Effects of CO₂ treatment on the interlayer bonding in 3D-printed alkali-activated concrete

Carbon dioxide (CO₂) treatment has demonstrated considerable potential for enhancing the interlayer bonding performance of 3D-printed concrete (3DPC). However, previous studies have mainly focused on homogeneous interlayer interfaces in Portland cement-based 3DPC. The effects and underlying mechanisms of controlled CO₂ treatment on alkali-activated 3DPC remain insufficiently understood, particularly for the heterogeneous interlayer interfaces formed between different alkali-activated concrete mixtures in functionally graded concrete (FGC). To address this, the present study systematically investigates the effects of different CO₂ concentrations on the interlayer performance of homogeneous interlayer interface (HOI) and heterogeneous interlayer interfaces (HEI) specimens composed of normal alkali-activated concrete (N-AAC), alkali-activated rubber concrete (R-AAC) and alkali-activated strain-hardening cementitious composites (SH-AAC). Interlayer bonding tests, XRD, DTG-TG, FTIR, and MIP were conducted to reveal the underlying mechanisms. The results indicate that the interlayer bonding strength exhibits a nonlinear trend with increasing CO₂ concentration, showing an initial decrease, followed by an increase and a subsequent decline. At the optimal CO₂ concentration (50%), the strength of HOI specimens remained below the uncarbonated specimens, whereas HEI specimens achieved strength enhancement (12%). CO₂ treatment promoted calcium carbonate formation, with an appropriate material combination in the HEI specimens favoring the formation of thermodyanmically stable calcite. Although carbonation reduced total porosity, excessive CO₂ concentration (80%) induced pore coarsening, increasing the proportion of mesopores in HEI specimens and macropores in HOI specimens.