Mechanical Properties of Basalt-Fiber-Reinforced Metakaolin–Slag–Fly Ash Geopolymer Mortar Characterized by 2D-DIC

Against the backdrop of rapid development in low-carbon building materials, geopolymer mortar has become a high-quality alternative to traditional cement-based materials due to its advantages of environmental friendliness, high strength, and excellent durability. However, its inherent brittleness and tendency to crack severely limit its widespread adoption and use in engineering. To mitigate the inherent brittleness of geopolymer mortar, this study developed a ternary binder system composed of metakaolin, slag, and fly ash. The effects of basalt fiber contents of 0%, 0.25%, 0.50%, 0.75%, 1.00%, and 1.25% by mass on the flowability, flexural strength, compressive strength, and deformation behavior of the geopolymer mortar were systematically investigated. The evolution of the displacement and strain fields during flexural and compressive loading was monitored in real time using two-dimensional digital image correlation (2D-DIC). The fiber-reinforcement mechanism was further examined by X-ray diffraction (XRD), scanning electron microscopy (SEM), and Fourier transform infrared spectroscopy (FTIR). The results show that basalt fiber reduces mortar flowability, and the reduction becomes more pronounced with increasing fiber content. The flexural strength first increased and then decreased with increasing fiber content; at 0.50% fiber content, the 28-day flexural strength reached 12.6 MPa, which was 8.2% higher than that of the fiber-free control. The compressive strength increased only slightly at a low fiber content of 0.25% and then decreased when the fiber content exceeded 0.50%. The 2D-DIC results indicate that a moderate fiber content (0.50–0.75%) markedly increased the ultimate displacement, delayed crack propagation, and enhanced the post-cracking deformation capacity. Microstructural observations revealed that an appropriate fiber content promoted good interfacial bonding with the matrix and enabled fiber bridging and crack resistance. In contrast, excessive fiber addition caused agglomeration-induced micropores and microcracks, thereby degrading mechanical properties. Overall, the recommended basalt fiber content is 0.25–0.50%. These findings provide a theoretical and experimental basis for optimizing high-performance, low-carbon geopolymer mortar for engineering applications.