The Effects of Synergistically Modified Basalt Fibers on the Durability and Microstructural Evolution of Cement-Based Composites

Basalt fiber (BF) has a smooth surface and poor hydrophilicity that leads to the formation of numerous pores in the interfacial transition zone (ITZ) when combined with cement. This prevents it from fully enhancing the durability of basalt fiber-reinforced cementitious composites (BFRCC). The ITZ is the weakest part of BFRCC and plays a decisive role in their durability. Therefore, three modification strategies were employed: (1) use of the silane coupling agents γ -aminopropyltriethoxysilane (AP), γ -glycidoxypropyltrimethoxysilane (GP), and γ -methacryloxypropyltrimethoxysilane (MA) to modify BF, producing AP/GP/MA-BF; (2) in situ growth of a nanoscale SiO 2 coating on the BF surface, producing NS-BF; (3) a dual, synergistic enhancement of BF, producing NS/AP/GP/MA-BF. These modifications aimed to improve the surface properties of BF, and their effects on the durability of BFRCC were studied. The results indicated that when nano- SiO 2 and GP were used for the synergistic enhancement of BF, BFRCC exhibited the lowest water absorption, chloride ion penetration depth, and migration coefficient, with reductions of 31.37%, 59.4%, and 51.6%, respectively, compared with those of the unmodified BF group. Additionally, under synergistic modification conditions, the BFRCC with modified BF endured the highest number of freeze-thaw cycles (120 cycles) and demonstrated smaller strength losses in compression and bending with respective reductions of 31.43% and 8.29%. Finally, microstructural analysis using X-ray diffraction, energy-dispersive spectroscopy, and scanning electron microscopy confirmed that after targeted regulation of the ITZ, the presence of nano- SiO 2 and GP on the BF surface induced the formation of hydration products such as Ca ( OH ) 2 , C─ S─ H gel, and AFt, which reduced interconnected pores and improved bonding. Mercury intrusion porosimetry (MIP) revealed that the synergistic modification of BF refined the pore structure above the nanoscale pores in BFRCC, considerably reducing the total pore volume before and after the freeze-thaw cycles. Overall, the synergistic modification of nano- SiO 2 and GP enhanced the BF, and the mechanism for enhancing the BFRCC durability was revealed through multiscale macro and microtargeted studies.