaCl DA-s. The significant deterioration in flexural toughness is attributed to the same mechanism as NaCl F-Cs. D-As equally accelerate cracking in the concrete matrix through their erosive effect, thereby degrading the flexural toughness of recycled aggregate concrete composite slab–column assemblies. Conversely, during D-As, the incorporation of 0.5–2.5 vol% basalt fibers enhanced the flexural toughness of recycled aggregate concrete composite slab–column assemblies. The maximum improvement of 254.7% was achieved at 2.5 vol% fibers after 20 D-As. This enhancement results from concrete cracking and fiber pull-out consuming fracture energy during bending, thereby increasing toughness. The magnitude of toughness enhancement exhibited strong dependence on basalt fiber content, with higher volumes (up to 2.5 vol%) generally yielding greater improvement. This content-dependent effect substantially increased resistance to deterioration-induced toughness loss.

3.2.4. Impact Toughness Analysis

Figure 11 and Figure 12, respectively, show the impact toughness of recycled aggregate concrete composite slab–column assemblies after NaCl solution D-As, along with the number of impacts and the impact toughness index. The results indicate that recycled aggregate concrete composite slab–column assemblies exhibited decreased impact toughness after 10–30 NaCl D-As. Meanwhile, the number of impacts to initial cracking, the number of impacts to failure, and the difference in impact counts between failure and initial cracking all decreased. The impact toughness degradation ranged from 3.3% to 28.3%. However, the impact toughness index (β, defined by Equation (3), as detailed in Section 3.1.4) increased with increasing D-As. Additionally, as shown in the figure, the incorporation of 0.5–2.5 vol% basalt fibers increased the impact toughness of recycled aggregate concrete composite slab–column assemblies by 5%–83.7%. The maximum improvement of 83.7% was achieved at 2.5 vol% fibers after 30 D-As. This improvement similarly originates from the effective suppression of crack initiation and propagation by basalt fibers within recycled aggregate concrete composite slab–column assemblies under impact loading, which enhances material toughness. The efficacy of impact toughness improvement was predominantly governed by basalt fiber content. Higher fiber volumes consistently improved crack initiation resistance and ultimate failure tolerance under impact loading, demonstrating content-controlled mitigation of D-A-induced deterioration. Collectively, under both NaCl F-C and D-A conditions, the incorporation of basalt fibers enhances the impact toughness of recycled aggregate concrete composite slab–column assemblies, increasing the number of impacts sustained prior to initial cracking and ultimate failure.

3.2.5. Microscopic Analysis

Figure 13a presents an SEM micrograph and corresponding EDS spectra of the concrete matrix in recycled aggregate concrete composite slab–column assembly specimens without fibers after NaCl D-As. The concrete microstructure exhibited a roughened surface morphology with progressively formed, interconnected crack networks, accompanied by the disintegration of the cementitious matrix around aggregates. This deterioration primarily stems from the cumulative damage effects of permeation pressure and chemical corrosion on the internal pore structure, which promotes interconnected crack formation and further aggravates degradation [65,66]. Moreover, after NaCl D-As, the Cl content (wt.%) in the concrete increased. NaCl D-As compromise the chloride erosion resistance of recycled aggregate concrete composite slab–column assemblies, facilitating continuous chloride ion penetration into concrete [67]. Additionally, comparative analysis of the Cl content disparities between Figure 13a and Figure 7b revealed that NaCl D-As exert a significantly stronger erosive effect on recycled aggregate concrete composite slab–column assemblies than NaCl F-Cs. Figure 13b depicts the EDS results of recycled aggregate concrete composite slab–column assemblies with 2.5 vol% basalt fibers after NaCl D-As. In contrast to specimens without fibers, the 2.5 vol% basalt fibers significantly reduced Cl content in the concrete matrix. This particularly pronounced reduction in chloride content, as shown in Figure 13b, exemplifies the significant impact of fiber content on suppressing chloride ion penetration. Such an effect confirms that basalt fibers effectively enhance the chloride erosion resistance of recycled aggregate concrete composite slab–column assemblies.

3.3. Comparative Analysis

The results presented in Section 3.1 and Section 3.2 reveal that both NaCl F-Cs and D-As induced significant deterioration in the properties of the recycled aggregate concrete composite slab–column assemblies. Compared to F-Cs, the composite slab–column assemblies exhibited more pronounced performance degradation after 10–30 D-As. This observation is consistent with existing studies, which suggest that the detrimental effect of coupled salt and D-As on concrete properties is more severe [68,69]. The incorporation of 0.5–2.5 vol% basalt fibers improved all performance metrics of the recycled aggregate concrete composite slab–column assemblies under both F-C and D-A conditions, with the most substantial enhancement observed at 2.5 vol% fiber content. This improvement corresponds to findings from a previous study, which indicates that basalt fiber addition enhances crack resistance and toughness, thereby improving concrete durability [70]. Moreover, although the improvement in certain individual properties was more significant under F-Cs, the enhancement in overall performance was consistently more pronounced under D-As. In summary, basalt fibers effectively mitigate the performance degradation of recycled aggregate concrete composite slab–column assemblies in chloride environments, with their protective effect on overall performance being more pronounced under NaCl D-As.

4. Conclusions

This paper systematically investigated the effect of basalt fibers (0.5–2.5 vol%) on the chloride salt resistance of recycled aggregate concrete composite slab–column assemblies and explored the underlying micro-mechanism. The following conclusions are summarized below.
(1) After 100–300 NaCl F-Cs and 10–30 NaCl D-As, the axial compressive strength of recycled aggregate concrete composite slab–column assemblies decreased by 4.4%–30.2% and 8.7%–32.1%, respectively. The incorporation of 0.5–2.5 vol% basalt fibers resulted in strength increases of 11.0%–87.1% and 12.4%–91.7% relative to corroded plain assemblies under identical exposure conditions. These strength enhancements peaked at 87.1% after 300 F-Cs and 91.7% after 30 D-As with 2.5 vol% fibers.
(2) After 100–300 NaCl F-Cs and 10–30 NaCl D-As, the MLR values of recycled aggregate concrete composite slab–column assemblies were 0.26%–1.13% and 0.33%–1.19%, while the RDME decreased to 73.4% and 72.7%. Under identical exposure conditions, the incorporation of 0.5–2.5 vol% basalt fibers reduced MLR to 0.08%–0.21% and 0.09%–0.23%, while enhancing RDME by 2.6%–19.9% and 3.5%–20.0% relative to corroded plain assemblies. The minimum MLR values of 0.08% (100 F-Cs) and 0.09% (10 D-As) were achieved at 2.5 vol% fibers, while peak RDME improvements reached 19.9% after 300 F-Cs and 20.0% after 30 D-As at 2.5 vol% fibers.
(3) After 100–300 NaCl F-Cs and 10–30 NaCl D-As, the flexural toughness of recycled aggregate concrete composite slab–column assemblies was significantly reduced, with impact toughness decreasing by 3.2%–19.4% and 3.3%–28.3%, respectively. The incorporation of 0.5–2.5 vol% basalt fibers increased impact toughness by 6.5%–62.0% under NaCl F-Cs and by 5.0%–83.7% under NaCl D-As. At 2.5 vol% fibers, impact toughness increased by up to 83.7% after 30 D-As, while maximum flexural toughness improvement reached 773.6% after 100 F-Cs. In addition, NaCl D-As had a more significant effect on chloride ion erosion in recycled aggregate concrete composite slab–column assemblies compared to NaCl F-Cs. The addition of basalt fibers significantly inhibited this erosive damage.
Comprehensive analysis indicates that basalt fiber content is the key controlling factor for enhancing chloride resistance performance of recycled aggregate concrete composite slab–column assemblies. Key performance indicators, such as axial compressive strength, RDME, and toughness, exhibit an apparent positive enhancement trend with increasing basalt fiber content under NaCl erosion environment. Among these, 2.5 vol% is the optimal content. Compared to NaCl F-Cs, the protective effect of basalt fiber is more pronounced in resisting NaCl D-As erosion. This study reveals the performance evolution of basalt fiber-reinforced recycled aggregate concrete composite slab–column assemblies in chloride salt environments, providing an important theoretical basis for developing fiber-reinforced design strategies targeting high-durability recycled aggregate concrete structures in coastal salt-erosion environments.

Author Contributions

Conceptualization, F.S. and Y.W.; methodology, D.W.; software, Z.X.; validation, H.W., C.L. and Z.X.; formal analysis, X.T.; investigation, Y.W.; resources, X.T.; data curation, N.X.; writing—original draft preparation, D.W.; writing—review and editing, D.W.; visualization, H.W.; supervision, Z.X.; project administration, D.W.; funding acquisition, H.W. and D.W. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by the 2023 Liaoning Provincial Science and Technology Programme Joint Programme Projects, grant number 2023JH2/101700001; the 2023 Liaoning Provincial Science and Technology Programme Joint Programme Projects, grant number 2023JH2/101700002; the 2024 Liaoning Provincial Department of Education Basic Research Project, grant number LJ212411779037; and the 2025 Liaoning Provincial Department of Education Basic Research Project, grant number LJ222511779001.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

The data used to support the findings of this study are available on request.

Conflicts of Interest

The authors declare no conflicts of interest.

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Figure 1. Test methods for composite slab–column assemblies: (a) displacement loading test; (b) impact test; (c) the test specimen.
Figure 1. Test methods for composite slab–column assemblies: (a) displacement loading test; (b) impact test; (c) the test specimen.
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Figure 2. The axial compressive strength of recycled aggregate concrete composite slab–column assemblies after NaCl F-Cs.
Figure 2. The axial compressive strength of recycled aggregate concrete composite slab–column assemblies after NaCl F-Cs.
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Figure 3. The MLR and RDME of recycled aggregate concrete composite slab–column assemblies after NaCl F-Cs: (a) MLR; (b) RDME.
Figure 3. The MLR and RDME of recycled aggregate concrete composite slab–column assemblies after NaCl F-Cs: (a) MLR; (b) RDME.
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Figure 4. The load-deflection curves and flexural toughness of recycled aggregate concrete composite slab–column assemblies after NaCl F-Cs: (a) load-deflection curves; (b) flexural toughness.
Figure 4. The load-deflection curves and flexural toughness of recycled aggregate concrete composite slab–column assemblies after NaCl F-Cs: (a) load-deflection curves; (b) flexural toughness.
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Figure 5. The impact toughness of recycled aggregate concrete composite slab–column assemblies after NaCl F-Cs.
Figure 5. The impact toughness of recycled aggregate concrete composite slab–column assemblies after NaCl F-Cs.
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Figure 6. The number of impacts and impact toughness index of recycled aggregate concrete composite slab–column assemblies after NaCl F-Cs: (a) number of impacts; (b) toughness index; (c) difference in impact counts.
Figure 6. The number of impacts and impact toughness index of recycled aggregate concrete composite slab–column assemblies after NaCl F-Cs: (a) number of impacts; (b) toughness index; (c) difference in impact counts.
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Figure 7. The SEM-EDS images of recycled aggregate concrete composite slab–column assemblies: (a) A1-before NaCl F-Cs; (b) A1-after NaCl F-Cs.
Figure 7. The SEM-EDS images of recycled aggregate concrete composite slab–column assemblies: (a) A1-before NaCl F-Cs; (b) A1-after NaCl F-Cs.
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Figure 8. The axial compressive strength of recycled aggregate concrete composite slab–column assemblies after NaCl D-As.
Figure 8. The axial compressive strength of recycled aggregate concrete composite slab–column assemblies after NaCl D-As.
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Figure 9. The MLR and RDME of recycled aggregate concrete composite slab–column assemblies after NaCl D-As: (a) MLR; (b) RDME.
Figure 9. The MLR and RDME of recycled aggregate concrete composite slab–column assemblies after NaCl D-As: (a) MLR; (b) RDME.
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Figure 10. The load-deflection curves and flexural toughness of recycled aggregate concrete composite slab–column assemblies after NaCl D-As: (a) load-deflection curves; (b) flexural toughness.
Figure 10. The load-deflection curves and flexural toughness of recycled aggregate concrete composite slab–column assemblies after NaCl D-As: (a) load-deflection curves; (b) flexural toughness.
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Figure 11. The impact toughness of recycled aggregate concrete composite slab–column assemblies after NaCl D-As.
Figure 11. The impact toughness of recycled aggregate concrete composite slab–column assemblies after NaCl D-As.
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Figure 12. The number of impacts and impact toughness index of recycled aggregate concrete composite slab–column assemblies after NaCl D-As: (a) number of impacts; (b) toughness index; (c) difference in impact counts.
Figure 12. The number of impacts and impact toughness index of recycled aggregate concrete composite slab–column assemblies after NaCl D-As: (a) number of impacts; (b) toughness index; (c) difference in impact counts.
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Figure 13. The SEM-EDS images of recycled aggregate concrete composite slab–column assemblies with and without fibers after NaCl D-As: (a) A1-after NaCl D-As; (b) A6-after NaCl D-As.
Figure 13. The SEM-EDS images of recycled aggregate concrete composite slab–column assemblies with and without fibers after NaCl D-As: (a) A1-after NaCl D-As; (b) A6-after NaCl D-As.
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Table 1. The chemical composition of OPC.
Table 1. The chemical composition of OPC.
Chemical Composition/%Median Particle Size
D50/μm
CaOSiO2Al2O3Fe2O3MgOMnOR2OSO3Loss on Ignition
62.5121.185.193.841.810.150.472.901.9518.6
Table 2. Relevant performance indicators of OPC.
Table 2. Relevant performance indicators of OPC.
TypeStrength Grade/MPaDensity/(g·cm−3)Initial Setting Time/minFinal Setting Time/min
OPC42.53.01112.3216
Table 3. Mix proportions for recycled aggregate concrete in composite slab–column assemblies (kg/m3).
Table 3. Mix proportions for recycled aggregate concrete in composite slab–column assemblies (kg/m3).
GroupWaterCementSandRecycled AggregateBasalt FibersWater
Reducing Agent		Basalt Fibers Content/%
A1175583.31648.50972.9500.750%
A2175583.31648.50972.9513.181.000.5%
A3175583.31648.50972.9526.351.251.0%
A4175583.31648.50972.9539.531.501.5%
A5175583.31648.50972.9552.701.752.0%
A6175583.31648.50972.9565.882.002.5%
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Wang, D.; Wu, Y.; Xu, Z.; Xu, N.; Li, C.; Tian, X.; Shi, F.; Wang, H. Effect of Basalt Fibers on the Performance of CO2-Cured Recycled Aggregate Concrete Composite Slab–Column Assemblies with Bolted Connections Under NaCl Erosion. Coatings 2025, 15, 1053. https://doi.org/10.3390/coatings15091053

AMA Style

Wang D, Wu Y, Xu Z, Xu N, Li C, Tian X, Shi F, Wang H. Effect of Basalt Fibers on the Performance of CO2-Cured Recycled Aggregate Concrete Composite Slab–Column Assemblies with Bolted Connections Under NaCl Erosion. Coatings. 2025; 15(9):1053. https://doi.org/10.3390/coatings15091053

Chicago/Turabian Style

Wang, Di, Yuanfeng Wu, Zhiqiang Xu, Na Xu, Chuanqi Li, Xu Tian, Feiting Shi, and Hui Wang. 2025. "Effect of Basalt Fibers on the Performance of CO2-Cured Recycled Aggregate Concrete Composite Slab–Column Assemblies with Bolted Connections Under NaCl Erosion" Coatings 15, no. 9: 1053. https://doi.org/10.3390/coatings15091053

APA Style

Wang, D., Wu, Y., Xu, Z., Xu, N., Li, C., Tian, X., Shi, F., & Wang, H. (2025). Effect of Basalt Fibers on the Performance of CO2-Cured Recycled Aggregate Concrete Composite Slab–Column Assemblies with Bolted Connections Under NaCl Erosion. Coatings, 15(9), 1053. https://doi.org/10.3390/coatings15091053

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