Ambient-cured ultra-ductile engineered geopolymer composites (UD-EGC): Crack characteristics, micromechanical mechanisms, and sustainability assessment

Achieving ultra-high tensile ductility together with high strength in engineered geopolymer composites (EGC) under ambient curing remains challenging. In this study, ultra-ductile engineered geopolymer composites (UD-EGC) were designed based on micromechanical principles by regulating the fly ash (FA)/ground granulated blast-furnace slag (GGBFS) ratio (7:3–3:7), water-to-binder ratio (0.30–0.35), and polyethylene (PE) fiber length (12–18 mm). Without heat curing, the optimized UD-EGC with W/B = 0.30 and 18 mm PE fibers achieved compressive strength up to 122.60 MPa, tensile strain capacity up to 12.08%, tensile strength up to 6.83 MPa, and energy absorption up to 572.61 kJ/m3. Crack evolution was characterized using digital image correlation (DIC) and X-ray computed tomography (CT). The results revealed a linear relationship between crack number and tensile strain, while individual crack widths (about 200 μm) and peak local crack strains rapidly stabilized. CT reconstructions demonstrated dense, tortuous, and spatially uniform internal crack networks, confirming effective strain redistribution. Single-crack tensile tests and matrix fracture toughness measurements verified that both strength and energy criteria were satisfied, with pseudo strain-hardening indices above critical thresholds. Sustainability assessment showed that UD-EGC, as a cleaner material, reduced embodied carbon, embodied energy, and material cost compared with conventional ECC, offering a pathway for high-performance, low-carbon strain-hardening composites.