A recent article published in Materials proposed modifying the surface of amorphous alloy fiber (AAF) with silane coupling agent (SCA) KH-550 to improve its interfacial bonding with the cement matrix, yielding ultra-high-performance concrete (UHPC).
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Background
UHPC provides exceptional strength, toughness, and durability, making it ideal for critical infrastructure projects like highways and bridges. Various fibers, including steel, carbon, polypropylene, and basalt, are essential components of UHPC, with their type and dosage significantly influencing the concrete's performance. However, each of these commonly used fibers has specific limitations.
AAF is a banded amorphous alloy material that lacks grain boundaries, dislocations, or slip surfaces, and its incorporation into concrete can enhance mechanical and corrosion resistance properties. However, the hydrophobic surface of AAF results in poor bonding performance with the cement matrix.
Surface modification is necessary to improve the interfacial bonding strength of AAF in concrete, with SCA being a commonly used surface modifier. Despite its potential, the incorporation of SCA-modified AAF into UHPC has been scarcely researched.
This study focused on examining the impact of AAF modified with SCA KH-550 on the mechanical properties of UHPC.
Methods
Different concrete mixes were prepared using ordinary Portland cement (OPC) with a water-binder ratio of 0.18, sand-binder ratio of 1.1, and silica fume-cementitious material ratio of 0.15. AAF was included in the mixes at volume contents of 0.4 %, 0.8 %, and 1.2 %, with the AAF modified using 5 %, 10 %, and 15 % KH-550 solutions before use.
The prepared concrete slurry was examined for fluidity and then molded into different-sized specimens. After curing for 48 hours, these specimens were placed in water at (20 ± 1) °C for 28 days. Subsequently, the fiber-cement matrix bonding performance was examined using a microcomputer-controlled electronic universal testing machine.
The specimens' surface morphology was investigated using a scanning electron microscope (SEM), and their surface element composition was analyzed using an X-ray energy dispersive spectrometer (EDS). Fourier-transform infrared (FTIR) spectroscopy was also performed to identify the main hydration products.
A laser confocal microscope was employed to characterize the roughness and three-dimensional morphology of the fiber surface. An electrochemical tester was used to evaluate the corrosion resistance of the AAF in a 3.5 % NaCl solution.
The flexural and compressive strengths of the concrete specimens were determined using an automatic compression and bending test machine. An electronic universal testing machine was also used to measure their tensile performance. Finally, the microstructure and structural composition of the interface between the fiber and cement matrix were investigated using SEM.
Results and Discussion
The unmodified AAF had a smooth surface, but after modification with KH-550, it was covered with a grey silane film. The thickness of the silane film and the surface roughness of the AAF increased as the concentration of the KH-550 solution was raised.
Notably, the thicker silane layer on the AAF enhanced its corrosion resistance, as evidenced by the polarization curve in a 3.5 % NaCl solution. Additionally, the contact angle of the modified AAF was reduced by 80.3 % compared to the unmodified fiber.
As the concentration of the SCA increased, so did the force required to pull the fiber from the cement matrix. The 15 % SCA solution resulted in the highest interfacial bonding strength between the fiber and cement matrix, measuring 3.29 MPa—over a 15 % improvement compared to the unmodified AAF. This enhanced bonding was attributed to the increased fiber activity due to the silane layer on its surface.
The modified AAF also significantly improved the mechanical properties of UHPC, with the compressive strength reaching a maximum of 133.6 MPa at 0.8 % fiber content.
This improvement was linked to the reaction of the silanol structure on the modified AAF with the hydroxyl group in hydrated calcium silicate (a cement hydration product), forming Si-O-Si bonds that strengthened the fiber-cement matrix bonding. However, the compressive strength decreased with increasing fiber content, likely due to fiber agglomeration.
On the other hand, the flexural and tensile strengths peaked at 1.2 % fiber content. At this level, the peak stress of UHPC with modified AAF was 4.3 % higher than that with unmodified AAF and 31.3 % higher than UHPC without AAF. Thus, the SCA-modified AAF effectively enhanced the impact resistance of the concrete.
Conclusion
The researchers effectively utilized the SCA KH-550 solution to modify AAF, resulting in significant enhancements to the mechanical properties of UHPC, including flexural and compressive strengths, tensile performance, corrosion resistance, and impact endurance.
The AAF modified with 15 % SCA KH-550 produced superior UHPC, achieving maximum interfacial bond strength with the cement matrix. The compressive strength, flexural strength, tensile strength, and peak stress of the UHPC with modified AAF reached 133.6 MPa, 25.5 MPa, 8.32 MPa, and 114.26 MPa, respectively—representing increases of 2.9 %, 6.3 %, 10.9 %, and 4.3 % compared to UHPC with unmodified AAF.
These improvements in concrete properties were attributed to the silane layer formed on the surface of the modified fiber, which enhanced its activity, durability, and interfacial bonding with the cement matrix.
Journal Reference
Wang, D., Liu, R., Wang, S., Ma, X. (2024). Mechanical Properties of Ultra-High-Performance Concrete with Amorphous Alloy Fiber: Surface Modification by Silane Coupling Agent KH-550. Materials. DOI: 10.3390/ma17164037, https://www.mdpi.com/1996-1944/17/16/4037.
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