Effect of Clay Brick Powder and Recycled Fine Aggregates on Properties of 3D Printed Concrete After High Temperature Exposure (2025-07)¶

To enhance the sustainability of 3D printed concrete (3DPC), this study investigates the synergistic utilization of recycled fine aggregate (RFA) and clay brick powder (CBP) as partial replacements for natural fine aggregate and cement, respectively. The primary objectives were to evaluate the effects of this combined substitution on the residual mechanical performance and crack evolution patterns of 3DPC specimens after exposure to elevated temperatures, aiming to develop a cost-effective, low-carbon 3DPC with enhanced thermal resilience. 3DPC mixtures and its mold-cast counterpart with 50 % RFA and 10 % CBP, alongside control specimens (without recycled materials), were exposed to temperatures ranging from 20 °C to 800 °C for 1 or 2 h. Post-exposure, residual compressive strength and crack patterns were analyzed. Microstructural evolution, including pore structure changes and phase transformations, was characterized using mercury intrusion porosimetry (MIP), sorptivity tests, and X-ray diffraction (XRD) to elucidate underlying degradation mechanisms. Test results demonstrate that although the incorporation of 10 % CBP and 50 % RFA reduced the compressive strength at ambient temperature, it significantly enhanced the residual strength retention rate after high-temperature exposure, due to the crack-inhibiting and internal curing effect provided by CBP and RFA. However, the internal curing effect was evident only at temperatures below 400 °C and for retaining times of 1 h or less. Although a similar degradation trend in the residual compressive strength with the increasing temperature was observed for 3DPC and mold-cast concrete having the same formulation, 3DPC exhibited unique cracking patterns, with cracks preferentially propagating along interlayer and interstrip interfaces due to steam channeling, contrasting with the dispersed cracking in mold-cast counterparts. Furthermore, the addition of recycled materials amplified the anisotropy of 3DPC at both ambient and elevated temperatures, with a minimum anisotropy observed at 200 °C due to directional strength variations. This work highlights the potential of using recycled materials in 3DPC to enhance its high temperature performance, providing valuable insights for sustainable 3D printing in construction.