The Bending Deformation Behavior and its Geometry Effect of High-Strength Engineered Geopolymer Composites with Varying Slag and Fly Ash Dosages

Geopolymer is a low-carbon and eco-friendly construction material whose microstructure and macroscopic properties are highly dependent on precursor composition. Such variations are expected to affect the deflection hardening and multiple cracking of engineered geopolymer composites (EGC). To systematically evaluate the effect of varying dosages of slag and fly ash on the flexural deformation behavior of high-strength EGC (HS-EGC), four-point bending tests were conducted on thin plates and prismatic beams, and the relationship between pore structure and flexural deformation behavior was further analyzed. Subsequently, the geometric effect on flexural toughness, ductility, and crack morphology was systematically investigated through image analysis and load–deflection ( 𝐹 − 𝛿 ) curve characterization. The results indicate that the 𝐹 − 𝛿 curves of the thin plate exhibited more frequent and stable load oscillations compared to those of the prismatic beam, accompanied by a higher deflection-to-span ratio. Correspondingly, the lower surface of the thin plate specimen subjected to bending failure remained intact with no primary cracks observed. The bending ductility and toughness of HS-EGC initially increased and then decreased with increasing slag dosage, peaking at 50 wt% slag, and showed a positive correlation with the porosity of 1–100 μm pores. For prismatic beam specimens, the flexural toughness and deflection were 1016.87 kJ/m³ and 7.31 mm, while those of thin plate specimens were 1120.91 and 53.00 mm, respectively. The pore structure of HS-EGC was associated with variations in its ductility and toughness at different slag dosages. The findings provide experimental evidence and a specific case study supporting the implementation of HS-EGC in resilient structures.