Benefits of Surface-Modified Steel Fibers on Enhancing the Mechanical Properties in Cement Matrix

Abstract

Steel fibers are widely used in cementitious composite materials to enhance their mechanical properties, such as tensile strength and toughness. However, the effectiveness of these fibers largely depends on their surface characteristics and bonding with the cement matrix. This study investigated the effects of various treatment processes on the microhardness and mechanical strength of steel fibers in cementitious composite materials. These methods include acetone and acid washing, silane coupling agent treatment, and nanosilica coating. Fibers washed with acetone exhibited a cleaner surface, primarily due to the removal of impurities. Acid treatment resulted in a notably roughened surface, which significantly enhanced mechanical interlocking with the surrounding matrix. Silane treatment led to an uneven surface with distinct vertical textures, potentially improving adhesion properties. Meanwhile, fibers treated with nanosilica displayed a coating of nanoparticles adhering to the surface, which may further influence the fiber–matrix interaction. The results of the mechanical properties tests indicated that nanosilica coating was the most effective in improving both the flexural and compressive strengths, especially in the early strengths in the cement matrix.

1. Introduction

Steel fibers have long been recognized as an essential reinforcement material in cementitious composite materials due to their ability to significantly enhance the mechanical properties of these composites, such as tensile strength and toughness [1,2,3]. However, the effectiveness of steel fibers in improving the performance of cementitious materials is largely contingent upon their surface characteristics and the quality of the bond they form with the cement matrix [4,5]. The surface condition of steel fibers plays a crucial role in determining the interfacial bond strength, which, in turn, affects the overall mechanical behavior of the composite. For example, Chun et al. examined the direct tensile behaviors of ultra-high-performance concrete (UHPC) reinforced with both plain and chemically modified steel fibers [6]. Their findings revealed that various chemical treatments can significantly enhance the tensile strength, tensile strain, and g-value of the material.

In recent years, numerous studies have explored various surface treatment methods to optimize the performance of steel fibers in cementitious composites [7,8]. These methods aim to improve the surface roughness, cleanliness, and chemical compatibility of the fibers with the cement matrix, thereby enhancing the mechanical properties of the composite materials [9]. Common surface treatment techniques include acetone and acid washing, silane coupling agent treatment, and nanosilica coating [10,11,12].

Acetone washing is a widely used method to remove surface impurities and contaminants from steel fibers, resulting in a cleaner surface that can improve the bond with the cement matrix [12,13]. Acid treatment, on the other hand, is known for its ability to roughen the surface of steel fibers, which enhances mechanical interlocking with the surrounding matrix [12,14]. Silane coupling agent treatment has been shown to create distinct vertical textures on the fiber surface, potentially improving adhesion properties [15,16]. Additionally, nanosilica coating has emerged as a promising method to further influence the fiber–matrix interaction by providing a layer of nanoparticles that can enhance the interfacial bond strength [17,18].

Despite the extensive research on surface treatment methods, there is still a need for a comprehensive study that better determines the effects of each method on the mechanical properties of steel fibers in cementitious composite materials. Understanding the methods optimized for cementitious composites is particularly important, as the results can vary significantly based on the specific matrix employed. This study aims to fill this gap by investigating the effects of various treatment processes, including acetone and acid washing, silane coupling agent treatment, and nanosilica coating, on the mechanical properties of steel fibers in cementitious composite materials. The results of this study are expected to provide valuable insights into the optimal surface treatment methods for steel fibers, ultimately leading to the development of cementitious composites with superior mechanical performance.

2. Materials and Methods

2.1. Mixture Proportioning Design

Cementitious materials are composed of cement, fly ash, quartz sand, and water. Fly ash serves as a filler, while quartz sand acts as the fine aggregate. The Portland cement used is P·O 42.5 ordinary Portland cement produced by Yangchun Cement Co., Ltd., in Zhucheng, Shandong, China. The specific surface area of the cement is 362 m²/kg. The fly ash used is Grade I fly ash produced by Henan Guyou Engineering Testing Materials Co., Ltd. (Zhengzhou, China), with a bulk density of 1.12 g/cm³. Quartz sand with a particle size of 110–160 mesh was employed. The superplasticizer used is a polycarboxylate high-performance water reducer produced by Shanxi Feike New Materials Technology Co., Ltd. (Yuncheng, China), and the water used is ordinary tap water from the laboratory. The water-to-binder ratio was 0.3. The standard curing conditions used in this experiment were a temperature of 20 ± 2 °C and a relative humidity of over 95%. Commercially available brass-coated steel straight steel fibers with a diameter of 0.12 mm and a length of 8 mm were used to explore changes in mechanical performance according to each surface treatment method. The physical properties of the steel fibers and detailed mixture proportion of the cementitious materials are summarized in

Table 1. Properties of steel fibers.

Properties	Fiber Diameter (mm)	Fiber Length (mm)	Aspect Ratio	Tensile Strength of Fiber (MPa)
Straight steel fiber	0.12	8	66.7	2850

and

Table 2. Standard mix proportion of cementitious materials.

Water-to-Binder Ratio	Mix Design (g)
	Cement	Silica Sand	Water	Fly Ash	Steel Fiber Content	Water Reducer
0.3	1062	625	375	188	30.1	3.3

, respectively.

2.2. Surface Modification Process

While steel fibers have garnered extensive attention and utilization in concrete applications, research focusing on enhancing their performance via surface modification remains relatively sparse, especially when contrasted with studies on organic and polymeric fibers. To systematically evaluate the impact of diverse surface modifications on steel fibers, four distinct surface treatments were meticulously applied. A detailed schematic of the surface modification process is provided in Figure 1. Prior to modification, all steel fibers underwent a preparatory copper removal treatment using a specialized stripping agent. This step was necessary because, while the copper plating on the surface of steel fibers effectively prevents rust, it also reduces the interfacial bonding performance, resulting in a weaker interface [19].

Acetone treatment: The steel fibers were degreased by immersion in a 95% (v/v) acetone solution for 5 min, followed by thorough rinsing with deionized water and ambient drying.

Acid pickling treatment: The steel fibers were etched in a 10% (v/v) hydrochloric acid solution for 5 min to remove surface oxides. After pickling, they were rinsed with deionized water and air-dried at room temperature.

Silane coupling agent treatment: The steel fibers were immersed in a 20% sodium hydroxide solution for 0.5 h. Subsequently, the fibers were washed with deionized water and dried at room temperature. A silane coupling agent hydrolyzate was then prepared, consisting of a mixture of silane-coupling agent KH-550 solution, anhydrous ethanol, and deionized water, with the KH550 solution accounting for 12% of the mixture. The alkali-treated steel fibers were soaked in the silane coupling agent hydrolyzate at 40 °C for 10 min, then removed and kept in an oven at 110 °C for 2 h.

Silica treatment: First, the steel fibers were subjected to alkali treatment, following the same procedure as in the silane coupling agent modification. Second, a mixture of 50% deionized water and 50% anhydrous ethanol was prepared. Then, 4.3 g of cetyltrimethylammonium bromide (CTAB) and 43.2 g of tetraethyl orthosilicate (TEOS) were added to the mixture to form a modification solution. The modified steel fibers were added to the mixed solution and treated in an ultrasonic cleaner for 0.5 h, during which ammonia was added to adjust the pH of the mixture to 9. The steel fiber mixture was then heated in a water bath at 60 °C for 6 h. After heating, the mixture containing the steel fibers was left to stand at room temperature for 24 h. Finally, the steel fibers were removed and kept in an oven at 105 °C for 2 h.

2.3. Surface Morphology and Elemental Composition

To comprehensively characterize the surface morphology and elemental composition of steel fibers after various surface modifications, field emission scanning electron microscopy (FE-SEM, Oberkochen, Germany) imaging and energy-dispersive X-ray spectroscopy (EDS) analysis were conducted using a ZEISS Sigma 500 microscope equipped with an EDS detector. The operating voltage was set at 5 kV. For FE-SEM observation, all specimens were first sputter-coated with a thin gold layer for 30 s to enhance conductivity, ensuring high-quality imaging.

To evaluate the reactivity of the SiO₂-modified layer, a saturated solution of Ca(OH)₂ was prepared to simulate a hydration environment. Steel fibers modified with SiO₂ were immersed in this solution at room temperature for 28 days. After immersion, the fibers were carefully removed and rinsed with deionized water to remove any residual solution. Subsequently, the fibers were examined using FE-SEM to observe their surface morphology and analyzed using EDS to determine their elemental composition.

2.4. Binary Data Analysis of Microscope

For a more in-depth analysis of the effects of the silica layer in the fiber–matrix interfacial transition zone, both untreated and SiO₂-modified steel fiber-reinforced cementitious composites were carefully sectioned using a precision saw. The samples were then meticulously polished using 60-grit and 120-grit sandpaper to ensure a smooth surface. Subsequently, the regions surrounding the fibers were examined in detail using an optical microscope (OM), specifically employing an Mshot MJ31 microscope (Guangzhou, China) to obtain high-resolution images and gain deeper insights into the microstructural characteristics.

In the OM images, the bright regions signify the dense phase, particularly at the interface where steel fibers bond with cement-based materials. These bright areas denote effective bonding with the fibers, whereas the dark regions indicate the presence of pores. To more accurately assess the bonding performance at the interface between fibers and cement-based materials, Avizo 2022 software was employed to quantify the image data. Threshold segmentation was utilized to distinguish pixels of varying colors within the image. In the segmented image, areas with superior bonding performance are rendered in black, while interface pores that exhibit weaker bonding are depicted in white. This color-coding makes it visually straightforward to discern the bonding quality at the interface between fibers and cement-based materials. Once the image was imported into Avizo, the “Ortho Slice” function was activated. Based on the imported image, the desired portion was extracted, and interactive thresholding was applied to the retained slice segment, which is essentially the threshold segmentation process. After threshold segmentation, the slices display only two colors: black and white. White is designated for pores, and black for the dense phase. Consequently, the bonding performance at the interface between steel fibers and cement-based materials can be intuitively evaluated.

2.5. Microhardness Test

Microhardness values provide insight into the quality of a microzone beneath the indenter, and are influenced by the surrounding areas [20]. This method is particularly suitable for characterizing the microzone quality of hardened cement paste, especially in the regions between rigid inclusions (such as aggregate and steel fibers) and the bulk paste. In this study, an i-Nano system (KLA, Milpitas, CA, USA) equipped with a Berkovich diamond tip was employed to measure the microhardness of the matrix, starting from the fiber edge and extending into the bulk paste. Here, HV represents the hardness of the sample under the applied load. The measurement points were carefully selected to avoid overlap, with the first point located 5 μm away from the fiber edge, the second point at 25 μm, and subsequent points measured at intervals of 25 μm. An applied load of 10 gf (0.098 N) and a holding time of 10 s were selected for the microhardness tests.

2.6. Flexural and Compression Strength Test

Three-point bending tests were performed on specimens measuring 40 mm × 40 mm × 160 mm to assess their flexural behavior. Each group consisted of three specimens, and the average value of the three tests was taken as the representative result for that group. Following the flexural tests, the specimens were subjected to compressive tests. The compressive strength was determined by averaging the six measurement values obtained from the three prisms. In accordance with the Chinese Standard GB/T 17671-2021 [21], the loading rates for the bending and compressive tests were set at 0.5 kN/s and 2.4 kN/s, respectively. The universal testing machine initially recorded the load-displacement data for both bending and compressive tests, from which the flexural strength and compressive strength were subsequently calculated. The plotting of error bars is based on the standard deviation of all data points, representing the associated uncertainty or variability in the measurements.

3. Results and Discussion

3.1. Surface Topography

The FE-SEM observation was performed after each step of steel fiber modification, including untreated steel fiber, hydrochloric acid and KH-550 solution-treated steel fiber, and nano-SiO₂-modified steel fiber. Figure 2 shows the surface topography of these fibers. Initially, the untreated steel fiber surface was neat and smooth, as shown in Figure 2a. Acetone was used to clean surface oil stains on the steel fibers, and the morphology was similar to the untreated steel fiber (Figure 2b). After 5 min of hydrochloric acid oxidation treatment, there were some shallow gullies generated on the steel fiber surface, as shown in Figure 2c,d, increasing the steel fiber surface area and roughness. After the modification of KH-550, a small amount of product was formed on the steel fiber surface, as shown in Figure 2e. The silane-coupling agent can improve the surface tension of steel fiber, which is favorable for the uniform development of the modified product on the fiber surface. After nano-SiO₂ modification, the steel fiber surface was grafted with some point particles, as shown in Figure 2f, which were the generated SiO₂ during the chemical modification of steel fiber surface. The element Si after KH-550 and nano-SiO₂ modification was validated by the point scanning method of EDS.

The microstructural characteristics and EDS results of the fiber hydration products in Ca(OH)₂-rich solution were also evaluated. Figure 3 shows the surface morphology of the nano-SiO₂ modified fiber immersed in saturated Ca(OH)₂ solution for 28 d. The diameter of the steel fiber was 124.8 μm, which was 4.8 μm bigger than the original fiber (Figure 3a). The Si/Ca ratio, which may be used to indirectly assess the composition of the cementitious matrix [22], was 2.475 (Figure 3b). This ratio enters the common Ca/Si range of calcium silicate hydrate (C-S-H) gel [23]. From the mapping EDS analysis, Si and Ca were observed and uniformly distributed on the fiber (Figure 3c,d). It can be concluded that the nano-SiO₂ coated on the steel fiber can react with Ca(OH)₂ to form an expanded volume of products, which is a type of C-S-H gel.

3.2. Microstructure of the Fiber–Matrix Interface

The typical morphology and binarized images of the microstructure at the steel fiber–matrix interface are shown in Figure 4. In the region near the unmodified fiber, it is evident that the matrix contains numerous pores, voids, and other defects (Figure 4a). The edge effect is the cause of this phenomenon: the addition of steel fibers, which are relatively large compared to the small particles such as cement, essentially “cuts” into the volume that the cement particles would otherwise occupy [24]. However, in reality, this does not happen. Instead, the random distribution and packing of the small cement particles are disturbed, resulting in higher porosity and smaller particle size near the fiber [25]. These defects near the fiber edges are often interconnected, leading to a very small contact area between the fiber and the cement matrix, and consequently, poor bonding between the fiber and the matrix.

In contrast, steel fibers modified with silica exhibit a strong bond with the cement matrix. The fiber–matrix interface is characterized by its dense and homogeneous structure, as shown in Figure 4c. This is because the silica-modified layer reacts with calcium hydroxide (CH) to form C-S-H products, which significantly reduce the number of defects in the region near the steel fiber and make the microstructure more dense [26]. After binarization image processing, this trend becomes even more apparent (Figure 4b,d). In the binary segmented images, white represents pores, while black represents the solid material. It can be observed that compared with the unmodified samples, the pores between the binarized circular fibers and the surrounding matrix in the modified samples gradually decrease, that is, the white circular rings become thinner and more incomplete. Compared with traditional backscattered electron (BSE) images required for observing the fiber–cementitious interface under a microscope [27,28], the sample preparation and image processing in this study are notably more streamlined and efficient. Moreover, after binarization processing, the images of the steel fiber–cementitious interface become more distinct, with defects around the fibers being more pronounced. In fact, the interfacial pores are “quantified”.

3.3. Microhardness of the Specimen

Microhardness testing was conducted to evaluate the interfacial properties between steel fibers and cementitious materials after 3 days of curing. As shown in Figure 5a, the microhardness profiles of the matrix surrounding the fibers were obtained. According to a previous study [18], the microhardness profiles around rigid inclusions in a cement matrix can be categorized into four types, as depicted in Figure 5b. This classification provides insights into the relative strengths and weaknesses of the matrix in proximity to and away from the inclusion, as well as the integrity of the bond between the inclusion and the matrix. In particular, steel fibers modified with silica exhibited a layer of silica on their surfaces, which reacts with the hydration products of the surrounding cementitious materials. This reaction not only fills the pores around the fibers but also significantly enhances the mechanical properties and strength of the material at the interface between the steel fibers and the cementitious matrix. In contrast, the untreated specimens exhibited Type IV microhardness profiles, indicative of a weak or nonexistent bond between the fibers and the matrix. The modified specimens, however, displayed Type I profiles, characterized by strong fiber–matrix bonding and matrix properties that are comparable to or even superior to those of the bulk paste. Moreover, the microhardness enhancement of the silica-modified specimens was more pronounced than that of the KH-550 modified specimens.

3.4. Flexural and Compression Strength

The flexural strength was determined using a three-point bending test, as depicted in Figure 6a. Neither acetone treatment nor pickling had a significant impact on the flexural strength at various curing ages (3, 7, and 28 days). In contrast, the application of a coupling agent and silica both led to an increase in flexural strength. Notably, silica contributed more to this improvement than the coupling agent. In particular, the flexural strength saw a substantial enhancement after silica modification, even at the early age of 3 days. As for the compressive strength, the results are presented in Figure 6b. At the early curing ages of 3 and 7 days, the compressive strength exhibited an increasing trend following modification with acetone, pickling, KH-550, and nano-SiO₂. However, at the age of 28 days, the improvement in compressive strength due to pickling, KH-550, and nano-SiO₂ was not as pronounced. The enhancement rate of surface modification on compressive strength was minimal and significantly lower than that on flexural strength. This is because the increase in compressive strength of cement-based composites is primarily attributed to improvements in microstructure and chemical composition, which are less influenced by fibers. The primary role of fibers in cement-based composites is to limit the formation of microcracks and the propagation of macrocracks by acting as bridges, thereby enhancing tensile and bending performance [29,30]. Among all the surface modifications, nano-SiO₂ had the most significant effect on both flexural and compressive strength, particularly at early curing ages.

The ratio of flexural–compressive strength (f_f/f_c) was calculated to evaluate the flexibility of the specimens and the results are provided in Figure 6c. The introduction of steel fibers not only significantly enhances the mechanical properties of cementitious materials, but more crucially, it significantly improves their toughness. This reinforcement effect enables the material to maintain a certain load-bearing capacity and continue to function even after failure occurs under critical load. The figure illustrates the variation trends of the f_f/f_c strength ratio of steel fiber-reinforced cementitious composite materials under different curing ages, including unmodified, acetone-washed, acid-washed, silane-coupling-agent-modified, and silica-modified steel fibers. It was found that the f_f/f_c strength ratio increases continuously with the extension of curing age under different modification methods, especially in the curing age range of 7 to 28 days, where a significant growth trend is observed for all modification methods. This phenomenon may be attributed to the continuous hydration reactions within the cementitious materials, a finding that aligns with previous research [31]. Under the same curing age conditions, the contributions to the f_f/f_c strength ratio, in descending order, are silica modification, silane-coupling-agent modification, acetone washing, unmodified, and acid washing. The silica particles on the surface of steel fibers can react with the surrounding cementitious materials to form a dense C-S-H phase, filling the micro-pore structure and thereby enhancing the bond between the fibers and the cementitious materials. Although acid washing can make the surface of steel fibers rougher, thereby improving their compressive and flexural strengths, it also leads to some degree of surface degradation of the steel fibers. As a result, the increase in flexural strength is far less than that of compressive strength, which reduces the overall toughness of the material. Acetone washing shows a similar f_f/f_c strength ratio to unmodified steel fibers, with little difference. The irregular protrusions formed on the surface of steel fibers after silane-coupling-agent modification also enhance the bond between the steel fibers and the cementitious interface. Moreover, these surface protrusions provide a certain degree of protection to the steel fibers, making the modified material more ductile than ordinary steel fiber-reinforced cementitious composite materials.

The latest studies have revealed that the incorporation of nano-silica significantly alters the concrete configuration, contributing substantially to the development of environmentally friendly, high-performance, and sustainable concrete [32]. In our future research, we will concentrate on exploring the long-term durability benefits and potential environmental challenges associated with the use of silane-coupling-agent modification in steel-reinforced cementitious materials.

3.5. Economic and Environmental Comparison Between the Different Surface Treatments

Based on the above findings, the economic and environmental comparison of different surface treatment methods for steel fibers in cementitious composites reveals a clear trade-off between cost, performance, and environmental impact. Acetone washing is the most cost-effective method, primarily removing surface impurities but offering minimal enhancement to the mechanical properties of the composite. However, acetone is a volatile organic compound that can evaporate into the atmosphere, contributing to air pollution and potentially forming smog, which is harmful to air quality and human health. Additionally, acetone is soluble in water and can contaminate water sources if not handled properly. Its use and disposal must comply with strict environmental regulations. Despite its cost-effectiveness, the environmental impact of acetone necessitates careful handling and adherence to regulatory standards.

Acid pickling increases surface roughness, which improves mechanical interlocking with the cement matrix, but it may cause some surface degradation, limiting the overall toughness of the composite. While more expensive than acetone washing, acid pickling provides a balance between cost and performance. However, acid wash solutions can pollute water bodies, alter water pH, release toxic metals, and harm aquatic life and soil ecosystems. The acid washing process must strictly follow environmental requirements for neutralization and waste treatment. While acid pickling offers a balance between cost and performance, the environmental risks associated with acid waste management must be carefully managed to mitigate potential harm.

Silane coupling agent treatment further enhances the bond strength between the steel fibers and the cement matrix by creating distinct vertical textures on the fiber surface, offering better performance than acetone washing and acid pickling, though at a higher cost. This method is relatively environmentally friendly, as it can enhance material performance and durability, reducing the need for frequent replacements. However, care must still be taken during use and disposal to prevent contamination of water and soil. Silane coupling agents offer a more sustainable option compared to acetone and acid washing, balancing performance enhancement with reduced environmental impact.

The most significant improvements in mechanical properties, especially in early strength development, are achieved through nanosilica coating, which forms a dense C-S-H phase that fills micro-pores and strengthens the fiber–matrix interface. Despite being the most expensive method, nanosilica coating provides the best performance enhancements, making it the most suitable choice for applications requiring high mechanical performance and early strength development. However, improper handling of nanosilica can lead to soil and water contamination, and its production process may also have environmental impacts. The use and disposal of nanosilica coatings must adhere to environmental regulations. Despite its high cost and potential environmental concerns, nanosilica coating provides the best performance enhancements, making it a viable option for applications where high mechanical performance is critical.

The selection of the surface treatment method should be guided by the specific requirements of the application, balancing the need for mechanical enhancement with cost and environmental considerations. For applications requiring significant mechanical enhancements, especially in early curing stages, nanosilica coating is the most effective despite its higher cost and environmental impact. For more cost-sensitive applications where moderate performance improvements are acceptable, silane coupling agent treatment or acid pickling may be more appropriate, provided that environmental regulations are strictly followed. Acetone washing is suitable for applications where surface cleanliness is the primary concern and significant mechanical enhancement is not required, but its environmental impact must be carefully managed.

4. Conclusions

The study explored the impact of different surface modifications on the mechanical properties of steel fiber-reinforced cementitious composites. Results indicated that nanosilica coating significantly enhanced the interfacial bond between steel fibers and the cement matrix, leading to notable improvements in flexural and compressive strengths, especially at early curing ages. The nano-SiO₂ on the surfaces of the modified steel fibers is chemically reactive and reacts with Ca(OH)₂, forming a type of C-S-H gel upon immersion in a saturated Ca(OH)₂ solution. Acid washing increased surface roughness but also caused some degradation, limiting the composite’s toughness. Acetone washing had minimal impact, while silane coupling agent treatment improved bond strength through surface protrusions. In summary, optimizing surface treatments, particularly nanosilica coating, can effectively improve the mechanical performance and durability of these composites. Future work should focus on long-term performance and broader applications of nanosilica-coated steel fibers in cementitious composites.