Unlocking the Potential of Lignin‐Based Modifiers: Improved Mechanical Robustness and Combustion Resistance in Basalt Fiber‐Reinforced Epoxy via Calcium Lignosulfonate Treatment

Basalt fiber (BF) is an emerging eco‐friendly, high‐performance reinforcing material. However, its inherently smooth and chemically inert surface hinders effective interfacial bonding with polymer matrices, limiting its widespread application in composite materials; this makes surface modification a key prerequisite for enhancing performance. To address this challenge, a sustainable surface modification strategy was proposed in this study, involving the deposition of a calcium lignosulfonate (CL) coating on the BF surface through a dipping process. Basalt fiber/epoxy composites were prepared using fibers treated with a 3 wt% CL solution at a pH of 7.0, resulting in significantly improved mechanical properties and flame retardancy. Specifically, the average transverse flexural strength increased to 530.60 MPa, which is 12.75% higher than that of untreated BF composites. Meanwhile, the tensile strength increased to 359.40 MPa, representing a 12.95% increase. The combustion rate of the modified material decreased to 0.0574 m/s, representing a 27.79% reduction compared to the unmodified composite. The proposed CL modification system is characterized by its simplicity, mild conditions, low cost, waste reuse, and renewability, offering a scalable and eco‐friendly route for functionalizing BFs and showing promise for extension to other inorganic or natural fiber systems.