Fracture damage mechanisms in ultra high performance geopolymer concrete: A multi-parameter acoustic emission study

Geopolymer binders offer a low-carbon alternative to Portland cement in Ultra-High-Performance Concrete (UHPC), yet whether they engage the same fiber–matrix fracture mechanisms as conventional UHPC remains unknown. This study reveals that they do not: cementitious and geopolymer fiber-reinforced UHPC employ fundamentally different energy dissipation pathways despite achieving comparable damage tolerance. Four UHPC configurations, a Portland cement reference (R), a slag/silica-fume geopolymer (G), and their steel fiber-reinforced counterparts (Rf, Gf) were tested under three-point bending at 24 h and 28 days with simultaneous acoustic emission (AE) monitoring. The cementitious composite (Rf) dissipates energy through a friction-dominated pathway, evidenced by 19.4% shear-type AE events and 18.4% long-duration signals (>500 µs) characteristic of frictional fiber pull-out within a dense interfacial transition zone. In contrast, the geopolymer composite (Gf) follows a debonding-dominated pathway, generating 91.9% tensile-type events with only 3.6% long-duration signals, reflecting progressive fiber–matrix debonding across a distributed crack network. A toughness crossover is identified: the geopolymer composite achieves superior energy absorption at 24 h (Gf: 30.2 J vs. Rf: 25.1 J) with a b-value trajectory that never drops below 1.0 indicating purely distributed damage without macro-crack localization but the cementitious system surpasses it by 28 days (Rf: 46.7 J vs. Gf: 35.0 J) as C–S–H densification strengthens the ITZ (+86% toughness gain vs. +16% for continued geopolymerization). Remarkably, both plain matrices exhibit identical toughness at 28 days (0.334 vs. 0.336 J) despite an 18 MPa compressive strength difference, demonstrating that fibers not the matrix alone activate the binder-specific fracture divergence. These findings establish that binder chemistry governs the energy dissipation mechanism rather than merely the strength level, with direct implications for the design of sustainable fiber-reinforced UHPC in precast and early-age applications.