Synergistic effects of secondary CO₂ mixing and different water reducing agents on mortar properties

The emergence of 3D concrete printing technology faces a fundamental challenge to achieve both pumpability and buildability in a single material system. This research investigates the strategic integration of secondary CO₂ mixing with water-reducing admixtures (WRAs) as a potential solution to this challenge. Systematic investigation of three WRA types (polycarboxylate ether-based (PCE), naphthalene sulfonate formaldehyde-based (NSF), and sulfonated melamine formaldehyde-based (SM)) indicates that CO₂ injection creates a mechanism where carbonation products interact with WRA molecules on cement particle surfaces, transforming flowable concrete into buildable material on demand. This interaction can be tuned to achieve optimal printing performance while potentially contributing to CO₂ utilization. Secondary CO₂ mixing significantly reduced workability and shortened setting time of mortar with WRAs (e.g., workability of mortar with high dosage of NSF reduced from 264 mm to 128 mm and setting time reduced by 69.5 %), while maintaining comparable 28-day compressive strength across most formulations. Although drying shrinkage increased by 13–26 %, this trade-off appears manageable through optimized mixture design. The mechanism where WRAs promote carbonation that subsequently modifies their own effectiveness becomes an advantage, enabling precise rheological control. This approach provides insights into controlled rheological modification for sustainable construction applications, positioning 3D printed concrete as both a structural solution and a potential carbon utilization technology.