Enhancing low-quality fly ash geopolymer paste with polyvinyl alcohol (PVA) fiber under steam curing: Toward improved mechanical performance and future applications of fibrous paste

The variability and low reactivity of low-quality fly ash (LQFA) limit its performance as a precursor in geopolymer binders. These limitations result in slow strength development, limiting its suitability for precast applications requiring sufficient handling strength and crack resistance. This study investigates the effectiveness of polyvinyl alcohol (PVA) fiber combined with steam curing in improving the fresh and hardened properties of LQFA-based geopolymer paste. Two fly ashes with distinct chemical and physical characteristics were characterized, followed by testing of fibrous paste using LQFA incorporating 0, 0.3%, 0.6%, and 0.9% PVA by volume. The results indicate that the fly ash with higher Calcium Oxide (CaO) content and finer particle size exhibited shorter setting time, better workability, and superior compressive strength. Although the incorporation of PVA fibers slightly reduced workability, it significantly enhanced mechanical performance. The mixture with 0.6% PVA achieved the highest compressive strength of 306.3 kg/cm² at 28 days. Flexural testing revealed improved post-cracking behavior and increased toughness due to effective crack-bridging, while excessive fiber content promoted agglomeration, reducing peak flexural load. Direct tensile tests showed an increase in tensile strength up to 2.2 MPa at 0.9% PVA. Ultrasonic Pulse Velocity (UPV) results demonstrated reduced pulse velocity at higher fiber contents, indicating increased internal heterogeneity caused by fiber clustering. Overall, the findings demonstrate that properly dispersed PVA fibers can effectively compensate for the brittleness and low reactivity of LQFA, enhancing its potential use in sustainable geopolymer-based non-structural precast elements, particularly thin-walled drainage components such as U-ditch segments.