Mechanical, Chloride Resistance, and Microstructural Properties of Basalt Fiber-Reinforced Fly Ash–Silica Fume Composite Concrete

Basalt fiber has advantages in enhancing the mechanical properties of concrete, but the comprehensive effects of fiber content and length, as well as the relationship between mechanical and impermeability performance, remain unclear and require systematic verification. This study aims to quantify the effects of basalt fiber content and length on mechanical properties (compressive strength, tensile strength, and flexural strength) and concrete permeability performance and reveal the underlying mechanisms. The macroscopic performance results indicate the following: (1) the optimum fiber content of compressive strength and flexural strength of basalt fiber-reinforced concrete is 1.5 kg/m3: (2) the optimum content of tensile strength is 1.0 kg/m3: and (3) the impermeability performance of the fiber-reinforced concrete is most significantly improved when the fiber content reaches 1.0 kg/m3 and the fiber length is 18 mm. During the permeability tests, a nonlinear functional relationship exists between two indicators, electric flux and chloride ion migration coefficient. Microscopic analysis showed that mineral admixtures (fly ash and silica fume) promoted the secondary hydration reaction in the cementitious material, generating a significant amount of C-(A)-S-H gels to increase the density of the concrete matrix. After incorporating basalt fibers, they tightly envelop the concrete matrix, reducing the number of internal voids and achieving a synergistic stress-bearing effect with the concrete, confirming that the addition of fibers optimizes the mechanical and impermeability properties of the concrete. This study provides a quantitative reference for the basalt fiber reinforcement design of engineering concrete structures and helps extend the service life of concrete buildings.