In Situ Alkali‐Induced Surface Modification of Basalt Fibers for Improved Interfacial Bonding in Geopolymer Composites

Basalt fibers (BFs) have emerged as sustainable, high‐performance reinforcements for geopolymers, providing an effective solution to their inherent drawbacks of severe shrinkage, pronounced brittleness, and limited toughness. However, the intrinsically smooth and chemically inert surface of BF limits interfacial bonding with the geopolymer matrix, thereby reducing their reinforcing efficiency. This study proposes an innovative in situ alkali pretreatment strategy, in which BFs are pretreated using the alkaline activator prior to mixing, thereby eliminating additional processing steps while enhancing the fiber‐matrix interface. Experimental results showed that alkali‐pretreated BF significantly improved the mechanical performance of metakaolin (MK)‐based geopolymers, with the flexural strength increasing by up to 71.7%, which is substantially higher than that achieved with untreated fibers (36.4%). Microscopic and spectroscopic analysis revealed that alkali treatment induced partial dissolution and cation reprecipitation on the fiber surface, resulting in a pronounced increase in surface roughness from R a = 12.1–45.6 nm, thereby enhancing mechanical interlocking. Simultaneously, cleavage of Si–O–T (T = Si or Al) bonds and the formation of Si–OH and Al–OH groups generated reactive sites that chemically bond with the geopolymer gels. The synergistic effects of mechanical interlocking and chemical coupling strengthened interfacial adhesion and overall structural integrity. This cost‐effective and sustainable pretreatment offers a practical route to optimize interfacial structures in fiber‐reinforced geopolymers, paving the way for high‐strength, durable, and low‐carbon construction materials.