Study on the Evolution Law of Mechanical Properties of the Modified High Strength BF-RCC Subjected to High Temperature

Abstract

Basalt fiber-reinforced cementitious composites (BF-RCC) have attracted considerable research interest in construction engineering owing to their excellent mechanical performance. However, some great challenges, such as limited ultimate tensile strain (typically less than 1%) and poor high-temperature resistance, have restricted its broader application. This study explores the influence of silane coupling agent (SCA) modification on the mechanical performance of the BF-RCC under high-temperature environments. The basalt fibers were treated with KH602 (SCA) to enhance interfacial bonding with the cement matrix under high-temperature environments. The mechanical performance of BF-RCC, including tensile strength, compressive strength, elastic modulus, crack propagation behavior and toughness index, was evaluated under different SCA concentrations (2.5% and 4.5%) and different temperatures (20 °C, 200 °C, 300 °C and 400 °C). The findings demonstrate that the tensile strength and compressive strength of the BF-RCC are elevated by 1.5 times and 1.7 times, respectively, while the toughness index and elastic modulus are enhanced by 1.6 times and 1.4 times, respectively. The incorporation of SCA significantly reduces the mass loss of the BF-RCC under high temperatures, with the 2.5% KH602 concentration exhibiting the optimal performance. However, when the temperature exceeds 300 °C, the mechanical properties of the BF-RCC deteriorate markedly. Digital image correlation (DIC) technology demonstrated that SCA-modified BF-RCC displays enhanced crack propagation resistance, with post-peak fracture energy showing a concentration-dependent increase, thereby reducing material brittleness.

1. Introduction

As a common binder in buildings, mortar is widely used in construction. However, ordinary Portland cement mortar exhibits strong brittleness and tends to develop cracks under significant external forces. Under sustained loading conditions, these cracks exhibit progressive propagation, resulting in accelerated deterioration of reinforced concrete structural performance [1]. To enhance the performance of cement mortar and address these limitations, researchers worldwide have conducted extensive studies and proposed incorporating various fiber materials, including polyvinyl alcohol [2], carbon fiber [3], glass fiber [4], and natural fiber [5], among others, to enhance the tensile strength, flexural performance, and fracture toughness of cement mortar. Notably, basalt fiber emerges as a green, environmentally benign, and sustainable high-tech material that harbors no environmental hazards. Its application is extensive across construction, bridge engineering, road pavements, and various other infrastructure endeavors. When juxtaposed with carbon fiber, glass fiber and aramid fiber, basalt fiber demonstrates enhanced mechanical properties, remarkable resistance to acidic and alkaline environments, superb electrical insulation, high wave transparency, a non-conductive nature, and superior sound and thermal insulation properties [6,7].

Basalt fiber (BF), characterized by its hydrophilic nature and a composition akin to that of the cement matrix, is capable of reacting with the cement matrix to produce higher chemical bond energy. Nonetheless, the smooth surface of BF [8], along with its scarcity of reactive functional groups [9] and a relatively small specific surface area, impedes the development of adequate adhesion with the cement matrix [10,11,12]. These attributes are disadvantageous for the mitigation of crack formation in basalt fiber-reinforced mortar. Branston et al. [12] explored the failure mechanisms of various fiber-reinforced concretes by using drop hammer impact tests. Their research uncovered that basalt fibers, in contrast to other fibers, primarily demonstrate a pull-out failure mode.

Comparative analysis reveals that basalt fiber-reinforced concrete (BF-RCC) exhibits a minimum tenfold enhancement in tensile strain capacity relative to conventional concrete [13]. Empirical research has shown that basalt fiber markedly improves the flexural and tensile strength of mortar [14], although it does not contribute to an enhancement in compressive strength [15]. Indeed, for basalt fiber mortar cured for 28 d, the compressive strength typically ranges 5–15% below standard mortar specifications, with this decrease being more significant when longer fibers were utilized [16]. Therefore, augmenting the mechanical performance is critical for reducing crack propagation in fiber-reinforced cement-based materials, boosting compressive strength, and expanding the potential applications of basalt fiber in cementitious composites.

Moreover, high temperatures significantly impact the strength of fiber-reinforced cement composites. For instance, PVA fibers undergo thermal degradation above 300 °C, resulting in the disappearance of strain-hardening behavior in ECC and significant reductions in tensile and compressive strength [17,18,19,20]. Zhang et al. [21] conducted split Hopkinson pressure bar (SHPB) tests on basalt fiber-reinforced RCC following high-temperature exposure. Their findings revealed that the dynamic compressive strength of the concrete initially rises but then falls as strain rates increase. Presently, little research has systematically examined RCC’s static and dynamic properties under high-temperature exposure. Nonetheless, as the use of RCC becomes more widespread, the potential for its exposure to fire or high-temperature scenarios grows. Therefore, it is imperative to analyze the performance of the RCC at high temperatures. The mechanical response and durability degradation of the RCC after exposure to elevated temperatures (or fire) have become focal points of research in this domain. Grasping these characteristics is vital for the further engineering advancement and application of the RCC.

At present, the exploration of chemical grafting at the fiber–cement interface is still in its infancy, with research predominantly emphasizing microscale analysis of basalt fibers [22,23]. Yet, a one-dimensional perspective frequently falls short of revealing the fundamental connection between material attributes and failure mechanisms. While there has been considerable investigation into the high-temperature behavior of natural fibers [24,25] and steel fibers [26] within cementitious composites, research into the elevated-temperature endurance of basalt fiber-reinforced cementitious materials is comparatively limited. Hence, to engineer basalt fiber-reinforced engineered cementitious composites (BF-RCC) with improved ductility and to analyze the influence of elevated temperatures on their mechanical performance, an integrated macro- and micro-level examination is indispensable.

In this investigation, the mechanical properties and failure mechanisms of BF-RCC were explored through the modification of basalt fiber surface roughness and the application of various temperature treatments. The central aim of this study is to fabricate BF-RCC with enhanced ductility. This study systematically investigated the effects of silane coupling agent (SCA) concentration and thermal treatment conditions on key mechanical properties (elastic modulus, compressive/tensile strength, and peak tensile strain). Digital image correlation (DIC) was employed to analyze strain distribution and fracture patterns in modified BF-RCC.

2. Experiment Preparation

2.1. Surface Modification of BF

Silane coupling agents (SCA) are characterized by the presence of two or more distinct reactive groups within their molecular structure, enabling them to form chemical bonds with both organic and inorganic materials. According to Chen’s research results [27], the effect of silane coupling agent KH602 is the best, so the modifier of this experiment is KH602. The modification process significantly reduced the concentration of -OH groups on the surface of basalt fiber (BF) while markedly increasing the presence of Si-Onb groups. This indicates that the -OH groups on the BF surface underwent hydrolysis and reacted with the KH602 solution. Concurrently, silicon from the KH602 solution adhered to the BF surface, forming a substantial number of Si-Onb groups. This resulted in an increase in Si-O polar bonds, effectively modifying the surface properties of the BF [28].

A homogeneous solution was prepared by mixing anhydrous ethanol with deionized water at a 3:7 volumetric ratio under room temperature conditions. Subsequently, a specific type of the SCA, namely KH602, is gradually introduced into the prepared anhydrous ethanol mixture under continuous and uniform stirring to facilitate complete dissolution of the SCA. Following the stirring process, the mixture stands for one hour to ensure a thorough reaction of the silane coupling agent. Subsequently, pre-prepared BF, as shown in Figure 1a, is incrementally added to the modified anhydrous ethanol solution, and the mixture undergoes uniform stirring for a duration of 40 min to ensure that each basalt fiber is adequately and uniformly exposed to the modification solution, thereby achieving effective surface modification of the fibers. Following the mixing process, the BF were separated from the solution and subjected to sequential distilled water washes to ensure complete removal of surface residues. The cleaned fibers underwent isothermal drying at 103 °C to eliminate all adsorbed surface water. Once dried, the modified basalt fibers are hermetically sealed and stored for subsequent experiments.

2.2. Preparation of the Modified BF-RCC

In this study, the mixing ratio used includes ordinary Portland cement, water, fly ash, a water-reducing agent, quartz sand and BF. The specific ratio is listed in

Table 1. Composition and proportion of BF-RCC (weight%).

Cement	Quartz Sand	Fly Ash	Water	Water Reducing Agent	BF
1	1.38	2.53	0.96	0.04	0.15

. The experimental steps are as shown in Figure 2: Firstly, the ordinary Portland cement, fly ash and quartz sand are weighed according to the predetermined proportion and put into the cement mixer. Subsequently, the mixer was set to manual mode and stirred at low speed for 40 s. Then, the water and water-reducing agent were weighed accurately, and in 30 s batches, evenly added to the mixer and continued to be mixed at low speed for a minute. After that, the stirring mode is switched to high speed until the cement mixture no longer adheres to the inner wall of the mixer. At this time, a small amount of 32 g of modified basalt fiber was added in batches and continuously stirred at high speed for 90 s. After completing the above steps, the homogeneous mixture was transferred into the mold and placed on the vibrating table for 20 s to eliminate the internal gas, and then the surface was flattened with a shovel. Finally, the formed specimens were moved into the maintenance room for 28 d of maintenance.

2.3. High Temperature Treatment of the BF-RCC

The specimens were divided into four temperature groups for testing: 20 °C, 200 °C, 300 °C and 400 °C. To mitigate the effect of random experimental errors and ensure data reliability, six specimens were prepared for each temperature condition. The specimens were exposed to high-temperature treatment in an XTM-800 high-temperature furnace for 3 h, using a controlled heating ramp of 5 °C/min, as demonstrated in Figure 3 and Figure 4.

2.4. Testing Method

In this study, compression tests were conducted on the modified BF-RCC using an electrohydraulic servo press at a fixed displacement speed of 0.2 mm/min to systematically evaluate the influence of elevated-temperature environments on their mechanical performance, as shown in Figure 5. The mechanical behavior was evaluated through comprehensive testing of compressive strength, tensile resistance, fracture toughness, and elastic modulus under different environmental/loading conditions. To further analyze the fracture behavior, a camera was employed for real-time image acquisition throughout the testing process. The crack evolution process under static loading was quantitatively characterized by employing the DIC method, with a focus on critical parameters such as crack initiation, propagation paths, and damage evolution. The macro-mechanical property enhancement derived from SCA-modified basalt fibers in cementitious composites was systematically evaluated through comparative testing.

3. The Quality Variation in the BF-RCC Under High Temperature

Elevated temperatures exert a profound detrimental effect on the mechanical strength of BF-RCC. Huang et al. noted that the structural integrity of the cement matrix was adversely affected following high-temperature exposure, with the extent of deterioration escalating as temperatures rose. This was substantiated by a marked decrease in specimen weight, as documented in their research [29]. To quantitatively assess the impact of elevated temperature on the BF-RCC, the weight and density of the specimens were recorded both prior to and subsequent to the thermal treatment. The formula employed for this evaluation is presented as follows:

$${\displaystyle \frac{M_{\mathrm{A}}-M_{\mathrm{B}}}{M_{\mathrm{A}}}}\times 100\%,$$

(1)

where M_A represents the quality of the dried specimen in the air, and M_B represents the quality after high temperature treatment.

$${\displaystyle \frac{{\rho }_{\mathrm{A}}-{\rho }_{\mathrm{B}}}{{\rho }_{\mathrm{A}}}}\times 100\%,$$

(2)

where ρ_A represents the quality of the dried specimen in the air and ρ_B represents the quality after high-temperature treatment.

To determine the initial dry weight, 28-day cured BF-RCC specimens were oven-dried at 105 °C following standard procedures. The drying was carried out in 8 h, repeated until the discrepancy in weight between two successive measurements fell below 0.5%. To maintain experimental reliability, triplicate specimens were tested for each condition, with mean values adopted for result interpretation.

The BF-RCC specimens underwent various temperature treatments, as depicted in the accompanying Figure 6 and Figure 7. The weight loss rates for cylindrical specimens treated with 4.5% KH602, 2.5% KH602, and those ordinary, at 200 °C, were 7.00%, 7.20%, and 7.65%, respectively. As demonstrated in Figure 6, there was a notable escalation in the weight loss rate across all specimens as the treatment temperatures increased. At the three treatment temperatures assessed, the ordinary specimens registered the highest weight loss rates, with values of 7.65%, 9.06%, and 11.62%, respectively. The reason is that the high temperature evaporates the water inside the BF-RCC, which causes the surface of the specimen to crack and fall off, resulting in mass loss [30], and this phenomenon is more obvious as the temperature rises. The addition of BF makes the cement base closely combined with the fiber surface, and the BF-RCC can still maintain a certain strength after the internal water evaporates, thereby reducing its mass loss rate. It can be seen from Figure 7 that the density loss rate of the Brazilian disk does not change much at 200 °C, but the density loss rate increases significantly above 300 °C. This is because the free water evaporation in the medium and low temperature range (20–300 °C) will cause a slight decrease in mass, but the volume change is small, so the increase in density loss rate is not obvious. In the high temperature range (above 300 °C), the dehydration reaction occurs inside the BF-RCC, and the hydration product Ca (OH)₂ generates CaO and H₂O under the action of high temperature. The C-S-H gel gradually loses the bound water, resulting in volume expansion and porosity increase [31,32]. Because of its increasing mass, the density loss rate suddenly changes at about 300 °C. It is worth noting that because the addition of BF inhibits high-temperature cracking and delays the dehydration rate of C-S-H gel, it reduces the increase in porosity, and the weight loss rate of ordinary cylindrical and Brazilian disk samples is always higher than that of modified samples. For example, the weight loss rates for cylinder specimens treated with 2.5% KH602 were 7.20%, 7.73%, and 10.32%, whereas those treated with 4.5% KH602 were 7.00%, 7.88%, and 8.24%, all of which were lower than the control group’s rates.

In general, the mechanical degradation of cement mortar under high-temperature conditions is primarily attributed to pore coarsening and the development of micro-cracks. These phenomena are driven by two main factors: thermal stress and pore pressure. Thermal stress arises from the primary distinction in thermal deformation characteristics between the cement paste and fine aggregates at high temperatures, leading to a loss of deformation compatibility and subsequent damage. However, the extent of damage propagation caused by thermal stress is relatively limited. On the other hand, pore pressure results from the continuous increase in internal vapor pressure within the specimen as the temperature rises. Pores are generated as illustrated in Figure 8. With progressive elevation of treatment temperature, the internal pore pressure within BF-RCC exhibited a consistent increase. This process results in progressive pore densification within the cementitious matrix, characterized by increased pore concentration per unit volume. At the same time, the diameter of the pore is also constantly expanding, so that the microstructure of the concrete has undergone significant changes. For high-strength concrete, which typically has a denser structure and lower porosity, the accumulation rate of internal vapor pressure is faster, making it more susceptible to explosive spalling, a sudden and highly destructive failure mode [33,34].

4. The BF-RCC Tensile Strength Test

Figure 9a showcases the impact of varying concentrations of the SCA and various temperature treatment conditions on the average tensile strength of the BF-RCC. As depicted in Figure 9b, the average tensile strength of the BF-RCC treated with the SCA displays a distinct pattern across different temperature environments. The experimental results demonstrate that the SCA significantly enhances the tensile strength of BF-RCC. It is found that the SCA can effectively improve the tensile strength of the BF-RCC. As depicted in Figure 10, upon examination through an industrial magnifying glass, the fracture of BF at cement-based cracks becomes evident (the area within the red circle is the fractured BF), suggesting that the alteration of SCA enhances the ruggedness of BF’s surface. In instances of tensile stress, even when the cement matrix fractures, BF and cement continue to uphold a favorable bonding condition. The tensile strength of BF-RCC is critically influenced by both the chemical composition and surface. Their characteristic changes significantly affect the internal structure and interaction mechanism of the BF-RCC, which in turn has a key impact on its tensile strength.

A comparative evaluation of tensile strength and peak tensile strain was conducted for BF-RCC specimens with varying modification concentrations under isothermal conditions. For example, at 20 °C, the tensile strains for the specimens with 0% KH602, 2.5% KH602, and 4.5% KH602 were recorded as 2.69 MPa, 3.82 MPa, and 3.56 MPa, respectively. The tensile strength of 2.5% KH602 and 4.3% KH602 is 1.42 times and 1.33 times the tensile strength of an ordinary specimen. The experimental results indicate that SCA incorporation effectively prevents BF fracture, substantially enhances the tensile strength of BF-RCC, and improves fiber bridging capacity within the cementitious matrix. These synergistic effects lead to superior mechanical performance of the composite.

Figure 11a demonstrates the influence of varying concentrations of the SCA and different temperature treatment conditions on the average peak tensile strain of the BF-RCC. Figure 11b illustrates the variation in peak tensile strain of modified BF-RCC. The experimental data demonstrate that SCA modification of basalt fibers substantially increases the peak tensile strain capacity of BF-RCC. As shown in Figure 11a, the BF-RCC treated with 2.5% KH602 displays a greater peak tensile strain than that treated with 4.5% KH602. At temperatures of 20 °C, 200 °C, 300 °C, and 400 °C, the average peak tensile strains were recorded as 3.52%, 1.33%, 1.28%, and 1.33%, respectively. This improvement is attributed to the weaker interfacial chemical bonding of the treated BF with the cement matrix, combined with an increased coefficient of sliding friction. These characteristics enable BF-RCC to undergo substantial deformation via sliding friction in regions adjacent to primary cracks, resulting in enhanced peak tensile strain and material toughness. For the same SCA concentration, the peak tensile strain decreases between 20 °C and 300 °C but exhibits a slight increase or stabilization when the temperature is elevated to 400 °C. Nonetheless, this strength does not increase with the strain rate, which may be linked to the formation of new C-S-H gel during the high-temperature treatment of the BF-RCC. This gel formation fills pores and cracks, partially restoring the material’s mechanical properties [35].

5. The BF-RCC Compressive Strength

5.1. Compressive Strength and Elastic Modulus

Figure 12a shows the influence of SCA concentration and thermal treatment conditions on the mean compressive strength of BF-RCC. Figure 12b presents the variation in compressive strength for SCA-modified BF-RCC across different temperature regimes. The experimental results confirm that SCA treatment enhances the compressive performance of BF-RCC, indicating that BF surface topography plays a pivotal role in governing the mechanical behavior of the cementitious composite. However, the concentration of the SCA exhibits a threshold effect on the improvement of compressive strength. For instance, at 20 °C, the compressive strengths of 0% KH602, 2.5% KH602, and 4.5% KH602 were measured as 19.46 MPa, 33.48 MPa, and 24.33 MPa, respectively. The results reveal that when the SCA concentration exceeds 2.5%, the average compressive strength decreases, although it remains higher than that of the ordinary BF-RCC.

For the BF-RCC material, the elastic modulus is another critical factor influencing the strength of building materials [36,37,38]. The elastic modulus, calculated as the stress–strain ratio in the linear elastic range, measures a material’s stiffness and its ability to resist reversible deformation. For cement-based materials, the elastic modulus is directly correlated with the deformation behavior and structural stability under load. Consequently, the elastic modulus constitutes an essential metric for characterizing the mechanical performance of cementitious composites while providing critical insights for structural integrity assessment in civil engineering applications.

Figure 13a demonstrates the influence of SCA concentration and thermal treatment parameters on the mean elastic modulus of BF-RCC. The results demonstrate that the modified BF significantly enhances the elastic modulus of basalt fibers, indicating an improved deformation resistance of the material. Although the elastic modulus of the modified BF-RCC decreases with increasing treatment temperature, it remains substantially higher than that of the ordinary BF-RCC. Figure 13b presents the variation in mean elastic modulus of SCA-modified BF-RCC across different thermal conditions. As illustrated in Figure 13a, a 2.5% KH602 concentration is identified as the optimal modification level, maximizing the elastic modulus of the BF-RCC. Furthermore, Figure 13b reveals that, compared to the ordinary BF-RCC, the elastic modulus of the BF-RCC treated with 2.5% KH602 at 20 °C, 200 °C, 300 °C, and 400 °C increased by 39.9%, 37.0%, 42.1%, and 34.2%, respectively. This improvement highlights the effectiveness of the SCA modification in enhancing the elastic modulus of the BF-RCC, particularly at lower temperatures, and underscores its potential for improving the structural performance of cement-based materials under varying environmental conditions.

5.2. Toughness Index

The toughness index (Tc) of cement mortar is a crucial parameter for assessing its resistance to cracking and fracture under stress, particularly its capacity to absorb energy during crack propagation. This study employed the ASTM C1018-97 [39] standard energy ratio method to calculate the toughness index, systematically evaluating the influence of SCA concentration and treatment temperature on the toughening behavior of BF-RCC composites. The toughness index (Tc) is calculated as the ratio of the area under the post-peak stress–strain curve (at 0.85 fp) to the corresponding pre-peak area, quantifying material ductility, as shown in Figure 14. This method provides a systematic approach to evaluating the energy absorption and crack resistance properties of the BF-RCC under different modification and temperature conditions.

Figure 15a illustrates the relationship between different concentrations of the SCA and temperatures with the mean Tc of the BF-RCC. Figure 15b depicts the change rate of the average Tc of the BF-RCC. The incorporation of basalt fibers into the cement matrix substantially enhances the energy dissipation capability of the composite, leading to improved fracture resistance in BF-RCC. For instance, at 20 °C, the average toughness indices for 0% KH602, 2.5% KH602, and 4.5% KH602 were 1.98, 2.2, and 3.2, respectively. This enhancement can be ascribed to the synergistic interaction between the hydrophilic wetting agent on the BF-RCC surface and the surface topography of BFs, which collectively facilitate the formation of hydration products at the fiber–matrix interface. This cooperative interaction leads to a marked enhancement of the interfacial adhesion between fibers and matrix. Under compressive load, stress is effectively transferred through the matrix–fiber interface, facilitating the formation of a uniformly distributed microcrack network within the specimen. Simultaneously, the bending deformation of the fibers absorbs and dissipates energy, significantly improving the compressive strength of the mortar specimen [40].

Furthermore, the modified BF-RCC exhibits a unique damage evolution mechanism. Upon exceeding the ultimate bearing capacity, the mortar matrix experiences progressive fracturing in the cracked zone, whereas the embedded BF retains residual load-bearing capacity owing to its exceptional mechanical properties. This stress redistribution mechanism results in the gradual expansion of crack width and length in the mid-span region, eventually extending to the top area of the mid-span. Notably, during the failure stage, the specimen retains structural integrity without brittle fracture, demonstrating the material’s significant toughness [27,40,41].

The test results reveal that the BF-RCC modified with 4.5% KH602 exhibits significant variations in toughness under different temperature gradients, with toughness indices of 3.20, 3.16, 2.07, and 1.88 at 20 °C, 200 °C, 300 °C, and 400 °C, respectively. The experimental data reveal a significant inverse relationship between BF-RCC toughness indices and thermal treatment temperature. When the temperature exceeds 300 °C, the difference in toughness between modified and ordinary BF-RCC diminishes significantly. Specifically, at 400 °C, the toughness indices of the BF-RCC specimens treated with 0% KH602, 2.5% KH602, and 4.5% KH602 were nearly identical, measuring 1.86, 1.87, and 1.88, respectively. This phenomenon may be attributed to the structural degradation of the interfacial transition zone and the weakening of the fiber–matrix interface performance under high-temperature conditions.

6. Crack Propagation Behavior Analysis

Digital image correlation (DIC) represents a state-of-the-art optical measurement methodology that employs digital image analysis to quantify full-field surface deformations with sub-pixel accuracy, facilitating detailed characterization of fracture development. This method employs a high-resolution camera to record stochastic speckle patterns deposited on the specimen surface, acquiring sequential images throughout the loading process for subsequent displacement field analysis. By analyzing the displacement fields of these speckle patterns, critical parameters of crack propagation, such as equivalent crack length, maximum principal strain, and displacement, can be accurately determined. These parameters provide essential experimental data for analyzing the mechanical properties of materials [42]. The DIC has been extensively applied in various fields, including geotechnical engineering [43], concrete structures [44], and tunnel engineering [45]. This investigation employed a 1920 × 1080 resolution camera for image capture.

6.1. Strain Field Distribution

As shown in Figure 16b, the BF-RCC modified with 4.5% KH602 exhibited remarkable strain concentration during the failure process. When the strain reached 2.592%, a distinct stripe emerged at the center of the specimen. The periphery of this stripe was yellow, while the interior was red, which clearly indicated the occurrence of strain concentration in this region. As shown in Figure 16, the evolution law of the strain band of the 4.5% KH602 modified BF-RCC at 20 °C could be distinctly observed. When the strain reached 2.676%, the concentrated strain area was initially formed at the center of the specimen (the blue area). As the pressure increased and the strain reached 3.307%, the concentrated strain area turned green, whereas the strain increment in the surrounding area was relatively small. Finally, the concentrated strain area kept expanding until the specimen ruptured when the strain reached 4.928%. As shown in Figure 17, for the BF-RCC modified with 2.5% KH602 at room temperature, the concentrated strain region formed when the strain reached 1.819%, and damage occurred when the strain reached 4.125%. In comparison, for the ordinary BF-RCC specimen at room temperature, the concentrated strain band area was initially formed when the strain reached 0.923%, and damage occurred when the strain reached 3.737%, as illustrated in Figure 18. Moreover, this phenomenon was verified at temperatures of 200 °C, 300 °C and 400 °C from Figure 16, Figure 17 and Figure 18. The SCA modification evidently enhances both the ductility of BF-RCC and its crack propagation displacement. Additionally, with the increase in the concentration of the SCA modification, the ductility of the BF-RCC is further improved, and the strain value at failure also increases correspondingly. This phenomenon demonstrates that the BF-RCC can effectively alleviate the brittleness of cement-based materials and cause them to exhibit distinct signs during the failure process. As a result, it provides a sufficient time window for appropriate responses when failure occurs.

At the same SCA modification concentration, the crack propagation behavior of the BF-RCC specimens at different treatment temperatures shows differences, as presented in Figure 16a–d, which display the strain magnitude and strain field distribution of 4.5% BF-RCC specimens treated at different temperatures during failure. The failure strains of the BF-RCC specimens treated at 20 °C, 200 °C, 300 °C and 400 °C are 4.928%, 4.537%, 4.358% and 4.098%, respectively. It can be seen that as the treatment temperature increases, the failure strain of the specimens decreases. This trend can also be observed in Figure 17a–d and Figure 18a–d. Furthermore, as the treatment temperature rises, the strain concentration in the specimens gradually diminishes. As presented in Figure 18c, when the treatment temperature reaches 300 °C, no obvious strain concentration area appears during specimen failure, showing typical brittle failure characteristics. The results indicate that high-temperature treatment significantly affects the mechanical properties of the BF-RCC specimens, not only reducing their ductility but also altering their failure mode.

In the compression experiment, the strain concentration area mainly appears on the upper and lower sides of the specimen, while the strain in the middle part shows a uniform increasing trend with the load, as shown in Figure 19a. As demonstrated in Figure 19b, taking the ordinary BF-RCC specimen as an example, when the displacement reached 0.8 mm at the beginning of the test, the initial formation of strain concentration zones became apparent and was mainly distributed around both sides of the specimen. As the applied load increased, the strain concentration zone progressively shifted toward the specimen’s lower region, whereas the mid-span area maintained comparatively homogeneous strain distribution. At the displacement of 1.2 mm, a distinct strain concentration zone emerged in the specimen’s upper region, exhibiting progressive expansion with further strain increment. Finally, when the displacement reached 1.4 mm, the specimen was destroyed in the strain concentration area. The experimental results demonstrate that incorporating modified BF significantly enhances the compressive ductility of the composite, consequently mitigating brittle fracture tendencies. Through the comparative observation of Figure 19a–d, it can be found that high temperature can not only reduce the compressive strength of the BF-RCC but also reduce its ductility. At the same time, the rule was verified in both the 2.5% concentration specimen and the 4.5% concentration specimen, as shown in Figure 20 and Figure 21.

6.2. Energy Evolution Law

The deformation and failure of solid materials fundamentally represent a process of energy input, energy storage, and energy release. In the Brazilian disk splitting test, it is assumed that no heat exchange occurs during the experiment. According to the first law of thermodynamics, the specimen constitutes an isolated system, and its internal energy remains conserved. As presented in Figure 22, the total input energy U (the area under the pre-peak curve) is transformed into the post-peak fracture energy Ue (the area under the post-peak descending curve) and the dissipated energy Ud. The relationship can be expressed by the following formula [46]:

$$U={\displaystyle {\int }_0^{{\epsilon }_{\mathrm{ap}}}{\sigma }_{1}d{\epsilon }_{\mathrm{a}}}+{\displaystyle {\int }_0^{{\epsilon }_{\mathrm{rp}}}{\sigma }_{2}d{\epsilon }_{\mathrm{r}}}+{\displaystyle {\int }_0^{{\epsilon }_{\mathrm{rp}}}{\sigma }_{3}d{\epsilon }_{\mathrm{r}}},$$

(3)

$$U_{\mathrm{e}}={\displaystyle {\int }_0^{{\epsilon }_{\mathrm{ad}}}{\sigma }_{1}d{\epsilon }_{\mathrm{r}}}+{\displaystyle {\int }_0^{{\epsilon }_{\mathrm{rd}}}{\sigma }_{2}d{\epsilon }_{\mathrm{r}}}+{\displaystyle {\int }_0^{{\epsilon }_{\mathrm{rd}}}{\sigma }_{3}d{\epsilon }_{\mathrm{r}}},$$

(4)

$$U_{\mathrm{d}}=U-U_{\mathrm{e}},$$

(5)

where σ₁, σ₂ and σ₃ are the principal stresses in three directions, respectively; ε_a and ε_r are axial strain and radial strain; ε_ap and ε_rp are axial and radial peak strain; and ε_ad and ε_rd are the axial and radial strain of the specimen after failure.

This test is a uniaxial compression experiment; the influence of lateral pressure is not considered, so σ₂ = σ₃ = 0. Total input energy U per unit volume; the formulas of post-peak fracture energy U_e and dissipation energy U_d can be described as the following:

$$U={\displaystyle {\int }_0^{{\epsilon }_{\mathrm{ap}}}{\sigma }_{1}d{\epsilon }_{\mathrm{a}}},$$

(6)

$$U_{\mathrm{e}}={\displaystyle {\int }_0^{{\epsilon }_{\mathrm{ad}}}{\sigma }_{1}d{\epsilon }_{\mathrm{a}}},$$

(7)

$$U_{\mathrm{d}}={\displaystyle {\int }_0^{{\epsilon }_{\mathrm{ap}}}{\sigma }_{1}d{\epsilon }_{\mathrm{a}}}-{\displaystyle {\int }_0^{{\epsilon }_{\mathrm{ad}}}{\sigma }_{1}d{\epsilon }_{\mathrm{a}}}.$$

(8)

As illustrated in Figure 23a, the total input energy of the specimen is highest for the modified BF-RCC with a 2.5% KH602 concentration under the same temperature, significantly exceeding that of the ordinary BF-RCC. This indicates that an appropriate amount of KH602 modifier can effectively boost the energy absorption capability of the material. Furthermore, under constant KH602 concentration, the specimens’ total energy absorption exhibits a negative correlation with treatment temperature. This phenomenon further confirms that elevated temperatures weaken the internal structure of the material, thereby reducing its tensile strength. Post-peak fracture energy, serving as a key toughness parameter, characterizes a material’s crack propagation resistance and energy dissipation capacity beyond peak stress. As presented in Figure 23b, at the same temperature, the post-peak fracture energy increases with higher KH602 modifier concentrations. This demonstrates that the KH602 modifier not only enhances the initial strength of the BF-RCC but also effectively improves its toughness, enabling the BF-RCC to maintain deformation capacity and energy absorption capability after reaching peak strength, thereby delaying the fracture process. However, at the same modification concentration, the post-peak fracture energy decreases with the temperature.

7. Conclusions

This research systematically examines how SCA-based surface treatment of basalt fibers affects the mechanical behavior of fiber-reinforced cementitious composites. The SCA treatment significantly enhanced the interfacial bonding between fibers and cement matrix, effectively mitigating the inherent brittleness of traditional cementitious materials. By systematically varying SCA concentrations, we achieved optimal improvement in the mechanical properties of BF-RCC. Comprehensive microstructural characterization was performed using the DIC and industrial magnification techniques, enabling detailed analysis of crack propagation mechanisms. The following meaningful conclusions can be obtained:

(1).

The addition of the modified BF can effectively reduce the mass loss of the BF-RCC at high temperature. Compared with the ordinary BF-RCC, the mass loss rate of the modified BF-RCC is reduced by 30%. The surface modification of BF by the SCA significantly improved the mechanical properties of BF-RCC. Among them, the tensile strength, compressive strength, elastic modulus and toughness index of the BF-RCC were 1.5 times, 1.7 times, 1.4 times and 1.6 times higher than those of the ordinary BF-RCC, respectively. The results show that when the concentration of SCA is 2.5% KH602, the elastic modulus, toughness index and comprehensive mechanical properties of the BF-RCC reach the maximum value.

(2).

Elevated temperatures significantly impact the mechanical strength of the BF-RCC. The SCA modification demonstrates effectiveness in enhancing material properties at temperatures below 300 °C, but this improvement diminishes substantially when exposed to higher temperatures. Particularly noteworthy is the progressive reduction in both elastic modulus and toughness index as temperature increases. These changes primarily result from structural deterioration within the interfacial transition zone and weakened interfacial bonding between fibers and matrix. At the extreme temperature of 400 °C, modified and unmodified BF-RCC specimens exhibit nearly identical values for toughness index and elastic modulus, clearly indicating the limited effectiveness of SCA modification under such severe thermal conditions.

(3).

The SCA modification enhances the interfacial bonding between basalt fibers and the cement matrix, thereby improving the crack resistance of the BF. DIC analysis reveals that the maximum strain of the BF-RCC specimens modified with 2.5% KH602 and 4.5% KH602 increased by 1.1 times and 1.3 times, respectively, compared to the ordinary BF-RCC. This demonstrates that after surface modification, the BF can maintain good bonding with the cement matrix even after matrix cracking occurs, resulting in increased crack propagation displacement.

(4).

The SCA treatment significantly increased the total energy absorption and post-peak fracture energy of BF-RCC. The post-peak fracture energy of the samples modified by 2.5% and 4.5% KH602 reached 5.27 times and 12.82 times at 400 °C, compared to unmodified BF-RCC. Moreover, the modified BF-RCC exhibits more uniform microcrack distribution and progressive fracture behavior, effectively preventing brittle failure and demonstrating superior ductility.