Effect of time interval on the interlayer adhesion of 3D printed concrete with recycled sand: Multi-factor influencing mechanisms and superabsorbent polymer enhancement

Interlayers have a significant impact on the mechanical properties and integrity of 3D printed concrete structures. The interlayer shear strength of 3D printed recycled mortar (3DPRM) was tested at interval times of 0 min-24 h, considering the effects of moisture, pore structure, and hydration products of the interlayer. Furthermore, the enhancement mechanism of superabsorbent polymer (SAP) incorporation on the interlayer adhesion of 3DPRM was also investigated. The findings indicated that as the time intervals increased, there was a corresponding increase in the yield stress of the bottom filament, leading to an increase in thickness and degradation of pore structures in the interlayer region. Additionally, there was a loss in moisture and a reduction in unhydrated particles, resulting in a decrease in the amount of C-S-H. As the bottom filament progressed from its fresh state to the final setting, the dominant factor in interlayer adhesion changed from moisture to pore structure. This conclusion provides a theoretical basis for solving the interlayer cold joint problem at different time intervals. The incorporation of 0.3% SAP completely offset the negative effect of recycled sand incorporation on interlayer adhesion strength and has good potential for practical application.