Influence of Steel and Polypropylene Fibers on Flexural Strength and Fracture Properties of Ambient-Cured Geopolymer Concrete

The environmental urgency of reducing Portland cement consumption has driven research into geopolymer concrete (GPC) as a sustainable alternative. However, its inherent brittleness limits structural applications. This study addresses this critical challenge by investigating the efficacy of steel fibers (SF) and polypropylene fibers (PP) in enhancing the mechanical properties of slag-based GPC. Thirteen mixtures, including a control, were designed with varying fiber types, lengths (35/60 mm for SF, 40/60 mm for PP) and dosages (25–75 kg/m3 for SF, 3–9 kg/m3 for PP). Comprehensive tests evaluated workability, flexural/compressive strength, and toughness. Results demonstrated that while both fibers reduced workability (PP > SF), they significantly improved ductility, with SFs increasing toughness by 6–15 times. A key finding was the time-dependent performance: SF enhanced early-age flexural strength by up to 38%, though this benefit declined at 28 days for most mixes under ambient curing. PP fibers reduced flexural strength by 25–40% at 28 days. Compressively, SF increased strength by up to 60%, while PP led to reductions up to 27%. The study conclusively establishes SF’s superiority due to its superior bonding and crack-bridging capabilities, providing essential insights for designing durable fiber-reinforced GPC. This research directly contributes to advancing sustainable construction materials by overcoming a fundamental limitation of geopolymers.