Pull-Out Behavior of Single-Hooked and Corrugated Steel Fibers Embedded in Slag-Based Geopolymer Mortars

Incorporating fibers into geopolymer concrete creates a sustainable composite with improved mechanical properties for structural applications. A key factor is the bond between fibers and the geopolymer matrix, which governs fiber efficiency—particularly since geopolymers differ from conventional concrete by lacking cement. This study investigated the pull-out behavior of two steel fiber types—single-hooked and corrugated—embedded in slag-based geopolymer mixes with varying silicate modulus (Ms = 0.8, 1.4, 2.0) and activator-to-binder ratios (A/B = 0.08, 0.11, 0.14). For comparison, two ordinary Portland cement (OPC) mortars (20 MPa and 50 MPa) were also tested. Fiber orientation effects were examined at 0°, 22.5°, and 45°. All mixes were evaluated for compressive and tensile strength, along with scanning electron microscope (SEM) analysis. Pull-out tests assessed bond-slip behavior, failure modes, pull-out energy, and slip resistance. Results showed that increasing Ms from 0.8 to 1.4 improved strength, ultimate load, and energy absorption, while further increase to 2.0 reduced performance. Higher A/B ratios consistently weakened mechanical properties due to increased porosity and reduced fiber–matrix bonding. Fiber type, orientation, and matrix strength significantly influenced failure modes. Fibers aligned at 0° failed by pull-out, at 22.5° sometimes ruptured in stronger matrices, and at 45° caused matrix breakout. Corrugated fibers provided better interfacial bonding than single-hooked fibers, resulting in higher load capacity and energy absorption. SEM/EDS analysis confirmed that optimal composition (Ms = 1.4, A/B = 0.08) produced a dense, uniform matrix with performance comparable to high-strength OPC.