Designing Printable Cement Composites with Porous Biochar Using an Extended Water Film Thickness Theory (2025-09)¶

Driven by the demand for sustainable construction solutions, there is a sparked interest in biochar-cement composites. Tailored biochar can modify the rheological and hardened properties of cement composites, potentially making them more suitable for 3D printing applications. This study examined the influence of wood waste biochar (WWBC) on the solid packing, water film thickness (WFT), printability, rheology, and microstructure of 3D printable cement composites (3DPCC). For designing 3DPCC with varying content of WWBC, the WFT parameter was the first time proposed to act as an indicator for adjusting the printability of WWBC-3DPCC, which we revealed to be within 0.040–0.055 μm. Adding WWBC facilitated the pumping and extrusion of 3DPCC during the printing window and maintained the shape stability of printed structures. This can be explained by the denser solid packing state, lower initial yield stress, higher thixotropy, and accelerated structural build-up kinetics of the mixture. All of these factors contributed to refining the microstructure and strengthening the interlayer bonding of 3DPCC. The results suggest the value-added utilisation of WWBC to improve the diverse properties of 3DPCC, and provides essential fundamentals for controlling the rheological behaviour of fresh cement-based mixtures containing porous particles.