A novel method to enhance interlayer bond in 3D-printed concrete using dry-powder application of supplementary cementitious materials

Extrusion-based 3D-printed concrete (3DPC) offers significant potential for automated construction but remains limited by weak interlayer bonding, which compromises structural reliability. This study investigates a low-complexity interlayer treatment in which dry pozzolanic supplementary cementitious material (SCM) powders, metakaolin (MK) and silica fume (SF), are applied between successive filaments during printing. This targets the weak interface by micro-filling and by inducing a localized pozzolanic reaction, without altering the main mix. Using a single baseline printable mortar, interlayer performance was evaluated through flexural and compressive strength testing, ultrasonic pulse velocity (UPV), scanning electron microscopy with EDS analysis, and drying shrinkage measurements. Both powders significantly enhanced interlayer cohesion but exhibited distinct performance trajectories. MK produced a rapid, localized effect, increasing flexural strength by 66% at 7 days while reducing drying shrinkage. In contrast, SF promoted progressive volumetric densification, achieving 51% flexural and 34% compressive strength increases at 56 days, accompanied by higher shrinkage. Microstructural and microchemical analyses revealed that MK concentrates pozzolanic reaction products at the interface, whereas SF drives pozzolanic activity into the bulk matrix. These findings demonstrate a clear strength-shrinkage trade-off and provide practical guidance for selecting interlayer treatments in 3D-printed concrete. This study presents the first systematic evaluation of dry-applied SCM powders as targeted interlayer modifiers in extrusion-based 3DPC, integrating mechanical, non-destructive, and microchemical evidence to validate a dual-action enhancement mechanism. By explicitly quantifying the strength-shrinkage trade-off and distinguishing localized and volumetric reactivity pathways, the work establishes a materials-based framework for rational selection of interlayer treatments while offering a low-complexity alternative to conventional wet-phase bonding systems. • Dry powder SCM application improves interlayer performance in 3D printed concrete. • Metakaolin enhances early-age interlayer strength while reducing drying shrinkage. • Silica fume increases long-term strength via volumetric pozzolanic densification. • SEM/EDS reveal localized versus bulk interlayer reaction mechanisms. • Strength-shrinkage trade-off guides material selection for layered concrete printing.