Experimental Study on the Damage Mechanism of Hybrid-Fiber-Reinforced Desert Sand Recycled Concrete Under Freeze–Thaw Cycles

Abstract

With the continuous growth of the demand for concrete in infrastructure construction, natural aggregate resources have become increasingly scarce. The preparation of concrete using desert sand and recycled aggregates has emerged as an effective approach to achieving the sustainable development of building materials. However, desert sand recycled concrete still confronts critical durability-related challenges when exposed to freeze–thaw conditions. We examined how hybrid fibers (steel fibers and hybrid PP fibers) affect the mechanical performance and freeze–thaw durability of desert sand recycled aggregate concrete, along with the underlying mechanisms. Mechanical properties (compressive, splitting tensile, flexural strength) and freeze–thaw damage indicators (mass loss, dynamic elastic modulus) were tested. The findings indicated that at a 30% desert sand replacement ratio, the concrete achieved optimal initial mechanical properties. For the hybrid fibers group (F0.15-S0.5) with 0.15% hybrid PP fibers and 0.5% steel fibers incorporated, relative to the control group, its compressive strength rose by 31.6%, while mechanical property loss was notably mitigated after 125 freeze–thaw cycles. Freeze–thaw damage models based on the exponential function and the Aas-Jakobsen function were established. Microscopic analysis indicated that the fibers effectively suppressed crack propagation and interfacial transition zone (ITZ) damage. This research offers critical experimental evidence and theoretical frameworks for the application of fiber-reinforced desert sand recycled concrete in cold-climate regions.

1. Introduction

The acceleration of the global urbanization process led to a sustained growth in the demand for concrete in infrastructure construction. Consequently, a series of resource and environmental issues arose, like the overharvesting of natural sand and gravel aggregates and the buildup of construction waste [1,2,3,4]. Against this backdrop, the preparation of concrete using desert sand and recycled construction aggregates offered an effective means of achieving the sustainable development of building materials. This approach not only alleviated the pressure of natural aggregate resource shortages but also contributed to reducing construction waste accumulation and land desertification, thereby demonstrating significant ecological and economic benefits [5,6,7,8,9,10,11].

However, the durability performance of desert sand recycled aggregate concrete (DSRAC) in cold regions, particularly its freeze–thaw resistance, confronted formidable challenges. The inherent pores and microcracks within the concrete deteriorated significantly under the action of freeze–thaw cycles. The stress generated by the repeated freezing and expansion of pore water caused microcracks to propagate from the surface into the interior, ultimately resulting in macroscopic cracking, spalling and even structural failure [12,13,14,15,16,17,18,19,20]. It is important to note that the long-term durability degradation of fiber-reinforced concrete under diverse severe service conditions represents a pervasive core challenge in the field. Cascardi et al. [21] demonstrated that alkaline exposure significantly weakens the interfacial bond between glass fibers and the cementitious matrix, resulting in a time-dependent decline in the constraint efficacy of fiber-reinforced composites. This further underscores the criticality of in-depth investigations into fiber–matrix interface damage mechanisms under specific environmental conditions, such as the freeze–thaw cycling addressed in this study, for ensuring the long-term structural integrity of fiber-reinforced concrete structures.

To enhance the freeze–thaw resistance of DSRAC, scholars explored various modification methods. Regarding fiber reinforcement, which was highly effective in inhibiting crack propagation, Ye et al. [22] verified that adding 0.15–0.20% basalt fibers notably enhanced the material’s overall performance under wind–sand erosion and salt–freeze cycles. The use of hybrid fibers was particularly promising due to the potential for synergistic effects; for instance, steel fibers excelled at bridging macro-cracks, while finer polymer fibers could control microcrack initiation. However, most existing research was limited to a single type of fiber. Separately, extensive research was conducted on the utilization of desert sand. At the macroscopic level, Zhang et al. [23] discovered through coupled experiments of chloride salt attack and freeze–thaw cycling that concrete with a 100% desert sand replacement rate exhibited superior performance compared to ordinary concrete after 200 cycles. Gong et al. [24,25] employed the Weibull distribution model to predict the freeze–thaw resistance life of desert sand concrete and identified that the optimal replacement rate range was 25–40%. Li et al. [26] examined the durability performance of aeolian sand concrete under the combined effect of carbonation and freeze–thaw cycling and developed a damage equation based on a parabolic model. Ji et al. [27] explored the impact of the combined application of recycled rubber and desert sand on the mechanical properties of concrete. At the microscopic scale, Luo et al. [28] confirmed that a 30% desert sand substitution ratio optimized the pore structure and reduced microcracks, and proposed a damage index based on fractal dimension. Yang et al. [29] revealed that 50% desert sand effectively suppressed the formation of gypsum and ettringite in the interfacial transition zone (ITZ) in a salt–freeze environment, enhancing its microscopic hardness and density. Bai et al. [30] developed a freeze–thaw damage degradation model linking macroscopic and microscopic perspectives, based on irreversible energy dissipation. Dong et al. [31] examined the capillary water absorption properties under the combined effect of a sulfate attack and freeze–thaw cycling via fractal theory. Wang et al. [32] explored the freeze–thaw resistance of desert sand in 3D-printed concrete.

Our previous research [33] systematically explored the mechanical performance of steel–hybrid PP fiber-reinforced DSRAC at an ambient temperature and determined that a 30% desert sand replacement rate and a hybrid combination of 0.15% hybrid PP fibers and 0.5% steel fibers (F0.15-S0.5) were the optimal parameters. However, this work had notable limitations. Firstly, the research was confined to a static normal-temperature environment, failing to unveil the performance degradation and damage evolution of the material under freeze–thaw cycles. Secondly, no quantitative damage model was established, making it challenging to support the durability design of structures in cold regions. Thirdly, the 30% desert sand replacement rate was optimized solely based on normal-temperature strength, and its durability performance under freeze–thaw conditions remained to be verified. These limitations, together with the inadequacies of the aforementioned single-fiber studies, collectively constituted the crucial research gaps in the current context.

Nevertheless, the utilization of desert sand was not a straightforward one-to-one substitution. Its ultra-fine particle size and relatively smooth surface morphology introduced distinct microstructural effects and interfacial challenges to concrete [34]. On one hand, fine particles might exert a positive micro-filling effect, refining the matrix pore structure; on the other hand, their high specific surface area could increase water demand or impair workability, while the smooth surface might weaken the chemo-mechanical bonding with cement paste, reducing the inherent strength of the interfacial transition zone (ITZ). In fiber-reinforced systems, the interplay between the weakening of the matrix’s native ITZ and the performance of the fiber–matrix interface emerged as a critical yet unresolved scientific issue [35,36]. Notably, in freeze–thaw environments, water migration was highly dependent on the material’s pore structure and interfacial defects [20]. Thus, the incorporation of desert sand might either improve the baseline freeze–thaw resistance by refining pores or exacerbate the accumulation of frost damage by introducing additional weak interfaces. The core challenge for enhancing the durability of desert sand recycled concrete was how to leverage the benefits while mitigating the drawbacks of desert sand via fiber reinforcement in such a complex interfacial matrix. Based on the above analysis, this study posited that simply optimizing the desert sand replacement ratio might be insufficient to offset the interfacial disadvantages it introduced. Fiber reinforcement, particularly hybrid fiber strategies, offered a viable pathway for actively regulating and strengthening this fragile system. Investigating whether and how hybrid fibers could overcome or compensate for interfacial defects induced by desert sand characteristics, thereby enhancing freeze–thaw durability, was one of the core mechanistic questions this study sought to address.

Extensive studies have confirmed that the composite incorporation of rigid fibers (e.g., steel fibers) and flexible fibers (e.g., polypropylene fibers, basalt fibers, PVA fibers) into concrete yielded significant synergistic effects. The general mechanism was as follows: rigid fibers could effectively bridge and constrain the propagation of macroscopic cracks, providing post-peak load-bearing capacity; flexible fibers, by contrast, could disperse stress, inhibit the initiation of microcracks and enhance matrix toughness. However, the existing research on hybrid fiber’s synergistic effects had predominantly focused on ordinary aggregate concrete or systems with specific single alternative materials. Concrete prepared from the complex multiphase system integrating desert sand (ultra-fine particles) and recycled coarse aggregates (coated with old mortar) exhibited numerous internal interfaces and defects, resulting in inherently poor freeze–thaw resistance. For such a fragile matrix, the following questions remained unsystematically addressed: whether the aforementioned synergistic enhancement principle still held; how to quantitatively characterize its enhancement efficiency; and what the specific mechanisms were by which fibers inhibited the freeze–thaw damage process. Therefore, to address this gap, this study systematically investigates the damage mechanism of hybrid-fiber-reinforced DSRAC under freeze–thaw cycles, based on the aforementioned optimal mix proportion. The specific research objectives are outlined below:

(1)

Evaluate and compare the freeze–thaw resistance of DSRAC with hybrid fibers against plain and single-fiber-reinforced counterparts, based on macroscopic performance indicators;

(2)

Establish freeze–thaw damage models built upon the exponential function and Aas-Jakobsen function, and characterize the stress–strain relationship evolution using Guo’s constitutive model;

(3)

Reveal the toughening and crack-arresting mechanisms of fibers at the microscopic scale. This work aims to provide a crucial theoretical basis and practical guidance for the engineering application of fiber-reinforced DSRAC in cold regions.

2. Materials and Methods

2.1. Material

The basic materials used in this research included P·O 42.5 ordinary Portland cement, 5–20 mm recycled coarse aggregates, sand from the Taklimakan Desert in Xinjiang, wave-milled steel fibers and hybrid PP fibers. Specifically, the cement met the requirements of ASTM C150 [37] for ordinary Portland cement. The chemical composition is presented in

Table 1. Cement chemical composition (mass fraction, %).

Composition	CaO	SiO2	SO3	Al2O3	MgO	Fe2O3	Other
Mass fraction/%	60.2	21.1	2.8	8.8	1.4	3.7	2.0

, below. To clearly demonstrate the crucial performance indicators of the materials employed in this research, their main physical and mechanical properties were systematically collated and summarized, as presented in

Table 2. Desert sand composition (mass fraction, %).

Composition	SiO2	CaO	Al2O3	Fe2O3	MgO	K2O	Na2O	Ti2O	Other
Content	56.42	15.30	10.27	2.53	2.13	2.33	2.10	0.35	8.57

,

Table 3. Coarse and fine aggregates’ physical properties and mechanical properties.

Category	Particle Size (mm)	Fineness	Apparent Density (kg/m3)	Bulk Density (kg/m3)	Mud Content (%)	Hygroscopic Rate (%)	Crushing Index (%)
Recycled aggregate	5–20	-	2650	1300	0.44	2.31	20.13
River sand	0–4.75	2.75	2580	1430	0.30	1.26	-
Desert sand	<0.18	0.26	2590	1570	0.17	1.97	-

and

Table 4. Physical parameters of fibers.

Steel Fiber		Hybrid PP Fiber
Length	30 mm	Length	54 mm
Type	wavy-milled type	Specific gravity	0.91
Tensile strength	600 MPa	Tensile strength	690 MPa
Elastic modulus	200 GPa	Elastic modulus	4.7 GPa
Aspect ratio	12	Aspect ratio	159

. For a more in-depth exploration of the parameters, interested readers were referred to our previously published work [33]. The synthetic fiber employed in this study was a blend of twisted-bundle macro-synthetic fibers and fibrillated polypropylene fibers, which were primarily composed of virgin copolymer and polypropylene. This fiber was designed to integrate the macroscopic bridging effect of coarse fibers and the microcrack-inhibiting capacity of fine fibers. For brevity in subsequent sections, it was abbreviated as “hybrid PP fibers”.

As presented in Table 2, the chemical composition of the employed desert sand is dominated by SiO₂, with notably high contents of CaO and Al₂O₃. This compositional characteristic implies potential pozzolanic activity [38]. While the present study primarily focuses on its macroscopic effects as an aggregate, its chemical properties are recognized as a critical context for subsequent in-depth investigation of microscale mechanisms.

2.2. Mix Design

The mix-proportion design of this study was founded upon the systematic parametric research that was previously published by the author’s research group [33]. In that earlier investigation, we systematically explored the impacts of various desert–sand replacement ratios (0%, 30%, 50%, 70%, 100%) and different fiber dosages (both single-fiber and hybrid-fiber configurations) on the mechanical properties of desert–sand recycled–aggregate concrete (DSRAC) at a normal temperature. The findings of this research clearly demonstrated that when 30% of the sand was replaced with desert sand, the concrete manifested the optimal baseline mechanical properties. Additionally, the group with hybrid fibers F0.15-S0.5 (comprising 0.15% hybrid PP fibers and 0.5% steel fibers) exhibited a significantly more pronounced reinforcement effect compared to any single-fiber type or other dosage combinations.

Consequently, the objective of this study was not to reiterate the parameter screening process but rather to conduct an in-depth exploration of the damage mechanism and performance evolution patterns of this material system under freeze–thaw cycles, building upon the optimally determined benchmarks from the previous research. Based on this premise, the experimental design of this study was characterized by a distinct focus and logical structure.

To accurately assess the sensitivity of the freeze–thaw performance to fluctuations in the desert–sand content around the pre-determined optimal value of 30%, we established three replacement ratio groups with 25%, 30% and 35% replacement levels. This approach, in contrast to including a group with a 0% replacement ratio (DS0), was more effective in uncovering the internal performance change patterns within the DSRAC system and precisely identifying the optimal desert–sand dosage under freeze–thaw conditions.

To directly evaluate the degree of improvement in the freeze–thaw durability and the underlying synergy mechanism upon the introduction of the optimal fiber combination into the pre-determined optimal matrix (DS30), we retained only the most effective hybrid-fiber group, F0.15-S0.5. This decision was made with the intention of emphasizing the verification of the effectiveness of this optimal combination in durability-related scenarios, rather than re-validating the already-established effects of single fibers.

In conclusion, the four mix-proportion designs in this study (DS25, DS30, DS35, F0.15-S0.5) were capable of effectively addressing the following core scientific inquiries: (1) Did the optimal desert–sand dosage remain optimal under freeze–thaw conditions? (2) Could the optimal fiber combination effectively enhance the durability of the optimal matrix in freeze–thaw environments? This focused experimental design enabled a more profound research approach and led to more conclusive and distinct findings. The experimental mix proportions are listed in

Table 5. Mixture proportion design.

Number	Mix Material Dosage (kg/m3)					Fiber Incorporation Amount (%)
	Cement	Water	Sand	Desert Sand	Recycled Aggregate	Steel Fiber	Hybrid PP Fiber
DS25	414	219	573	191	1342	-	-
DS30	414	219	534.8	229.2	1342	-	-
DS35	414	219	496.6	267.4	1342	-	-
F0.15-S0.5	414	219	534.8	229.2	1342	0.5	0.15

Note: In “DS30”, the “30” denoted that the replacement ratio of the desert sand was 30%. The same logic applied to other designations. In the label “F0.15-S0.5”, “F0.15” denotes a 0.15% volumetric dosage of hybrid PP fibers in desert sand recycled aggregate concrete (DSRAC) and “S0.5” represents a 0.5% volumetric dosage of steel fibers in the same material.

.

Given that the specific gravity of the employed hybrid PP fibers (0.91) was lower than that of water, a standardized mixing protocol was implemented in this study to prevent fiber floating and segregation during mixing, as well as to ensure uniform fiber dispersion within the concrete matrix. The protocol was specified as follows: a forced-action mixer was utilized; feeding sequence: all dry ingredients (cement, sand, desert sand, recycled coarse aggregate) were first introduced into the mixer and dry-mixed for 60 s to achieve homogeneous blending; approximately 80% of the mixing water was then added, followed by wet-mixing for 90 s; hybrid PP fibers and steel fibers were uniformly dispersed and sprinkled into the mixer, with mixing being continued for 60 s; finally, the remaining 20% of the mixing water (intended to rinse fibers adhering to the mixing blades and drum inner walls) was added, and mixing was prolonged for 120 s until a homogeneous mixture free of fiber agglomeration was obtained. The total duration of the mixing process was strictly controlled. Observations indicated that this protocol effectively mitigated fiber floating; the molded specimens exhibited a uniform appearance, with no evident fiber stratification or localized enrichment.

2.3. Specimen Preparation

This study followed the criteria outlined in the standard “GB/T 50082-2009” [39]. The rapid freeze–thaw protocol was employed to perform freeze–thaw cycling tests on desert sand recycled aggregate concrete (DSRAC) specimens. The cycle counts were set to 0, 25, 50, 75, 100 and 125, respectively. After 24 days of standard curing, the specimens were extracted and soaked in 20 °C water for 4 days—during immersion, the water level was consistently kept 20–30 mm above the specimen top surface. At the 28-day curing age, the specimens were taken out of the water and their surface moisture was wiped dry. An ACS-30 electronic balance (precision: 0.1 g) was used to weigh the specimens. Meanwhile, a DT-20 dynamic elastic modulus tester was utilized to measure the specimens’ natural vibration frequency via the forced resonance method. Next, the specimens were placed into a TDR-3 rapid freeze–thaw testing device (Figure 1) for cycling. Throughout the experiment, the core temperature was regulated to fluctuate between −17.0 °C ± 2.0 °C and 8.0 °C ± 2.0 °C. After every 25 freeze–thaw cycles, each group of specimens was tested for mass loss rate, dynamic elastic modulus, compressive strength, splitting tensile strength and flexural strength.

2.4. Test Methods

Following the guidelines of GB/T 50081-2019 [39], this research utilized 100 × 100 × 100 mm cubic specimens for both the compressive strength and splitting tensile strength tests. For the flexural strength test, 100 × 100 × 400 mm prismatic specimens were used; the test loading rate, along with the mechanical property-testing instruments and loading setups, were consistent with the parameters employed in our team’s prior study [33].

The mass loss rate calculation formula for DSRAC post-freeze–thaw cycles is given in Equation (1).

$$\mathsf{\Delta }{W}_{\mathrm{ni}}={\displaystyle \frac{W_{0i}-W_{\mathrm{ni}}}{W_{0i}}}·100$$

(1)

where $\mathsf{\Delta }{W}_{\mathrm{ni}}$ represents the mass loss rate of the i-th concrete specimen post N freeze–thaw cycles (%); $W_{0i}$ represents the initial mass of the i-th concrete specimen before freeze–thaw cycle testing (g); and $W_{\mathrm{ni}}$ represents the post-N-freeze–thaw-cycle mass of the i-th concrete specimen (g).

The calculation methodology for the relative dynamic elastic modulus of desert sand recycled concrete following freeze–thaw cycles was presented as Equation (2) [39].

$$P_i={\displaystyle \frac{f_{ni}^2}{f_{0i}^2}}·100$$

(2)

In the equation: $P_i$ denotes the relative dynamic elastic modulus of the i-th concrete specimen post N freeze–thaw cycles; $f_{ni}^2$ represents the transverse fundamental frequency (Hz) of the i-th concrete specimen post N freeze–thaw cycles; and $f_{0i}^2$ represents the initial transverse fundamental frequency (Hz) of the i-th concrete specimen pre freeze–thaw cycle testing.

The relative dynamic elastic modulus calculation formula for desert sand recycled aggregate concrete (DSRAC) post-freeze–thaw cycles was provided in Equation (3) [39].

$$E_{\mathrm{dtn}}=9.464\times {10}^{-7}{\displaystyle \frac{W_{\mathrm{n}}{L}_{\mathrm{n}}^{3}T}{b_{\mathrm{n}}{h}_{\mathrm{n}}^3}}·f_{\mathrm{n}}^2$$

(3)

where f represents the natural vibration frequency (unit: Hz) and W denotes the specimen mass (unit: kg). Here, the subscript “0” indicates 0 freeze–thaw cycles and “n” denotes the freeze–thaw cycle count; L, b and h correspond to the specimen’s length, width and height, respectively (unit: mm); T is a size- and Poisson’s ratio-related correction factor for the specimen. According to the specification [39], T took the value of 1.40.

2.5. Damage Model and Analytical Method

2.5.1. Exponential Function Freeze–Thaw Damage Model

Through analysis of the relative dynamic elastic modulus variation pattern for hybrid-fiber-reinforced DSRAC specimens, it was found that the relative dynamic elastic modulus exhibited a decreasing trend as the number of freeze–thaw cycles increased and the curve exhibited certain non-linear characteristics. Therefore, an exponential decay function was employed to construct the freeze–thaw damage model. The specific formula was as shown in Equation (4).

$$P(n)=A{e}^{-{\displaystyle \frac{n}{B}}}+C$$

(4)

where P(n) represents the specimen’s relative dynamic elastic modulus post-n freeze–thaw cycles, in percentage (%) (dimensionless parameter, reflecting the integrity of the material internal structure after freeze–thaw); n represents the freeze–thaw cycle count, in cycle (experimental variable, with values of 0, 25, 50, 75, 100, 125 cycles in this study); A represents the initial difference relative to the dynamic elastic modulus (when n = 0) and the final stable value, in percentage (%) (dimensionless parameter, reflecting the total attenuation amplitude of the material’s elastic modulus from the initial state to the stable state after late freeze–thaw); B represents the attenuation constant, in cycle (determining the rate at which the relative dynamic elastic modulus declines as the freeze–thaw cycle count increases; a higher B value denotes a slower attenuation rate and superior freeze–thaw resistance of the material); C represented the relative dynamic elastic modulus’ stable value in the late freeze–thaw cycle stage, in percentage (%) (dimensionless parameter, which reflects the lower limit of the relative dynamic elastic modulus when the material’s internal damage saturates, following repeated freeze–thaw cycles).

2.5.2. Aas-Jakobsen Freeze–Thaw Damage Model

Compared with other existing freeze–thaw damage models, such as the empirical models, probabilistic damage models and fatigue damage models proposed in previous studies [40,41,42,43,44,45,46,47], the Aas-Jakobsen function model [48] was a classic form of the freeze–thaw damage model. Its core concept is to characterize the evolution law of damage parameters with a freeze–thaw cycle count via a power–law relationship. Considering the characteristics of hybrid-fiber-reinforced DSRAC, by introducing influence parameters such as fiber type, dosage and recycled aggregate replacement rate, a non-linear correlation between the damage parameter and freeze–thaw cycle count was constructed. The model was shown in Equation (5), as follows.

$${\psi }_n={\displaystyle \frac{1}{1+{(\frac{n}{n0})}^{-m}}}\times 100\%$$

(5)

In Equation (5), ${\psi }_n$ denoted the dynamic elastic modulus loss rate, n represented the freeze–thaw cycle count, n0 represented the characteristic number of cycles and m was the shape parameter.

2.5.3. Stress–Strain Constitutive Relationship

Prior to this, researchers worldwide had obtained extensive findings regarding the mathematical modeling of concrete’s uniaxial compressive stress–strain relationship under ambient temperature conditions. Representative models included those proposed by Hognestad, Guo and Saenz et al. [48,49,50,51,52,53,54]. Among these, the constitutive model developed by Guo [49] gained the broadest applications. For the stress–strain relationship of recycled desert sand concrete, this characterization approach delivered higher precision and could more effectively capture the material’s actual mechanical behavior under diverse working conditions. Consequently, it offered a more robust foundation for assessing and forecasting the performance of hybrid-fiber-reinforced recycled DSRAC.

The detailed model expression was presented in Equation (6).

$$y=\left\{\begin{array}{c}ax+(3-2a)x^2+(a-2)x^3, 0\le x\le 1\\ {\displaystyle \frac{x}{b{(x-1)}^2+x}}, x\ge 1\end{array}\right.$$

(6)

In the equation, $x=\epsilon /{\epsilon }_c, y=\sigma /{\sigma }_c$, where ${\epsilon }_c$ represented the strain and ${\sigma }_c$ represented the stress.

2.6. Microscopic Morphology Analysis

To explore the influence of freeze–thaw cycles on the microstructural characteristics of materials, this study employed scanning electron microscopy (SEM) to perform micromorphological observations on the selected test specimens. The samples for SEM analysis were sourced from the inner core regions of the specimens following mechanical property tests. This approach was adopted to mitigate the influence of the cutting surface. The samples underwent a series of preparatory treatments, including drying and sputter coating with gold, before being positioned within the vacuum sample chamber. During SEM observation, the main accelerating voltage was adjusted to 15.0 kV, while the working distance was kept at roughly 15 mm. This study focused primarily on the microstructural evolution of the hybrid-fiber-reinforced group (F0.15-S0.5) under different freeze–thaw cycle counts (0, 25, 50, 75, 100, 125 cycles). For the sample that had not undergone any freeze–thaw cycles (0 cycles), a magnification of 500× was employed to display the initial microstructure. For specimens that had undergone freeze–thaw cycles, all observations were performed at a consistent magnification of 200×. This standardization was implemented to guarantee the direct comparability of damage patterns across different freeze–thaw cycles.

3. Results and Discussion

3.1. Analysis and Discussion of the Results of Mechanical Properties

Drawing on the mechanical property test data of DSRAC following freeze–thaw cycles (

Table 6. Mechanical property test data of DSRAC following freeze–thaw cycles.

Sample	Freeze–Thaw Cycle Count	Compressive Strength (MPa)	Standard Deviations	Splitting Tensile Strength (MPa)	Standard Deviations	Flexural Strength (MPa)	Standard Deviations
DS25	0	37.9	0.6	2.78	0.08	5.43	0.15
	25	30.4	0.8	2.28	0.07	4.78	0.12
	50	24.9	1.0	1.87	0.09	4.47	0.18
	75	14.8	1.2	1.44	0.1	4.43	0.2
	100	8.1	0.9	1.37	0.12	3.44	0.25
	125	5.4	0.7	1.05	0.11	2.67	0.22
DS30	0	39.0	0.5	3.04	0.09	5.95	0.14
	25	33.3	0.7	2.89	0.08	5.08	0.13
	50	30.0	0.9	2.67	0.1	4.74	0.16
	75	18.7	1.1	1.77	0.11	4.47	0.19
	100	10.8	1.0	1.67	0.13	3.52	0.24
	125	7.1	0.8	1.10	0.1	2.58	0.21
DS35	0	37.4	0.7	2.55	0.1	5.35	0.16
	25	25.1	1.0	1.78	0.09	4.78	0.17
	50	17.2	1.3	1.51	0.11	3.73	0.22
	75	9.5	1.1	1.33	0.12	3.29	0.26
	100	7.0	0.9	1.26	0.14	2.09	0.28
	125	4.3	0.6	1.02	0.11	1.01	0.2
F0.15-S0.5	0	42.1	0.8	4.31	0.12	6.9	0.18
	25	40.1	0.9	4.07	0.11	5.9	0.16
	50	37.7	1.0	3.62	0.13	5.3	0.19
	75	31.6	1.2	3.24	0.14	4.8	0.21
	100	24.2	1.3	2.62	0.15	4.0	0.25
	125	11.3	1.1	1.72	0.13	3.7	0.23

, Figure 2), this study systematically uncovered the influence patterns of varying desert sand substitution rates and hybrid fiber reinforcement on the freeze–thaw durability of the material. Herein, DS25, DS30 and DS35 represented the control groups with desert sand replacement ratios of 25%, 30% and 35%, respectively (without fiber incorporation), while F0.15-S0.5 denoted the enhanced group with the incorporation of 0.15% hybrid PP fibers and 0.5% steel fibers, based on the 30% replacement ratio.

Firstly, novel findings of engineering significance emerged in the discussion regarding the optimal desert sand replacement rate. In the initial state, the DS30 group did indeed demonstrate the most favorable baseline mechanical properties (compressive strength of 39.0 MPa). This was consistent with the conclusions of our previous room-temperature mechanical studies [33], indicating that a 30% desert sand content could effectively optimize the particle gradation and enhance the initial compactness. However, in the freeze–thaw environment, a crucial shift occurred in the material’s performance degradation pattern. Although the DS30 group exhibited the highest initial strength, its performance degradation rate notably accelerated in the middle–late phase of the freeze–thaw cycles (following 75 cycles). Eventually, after 125 cycles, its strength-loss rate (81.8%) was comparable to that of the DS25 group (85.8%). The DS25 group, on the other hand, demonstrated a more stable degradation trend throughout the entire freeze–thaw process. This phenomenon implied that for engineering scenarios that prioritized long-term durability over mere initial strength, a 25% desert sand replacement rate might be a more reliable option. It was hypothesized that the slightly lower desert sand content reduced the number of capillary pores introduced by ultra-fine particles, thereby decreasing the source of frost heave stress. In stark contrast, the DS35 group performed the poorest across all freeze–thaw stages, clearly indicating that an overly high desert sand content (35%) could severely impair the concrete’s freeze–thaw resistance, due to excessive cement paste and unbalanced gradation.

Secondly, the core finding and principal contention of this study were that the incorporation of hybrid fibers (F0.15-S0.5) had realized a synergistic improvement in both the mechanical properties and freeze–thaw durability of DSRAC. The addition of fibers led to a 31.6% increase in the initial compressive strength. This may be ascribed to the bridging effect of the fibers within the matrix, which effectively transferred the load. After 125 freeze–thaw cycles, all strength indicators for the F0.15-S0.5 group were significantly higher than those of all control groups. More significantly, its strength-loss rates (compressive strength loss of 73.2%, splitting tensile strength loss of 60.1%, flexural strength loss of 46.4%) were minimized. As is evident from Figure 2, in the late phase of the freeze–thaw cycles, the splitting tensile strength and flexural strength of the fiber-reinforced group exhibited optimal retention. This provided compelling evidence that the fibers, through their “bridging” mechanism, effectively suppressed the propagation and connection of microcracks, delaying the transition of the material from ductility to brittleness and thus manifesting excellent mechanical properties at the macroscopic level.

The F0.15-S0.5 group exhibited an extremely low mass-loss rate (0.34%) and a relatively high residual dynamic elastic modulus, which reveals the synergistic protective effect of hybrid fibers under freeze–thaw conditions. This synergistic effect arises from the complementarity of the two fibers in modulus, size and function; steel fibers, as a rigid reinforcing phase, primarily serve for internal confinement. They can effectively bridge and span potentially propagating macroscopic cracks, transferring stress from cracked regions to the undamaged matrix, thereby maintaining the overall structural continuity and load-bearing capacity of the specimens during freeze–thaw cycles. Hybrid PP fibers, as a flexible reinforcing phase, mainly function in microcrack control for the surface and near-surface regions. Their lower elastic modulus allows for activation at smaller strains, effectively dispersing the freeze–thaw stress and inhibiting the initiation and early propagation of microcracks within the mortar matrix. The random distribution of a large number of fine fibers in the surface layer forms a dense micro-reinforcement network, which significantly blocks crack connectivity and the sheet-like spalling of surface mortar, directly contributing to the extremely low mass loss. The stability of the “macroscopic skeleton” provided by the steel fibers offers a solid substrate for the surface hybrid PP fiber network to exert its micro-damage suppression effect. The combined action of both enables multi-scale damage control from macro to micro and from interior to surface.

In conclusion, through a systematic analysis of the mechanical properties that followed freeze–thaw cycles, this study unveiled the following core principles: Although the DS30 group exhibited the optimal initial mechanical properties, its performance degradation rate during the mid-to-late stages of freeze–thaw cycles was relatively rapid. This indicated that in scenarios where the utmost durability was sought, DS25 might represent a more robust alternative. The comprehensive deterioration of the DS35 group served as a cautionary reminder of the adverse impact of an overly high desert sand replacement ratio on freeze–thaw resistance. More significantly, by directly comparing the hybrid-fiber-reinforced group (F0.15-S0.5) with its corresponding reference group (DS30), this study provided conclusive evidence of the synergistic reinforcement effect of fibers. After undergoing 125 rigorous freeze–thaw cycles, the residual compressive strength of the F0.15-S0.5 group (11.3 MPa) was 59.2% higher than that of the DS30 group (7.1 MPa). Moreover, its residual splitting tensile strength (1.72 MPa) was 56.4% higher than that of the DS30 group (1.10 MPa). Such a substantial improvement in the performance retention rate was not merely a marginal enhancement but rather a testament to the remarkable effectiveness of this specific fiber combination in inhibiting freeze–thaw damage. Consequently, the central conclusion of this section was that in the optimized desert sand recycled concrete matrix (with a 30% replacement rate), the incorporation of F0.15-S0.5 hybrid fibers could generate a significant synergistic effect through their bridging and crack-arresting mechanisms. This effectively enhanced the material’s long-term ability to maintain mechanical properties in freeze–thaw environments. This finding provided a critical technical foundation for the application of fiber-reinforced desert sand recycled concrete in cold regions.

3.2. Surface Deterioration Characteristics After Freeze–Thaw Cycles

Based on the observations of the surface damage characteristics of recycled concrete with a 30% desert sand content under different freeze–thaw cycle counts (Figure 3), it was possible to clearly identify the process by which surface deterioration gradually intensified as the number of cycles increased. During the initial stage of the freeze–thaw cycles (25 cycles), microcracks and local minor spalling began to emerge on the surface of the specimens. When the cycle count reached 50, the surface roughness rose sharply, while the severity of crack propagation and spalling became more distinct. By 75 and 100 cycles, further damage accumulation was evident, characterized by the extensive exposure of aggregates and severe spalling of the surface mortar layer. Finally, after 125 freeze–thaw cycles, the specimen surfaces were almost completely destroyed and the structural integrity was lost. This phenomenon clearly demonstrates the cumulative damage mechanism of freeze–thaw actions on the surface properties of DSRAC. The destruction process was initiated with the formation and propagation of microcracks, ultimately leading to the failure of the macroscopic structure.

Figure 4 illustrates the surface damage evolution of hybrid-fiber-reinforced DSRAC (F0.15-S0.5) throughout the freeze–thaw cycling process. As the freeze–thaw cycle count increased, the specimen surfaces gradually shifted from mild to moderate deterioration. Nevertheless, the overall structure maintained a relatively high level of integrity throughout the process. In the early phase of freeze–thaw cycles (at 25 cycles), merely a few fine cracks appeared in the fiber-reinforced group, with no distinct spalling detected. By the 50-cycle mark, although the cracks had developed to a certain extent, the spalling phenomenon remained insignificant. After undergoing 75–100 cycles, minor surface spalling occurred locally, yet no large-scale structural damage was detected. Even when subjected to 125 freeze–thaw cycles, the specimen surfaces still retained a relatively intact state. The main manifestations were local mortar abrasion and limited crack propagation, and the overall morphology was far superior to that of ordinary DSRAC. These phenomena suggested that the addition of hybrid fibers notably suppressed the progression of freeze–thaw-induced surface damage. Through mechanisms such as bridging and restricting the propagation of microcracks and dispersing the freeze–thaw stress, the fibers effectively delayed the occurrence of surface spalling and the spread of cracks, thus maintaining the apparent form and structural durability of the concrete.

3.3. Analysis of the Mass Loss Ratio of Hybrid-Fiber-Reinforced DSRAC After Freeze–Thaw Cycles

Drawing on the post-freeze–thaw mass loss rate data of DSRAC in

Table 7. Mass-loss rate of desert sand recycled concrete, post-freeze–thaw cycling.

Number of Freeze–Thaw Cycles	Percentage Mass Loss (%)
	DS30	Standard Deviations	F0.15-S0.5	Standard Deviations
0	0	0	0	0
25	−0.06	0.02	−0.10	0.03
50	0.43	0.05	0.03	0.02
75	0.92	0.08	0.06	0.02
100	1.35	0.12	0.11	0.03
125	2.83	0.25	0.34	0.05

and the freeze–thaw cycle count-mass loss rate correlation for DSRAC, as illustrated in Figure 5, during the early freeze–thaw phase (25 cycles), both DS30 and F0.15-S0.5 specimens showed mild negative mass losses (−0.06% and −0.10%, respectively). This could be attributed to the temporary retention of moisture on the concrete surface caused by freeze–thaw migration, which led to a slight increase in the apparent mass. The incorporation of fibers enhanced the compactness of the matrix to a certain degree, rendering this phenomenon more evident. With the increase in the freeze–thaw cycle count, the internal damage of the material grew more severe, and the mass losses all changed from negative to positive. For the DS30 group without fiber reinforcement, the mass loss increased significantly, reaching 2.83% after 125 cycles. This indicated that the internal cracks continued to expand and the surface mortar spalled severely. In contrast, the fiber-reinforced group (F0.15-S0.5) experienced an extremely slow mass loss throughout the entire freeze–thaw process. After 125 cycles, the loss was only 0.34%, demonstrating excellent anti-spalling properties and durability. These findings suggested that the hybrid fibers effectively suppressed the initiation and propagation of cracks, notably alleviating the material loss caused by the freeze–thaw actions. As a result, they enhanced the mass stability and long-term durability of DSRAC in freeze–thaw environments.

3.4. Analysis of the Relative Dynamic Elastic Modulus

Drawing on the post-freeze–thaw relative dynamic elastic modulus test data of DSRAC (

Table 8. Post-freeze–thaw relative dynamic elastic modulus test data of DSRAC.

Number of Freeze–Thaw Cycles	Relative Dynamic Modulus of Elasticity (%)
	DS30	Standard Deviations	F0.15-S0.5	Standard Deviations
0	100	0	100	0
25	93.0	1.2	96.9	0.8
50	83.2	1.8	94.3	1.0
75	82.6	2.1	86.6	1.3
100	75.9	2.5	83.0	1.5
125	65.7	3.0	72.9	1.8

, Figure 6), it can be concluded that throughout the freeze–thaw cycling process, the relative dynamic elastic modulus of specimens across all groups decreased gradually as the number of cycles increased. However, the decline rate of the fiber-reinforced group (F0.15-S0.5) was notably lower than that of the fiber-free reference group (DS30). After 125 freeze–thaw cycles, the relative dynamic elastic modulus of the DS30 group dropped to 65.7%, with a cumulative loss rate of 34.3%. In comparison, the F0.15-S0.5 group still maintained a value of 72.9%, corresponding to a cumulative loss of only 27.1%. Particularly during the early to middle stages of freeze–thaw cycling (25–75 cycles), the modulus retention rate of the fiber-reinforced group was significantly higher, which indicates that the degree of internal structural damage was milder.

3.5. Analysis of the Dynamic Elastic Modulus

Based on the post-freeze–thaw dynamic elastic modulus test data of DSRAC (

Table 9. Post-freeze–thaw dynamic elastic modulus test data of DSRAC.

Number of Freeze–Thaw Cycles	Dynamic Elastic Modulus (GPa)
	DS30	F0.15-S0.5
0	32.43	34.17
25	30.77	33.12
50	26.98	32.23
75	26.78	29.60
100	24.62	28.36
125	21.32	24.91

, Figure 7), prior to freeze–thaw cycling, the dynamic elastic modulus of the fiber-reinforced group (F0.15-S0.5) was 34.17 GPa, which exceeded the 32.43 GPa of the fiber-free group (DS30). This suggests that fiber incorporation enhanced the initial stiffness of the material. As freeze–thaw cycling proceeded, the dynamic elastic moduli of both specimen groups decreased gradually, yet the reduction rate of the fiber-reinforced group was notably smaller. After 125 cycles, the dynamic elastic modulus of the DS30 group dropped to 21.32 GPa, corresponding to a cumulative loss rate of 34.3%. By contrast, the F0.15-S0.5 group maintained a value of 24.91 GPa, with a loss rate of 27.1%. Particularly in the middle to late stages of freeze–thaw cycling (after 50 cycles), the modulus retention rate of the fiber-reinforced group was significantly higher than that of the reference group, indicating slower progression of internal damage.

3.6. Freeze–Thaw Damage Model of Exponential Function

By means of Equation (4), an exponential freeze–thaw damage model was derived for hybrid-fiber-reinforced desert sand recycled concrete.

Table 10. Parameter set of the freeze–thaw damage model with exponential function for hybrid-fiber-reinforced DSRAC.

Sample	Fitting Parameters			R2
	A	B	C
DS30	−248.46	1034.29	347.557	0.96
F0.15-S0.5	−9.28	92.08	109.273	0.98

presents the parameter values of this exponential freeze–thaw damage model corresponding to the aforementioned concrete. Figure 8 illustrates the fitting curve between the freeze–thaw cycle count and relative dynamic elastic modulus. As can be inferred from the table and figure, after freeze–thaw cycles, the experimental data points of the desert sand recycled concrete specimens, either without fiber addition or with fiber reinforcement, are evenly distributed on both sides of the fitting curve. The coefficients of determination (R²) are 0.966 and 0.989, respectively. This suggests that the exponential decay-based freeze–thaw damage model can precisely predict the correlation between the freeze–thaw cycle count and the relative dynamic elastic modulus of fiber-reinforced desert sand recycled concrete. Clearly, the model exhibits strong explanatory power and sound reliability.

3.7. Freeze–Thaw Damage Model of Hybrid-Fiber-Reinforced DSRAC, Based on the Aas-Jakobsen Function

Based on Equation (5), through non-linear fitting, the parameters were as shown in

Table 11. Parameter set of Aas-Jakobsen function-based freeze–thaw damage model.

Sample	n0	m	R2
DS30	150	2.5	0.96
F0.15-S0.5	280.26	1.908	0.98

below. Figure 9 illustrates the fitting curve for freeze–thaw cycle count versus the dynamic elastic modulus loss rate. Fitting analysis based on the Aas-Jakobsen function revealed that incorporating 0.15% hybrid PP fiber and 0.5% steel fiber into DSRAC notably enhanced its freeze–thaw durability. Specifically, through the comparison between the experimental data and the theoretical model, it was found that the un-reinforced concrete began to show significant damage after about 150 freeze–thaw cycles. However, after adding the hybrid fibers, this threshold was increased to about 280 cycles. At the same time, the rate of damage development was also lower, which was reflected in the shape parameter of the fitting curve, decreasing from −2.5 to −1.908. In addition, the R² coefficients of the fitting curve (0.96 for specimens without fibers and 0.98 for specimens with fibers) proved the predictive accuracy of the model for the dynamic elastic modulus loss rate under both scenarios.

3.8. Freeze–Thaw Damage Constitutive Relationship of Hybrid-Fiber-Reinforced DSRAC

Fitting analysis was performed on the stress–strain relationship of the fiber-free DSRAC after freeze–thaw cycles, with reference to Guo’s constitutive model. The results were shown in Figure 10 and

Table 12. The fitting results of the freeze–thaw constitutive parameters for DSRAC without fiber addition.

Sample	a1	R2	b1	R2
DS30-0	2.065	0.9997	2.2029	0.9998
DS30-25	2.332	0.9998	6.7314	0.9793
DS30-50	2.5118	0.9999	2.1038	0.9968
DS30-75	2.3332	0.9995	7.6638	0.9912
DS30-100	1.2778	0.9980	7.6551	0.9906
DS30-125	1.4398	0.9843	6.2385	0.9715

, indicating that this model had high applicability. The fitting determination coefficient R2 under each freeze–thaw cycle was higher than 0.97. Overall, with the development of freeze–thaw damage, DSRAC presented obvious characteristics of strength and stiffness degradation and brittle failure.

With reference to Guo’s constitutive model, systematic fitting was conducted on the stress–strain relationship of hybrid-fiber-reinforced DSRAC under freeze–thaw cycling, as illustrated in Figure 11; the corresponding model parameters are tabulated in

Table 13. The fitting results of the freeze–thaw constitutive parameters for hybrid-fiber-reinforced DSRAC.

Sample	a2	R2	b2	R2
F0.15-S0.5-0	4.0972	0.9996	3.5537	0.9924
F0.15-S0.5-25	1.7607	0.9999	4.4590	0.9977
F0.15-S0.5-50	2.7612	0.9997	2.4284	0.9987
F0.15-S0.5-75	5.7909	0.9971	3.6972	0.9993
F0.15-S0.5-100	5.7091	0.9995	2.9385	0.9982
F0.15-S0.5-125	1.6535	0.9843	4.9542	0.9976

. The fitting results revealed that this constitutive model can highly accurately characterize the mechanical response of the material across varying freeze–thaw cycle counts. All fitting curves yielded coefficients of determination (R²) exceeding 0.984, verifying the model’s excellent reliability and predictive capacity. Notably, even as the freeze–thaw cycle numbers increased, the fitting curves remained in good alignment with experimental data—indicating the model’s effectiveness in capturing the damage evolution process of fiber-reinforced concrete under freeze–thaw exposure. Additionally, fiber incorporation was found to significantly enhance the material’s post-peak deformation performance, retard crack propagation and boost toughness, thereby effectively mitigating the brittle failure tendency induced by freeze–thaw cycling.

4. Microscopic Research

Figure 12 displays scanning electron microscope (SEM) micrographs of hybrid-fiber-reinforced DSRAC after varying freeze–thaw cycle counts. From the perspective of microstructural evolution, it is clear that as the number of freeze–thaw cycles increases, the material undergoes a distinct degradation process. When the specimen had not experienced any freeze–thaw cycles (Figure 12a), its microstructure was relatively compact, with a low porosity. The interfacial transition zone (ITZ) between cement paste and aggregates remained continuous and structurally intact, demonstrating an excellent initial condition. After 25 freeze–thaw cycles (Figure 12b), microcracks started to emerge in the ITZ region, indicating that the freeze–thaw action had induced the initial damage. After 50 freeze–thaw cycles (Figure 12c), these microcracks further propagated and interconnected. The porosity increased, and the continuity of the ITZ was disrupted, signifying a gradual accumulation of internal damage within the material. Upon reaching 75 freeze–thaw cycles (Figure 12d), the number of cracks increased markedly, and their widths expanded. In some local areas, the cement paste peeled off, the surface roughness increased and the overall structural performance deteriorated. When the number of freeze–thaw cycles reached 100 (Figure 12e) and 125 (Figure 12f), the damage to the microstructure intensified. Cracks were extensively distributed and interconnected. The ITZ was severely damaged and the porosity rose significantly. Some regions exhibited a loose and peeling state, indicating that the microstructure of the material was on the verge of failure.

The aforementioned SEM characterization results indicated that with increasing freeze–thaw cycle counts, the microstructure of hybrid-fiber-reinforced DSRAC gradually transitioned from a dense morphology to a state marked by extensive cracking and elevated porosity. This degradation at the microscopic scale directly gave rise to a decline in the macroscopic mechanical properties. The addition of fibers had, to some extent, retarded the propagation of cracks. However, it had not been able to completely inhibit the cumulative damage induced by the freeze–thaw actions. These findings clearly elucidated the microscopic mechanism underlying the degradation of the material’s durability in a freeze–thaw environment. This provided a theoretical foundation for further enhancing frost resistance through material design strategies, such as optimizing the fiber composition, incorporating air entraining agents and other related measures.

The SEM analysis presented in this paper was primarily centered on uncovering the sequence of the microstructure evolution of the hybrid-fiber-reinforced group under the influence of freeze–thaw cycles. Notwithstanding the fact that no concurrent SEM observations were conducted on the fiber-free control group, the systematic sampling carried out on the fiber-reinforced specimens (with a minimum of three samples for each freeze–thaw cycle) and the observations made at a consistent magnification (200×, except for the initial state where 500× was used) clearly illuminated the mechanism by which fibers impeded the propagation of microcracks. These qualitative findings, when combined with the macroscopic performance data, collectively provided strong evidence for the reinforcing effect of the fibers. Future research endeavors will encompass direct microscopic comparisons between fiber-containing and fiber-free specimens. Additionally, image quantification techniques will be employed to enable more refined and in-depth characterizations.

Through scanning electron microscopy (SEM) observations, this study focused on revealing the inhibitory mechanisms of fibers against physical damage under freeze–thaw cycles. However, the unique chemical composition of desert sand might induce chemical microstructure evolution (e.g., chemical alterations of hydration products at the ITZ), which also constituted a potential factor influencing long-term durability. Given that this study primarily focused on the degradation of physical–mechanical properties and the physical crack-arresting effect of fibers, the quantitative characterization of chemical interactions represented a critical direction for future in-depth investigations.

5. Conclusions

This study systematically investigated the performance evolution and damage mechanism of hybrid-fiber-reinforced DSRAC under freeze–thaw cycling. The exploration was carried out through macroscopic mechanical experiments, freeze–thaw durability tests, damage model establishment and microstructure analysis. The main findings were as follows:

(1) At a desert sand replacement ratio of 30%, the initial mechanical properties of DSRAC were found to be optimal; however, its freeze–thaw resistance performance was relatively poor. When the replacement ratio was 25%, although the initial strength was slightly lower, the performance degradation during the freeze–thaw process was observed to be more gradual, indicating better durability.

(2) The incorporation of hybrid fibers (F0.15-S0.5) was shown to significantly enhance the mechanical properties and freeze–thaw resistance of DSRAC. The compressive strength of specimens in this group was measured to be 31.6% higher than that of the fiber-free control group. After 125 freeze–thaw cycles, the loss rates of compressive strength, splitting tensile strength and flexural strength dropped to 73.2%, 60.1% and 46.4%, respectively—values that are significantly lower than those of the reference group.

(3) The fibers were proven to effectively mitigate the mass loss and the decline in the dynamic elastic modulus during the freeze–thaw process. After 125 freeze–thaw cycles, the mass loss of the fiber-added group was only 0.34%, and the dynamic elastic modulus remained at 72.9%, demonstrating excellent durability performance.

(4) The freeze–thaw damage models constructed using the exponential function and Aas-Jakobsen function were verified to precisely characterize the evolution pattern of the relative dynamic elastic modulus with freeze–thaw cycle counts, with coefficients of determination R² ≥ 0.96. Guo’s constitutive model was verified to effectively represent the changes in the stress–strain relationship before and after freeze–thaw, and the goodness-of-fit was higher than 0.97 for all cases.

(5) SEM analysis revealed that as the number of freeze–thaw cycles increased, the microstructure of the concrete gradually evolved from a dense state to a state with multiple cracks and high porosity. The addition of the fibers was shown to significantly retard the damage of the interfacial transition zone and crack propagation behavior, thus enhancing the macroscopic freeze–thaw resistance and ductility.

Freeze–thaw tests in this study were conducted in a pure water environment, and no quantitative characterization of the concrete’s pore system was performed. Future research should integrate ASTM C457 standard [55] analysis to comprehensively evaluate the synergistic durability enhancement mechanism of air-entrainment and fiber-hybridization strategies for desert sand recycled concrete under multi-factor environments (e.g., salt–frost coupling). Furthermore, this research was predominantly centered on the material scale. As such, the mechanical properties of full-scale structural members remain to be further validated. Consequently, future research endeavors should prioritize investigations into durability under the coupled action of multiple corrosion factors. Additionally, experimental verification of the findings of this study through tests on structural components is essential. Such efforts will facilitate the translation of these research outcomes into practical engineering applications. Performance analysis in this study relies primarily on mean values and standard deviations. Future research will leverage larger sample sizes (n ≥ 6) and employ more diverse statistical visualization tools, such as boxplots, to conduct a more thorough analysis of the distributional characteristics of performance data, thereby providing a more comprehensive reliability assessment.