Low carbon and high performance have become key trends in the development of construction materials. Understanding the mechanism by which curing conditions affect the mechanical properties of high-ductility geopolymer concrete (HDGC) is of significant importance. This study investigated three sealing curing temperatures (room temperature, 45 °C, and 60 °C) and four curing durations (1 day, 3 days, 5 days, and 7 days), while considering two final curing ages (7 days and 28 days) to explore their effects on the axial tensile and compressive properties of HDGC. The results showed that both 45 °C and 60 °C could improve the brittle failure of HDGC under axial compressive loading. However, curing at 60 °C and for durations longer than 1 day in an oven would catalyze the formation of eight-faced zeolite crystals within the slag–fly ash geopolymer matrix, and it could weaken the matrix’s pore structure and subsequently affect the material’s later strength development. Nevertheless, oven heat curing enhanced the bridging effect between the fibers and the matrix, partially compensating for the reduction in the initial tensile strength of HDGC. This follows the pseudo-strain-hardening material’s saturation cracking criterion to enhance the strain-hardening behavior of HDGC and improve its tensile energy absorption capacity. A curing condition of 45 °C for 5 days is recommended to maximize the ductility of HDGC. This study provides important theoretical support for the design and promotion of green, low-carbon, high-ductility composite materials.

The application of alkali-activated concrete (AAC) shows promise in reducing carbon emissions within the construction industry. However, the pursuit of enhanced performance of AAC has led to a notable increase in carbon emissions, with alkali activators identified as the primary contributors. In an effort to mitigate carbon emissions, this study introduces Na₂CO₃ as a supplementary activator, partially replacing sodium silicate. The objective is to develop a low-carbon alkali-activated-strain-hardening-ultra-high-performance concrete (ASUHPC). The experimental investigation explores the impact of varying levels of Na₂CO₃ replacement quantity (0, 0.75 Na₂O%, and 1.5 Na₂O%) and activator modulus (1.35, 1.5, and 1.65) on the fresh and hardened properties of ASUHPC. The augmentation of Na₂CO₃ replacement quantity and activator modulus are observed to extend the setting time of the paste, indicating an increase in the modulus of the activator and Na₂CO₃ replacement quantity would delay the setting time. While the use of Na₂CO₃ intensifies clustering in the fresh paste, it optimizes particle grading, resulting in higher compressive strength of ASUHPC. The tensile crack width of ASUHPC conforms to the Weibull distribution. ASUHPC with a Na₂CO₃ replacement quantity of 0.75 Na₂O% exhibits superior crack control capabilities, maintaining a mean crack width during tension below 65.78 μm. The tensile properties of ASUHPC exhibit improvement with increasing Na₂CO₃ replacement quantity and activator modulus, achieving a tensile strength exceeding 9 MPa; otherwise, increasing the activator modulus to 1.5 improves the deformation capacity, reaching 8.58%. Moreover, it is observed that incorporating Na₂CO₃ as a supplementary activator reduces the carbon emissions of ASUHPC. After considering the tensile performance indicators, increasing the activator modulus can significantly improve environmental performance. The outcomes of this study establish a theoretical foundation for the design of low-carbon, high-performance-alkali-activated-strain-hardening-ultra—high-performance concrete.

The utilization of polyethylene terephthalate (PET) powder as aggregate in the development of environmentally friendly high-ductility composites (P-EHDC) offers a promising pathway for advancing sustainable and high-performance concrete materials. Despite its potential, the fracture behavior of P-EHDC—particularly under the influence of alkali-activated precursors—remains insufficiently explored. In this study, the fracture performance of P-EHDC was evaluated by varying the precursor composition ratios (GGBS:FA = 4:6, 3:7, and 2:8) and PET powder replacement ratios (0%, 15%, 30%, and 45% by volume). Fracture modes, Mode I fracture energy (G_F), and crack propagation behavior were analyzed using the J-integral method. All specimens exhibited ductile fracture characteristics, a clear contrast to the brittle failure observed in conventional concrete. The replacement of 15 vol% PET powder significantly increased G_F in precursor systems with higher GGBS content (4:6 and 3:7), and 30 vol% was more effective in fly ash-rich systems (2:8). The J-integral method, which offers broader applicability compared to conventional methods such as the double-K fracture model, provided a more comprehensive understanding of the fracture behavior. The results showed that PET powder reduced the matrix fracture toughness, promoted matrix cracking, and weakened the fiber-bridging effect, leading to enhanced energy absorption via fiber pull-out. At low PET powder replacement ratios (e.g., 15 vol%), the cracking threshold of the matrix was not significantly reduced, while more fibers engaged during the crack instability stage to absorb fracture energy through pull-out. This behavior highlights the synergistic toughening effect between PET powder and fibers in the P-EHDC system. The effect became more pronounced when the PET content was below 45 vol% and the precursor matrix contained a higher proportion of GGBS, leading to enhanced ductility. This study introduces a novel approach to fracture behavior analysis in PET-modified alkali-activated composites and provides theoretical support for the toughening design of high-performance, low-carbon concrete materials.