Mechanical Performance of Polypropylene Fiber–Reinforced Fly Ash and Metakaolin Geopolymers under Different Alkaline Molarities

This study evaluates the effect of polypropylene (PP) fiber addition on the mechanical performance of fly ash (FA)- and metakaolin (MK)-based geopolymer composites activated with 8M, 10M, and 12M sodium hydroxide. Mixes were cured at 60 °C for 24 h to obtain consistent early-age strength. Mechanical tests included compressive strength, three-point flexural strength, and splitting tensile strength. For FA-based mixes at 12M NaOH, PP fibers at 900 g/m³ substantially improved performance: compressive strength increased from 300 kg/cm² to 416 kg/cm², (≈38.7% gain), flexural strength increased from 33 kg/cm² to 62 kg/cm², and splitting tensile strength increased from 8.8 kg/cm² to 20.4 kg/cm². In contrast, MK-based mixes exhibited lower absolute strength and only limited gains after PP fiber addition. For MK-based geopolymers, adding PP fibers produced only limited gains in flexural and splitting tensile strength, indicating less efficient stress transfer between fiber and matrix compared with FA-based mixes. Monte Carlo simulations were employed to model the variability of compressive strength across the different mixes, providing probabilistic insight into performance reliability rather than relying solely on mean values. The statistical analysis indicates that FA-based geopolymer reinforced with PP fibers delivers both higher strength and higher reliability, supporting its viability as a structural alternative to Portland cement concrete in applications that demand improved flexural and tensile resistance. At the same time, the results show that MK-based systems are less responsive to PP fibers at the tested dosage, suggesting that improved reinforcement strategies (e.g., different fiber type, higher fiber content, or modified curing conditions) are required to enhance the mechanical efficiency of MK-based geopolymers. The results inform sustainable geopolymer binder design and elucidate how precursor chemistry governs the effectiveness of PP fibers at the fiber matrix interface.