Mechanical Properties, Microstructure, and Water Absorption of Concrete Reinforced with Surface-Treated Basalt Fibers and Nanosilica

The development of fiber- and nanomaterial-modified cementitious composites has attracted increasing attention for their potential to enhance concrete performance and durability, contributing to more efficient and sustainable construction materials. This study investigated the influence of basalt fiber (BF), fiber surface treatment, and nanosilica (NS) on the mechanical properties, workability, microstructure, and capillary water absorption of concrete. Initially, basalt fiber-reinforced concrete (BFRC) mixtures containing fibers with lengths of 3, 6, and 9 mm at contents of 0.1%, 0.3%, and 0.5% by mass of cement were evaluated. Then, the selected 9 mm basalt fibers were surface-treated with HCl, followed by the incorporation of colloidal nanosilica at 1.5% and 3.0% by mass of cement. Compressive and flexural strengths, capillary water absorption, and scanning electron microscopy (SEM) were evaluated. The results showed that higher fiber contents negatively affected workability and mechanical performance, whereas nanosilica improved the performance of concrete containing treated basalt fibers. The highest compressive and flexural strengths among the investigated mixtures were obtained with 0.3% treated 9 mm basalt fibers combined with 3.0% nanosilica. At 63 days, this mixture reached compressive and flexural strengths of 34.87 and 7.57 MPa, representing an increase of 28.2% and 8.3%, respectively, compared with the reference concrete. The incorporation of 3.0% nanosilica also reduced capillary water absorption, particularly at fiber contents of 0.3% and 0.5%. SEM images suggested a denser cementitious matrix and improved fiber–matrix contact. The combined use of treated basalt fibers and nanosilica improved mechanical and microstructural performance while reducing capillary water absorption.