Mechanical and microstructural characterization of interlayer bonding in multi-material 3D-Printed concrete

Layer-by-layer deposition in 3D-printed concrete (3DPC) facilitates functionally graded concrete (FGC) structures for sustainable construction. This study addresses the critical interfacial bonding challenge in multi-material systems by investigating printable alkali-activated concrete (AAC), normal concrete (NC), and engineered cementitious composite (ECC). Homogeneous concrete (HGC) and FGC specimens were fabricated, evaluating bonding strength evolution at 0–60 min intervals. Surface moisture content trends were monitored, revealing an overall decline with time, interrupted by a rebound at 30–45 min due to internal moisture redistribution. Results showed that the bonding strength in FGC specimens decreased linearly over time, whereas HGC specimens showed partial recovery at 45 min. Results quantified a linear decline in bonding strength for FGC specimens, with strength reduction reaching up to 32.25 % at 60 min compared to initial values. In contrast, HGC specimens exhibited partial recovery at 45 min, demonstrating strength restoration of up to 17.34 % relative to the 30 min interval. A multiscale analytical framework—combining molecular dynamics (MD), mercury intrusion porosimetry (MIP), and backscattered electron microscopy (BSE)—was employed to elucidate bonding mechanisms. MD simulations highlighted the importance of surface moisture for molecular-scale adhesion. MIP and BSE results confirmed that concrete type and interfacial moisture significantly influence pore structure and hydration, directly affecting bond strength. These findings offer critical insights into compatibility and time-dependent degradation in multi-material 3DPC.