Enhancing Alkali Resistance of rGFRP Fiber and Mechanical Properties of Cementitious Composites through Sol-Gel Synthesized Nano-ZrO2 Coatings

Abstract Incorporation of the recycled glass fiber reinforced polymer (rGFRP) fibers into cementitious composites offers a sustainable pathway for the waste wind turbine blades. However, the inherent susceptibility of rGFRP fibers to alkali-induced degradation in cementitious environments severely limits their long-term performance. This study compared the effectiveness of two sol-gel synthesized nano- ZrO 2 coatings in enhancing the alkali resistance of rGFRP fibers through accelerated corrosion tests, single fiber pull-out test, four-point bending test, impact tests, accelerated aging test, scanning electron microscopy - energy dispersive X-ray spectroscopy (SEM-EDS) and nanoindentation test. Compared to the zirconium oxychloride octahydrate ( ZrOCl 2 · 8 H 2 O ) coating, the aminopropyltriethoxysilane (APTES)- ZrO 2 coating exhibited superior alkali resistance through the formation of a dense, chemically stable layer, and improved interfacial bonding. APTES- ZrO 2 coating reduced the mass loss of rGFRP fibers from 5.0% to 3.4% after 35 days of immersion in an alkaline solution, approaching the performance of alkali-resistant glass fibers (2.88%). The bonding strength of APTES- ZrO 2 coated fibers to calcium sulphoaluminate cement (SAC), ordinary portland cement (OPC) and geopolymer matrix was 2.7%, 17.8%, and 19.5% higher than that of uncoated fibers, respectively. The APTES- ZrO 2 coating markedly reduced the aging-induced flexural strength loss, with losses of only 3.2%, 3.6%, and 8.5% for the SAC, OPC, and geopolymer mortars, respectively, compared to substantially higher corresponding losses of 11.1%, 11.7%, and 15.6% with uncoated fibers. This work provides a practical strategy to enhance the durability of rGFRP fiber reinforced cementitious materials.