Effect of Na2CO3 Replacement Quantity and Activator Modulus on Static Mechanical and Environmental Behaviours of Alkali-Activated-Strain-Hardening-Ultra-High-Performance Concrete

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.