Potential of rice husk biochar in 3D concrete printing: an analysis of fresh-state rheology, strength retention, and carbon sequestration

This study evaluates rice husk biochar (RHB) as a partial cement replacement in 3D concrete printing (3DCP). RHB was incorporated at 2.5–15 vol.% and its effects on extrudability, buildability, rheology, mechanical strength, microstructure, binder-normalised CO₂ uptake (thermogravimetric analysis), and total CO₂ emissions were investigated. Buildability increased with dosage and peaked at 10 vol.% (26 layers), while 15 vol.% failed during initial deposition. Compressive strength at 28d showed ≈3% higher than CTRL at 2.5 vol.% but ≈8% lower at 5 vol.%, whereas 10 vol.% reduced strength by ≈13%. Flexural strength was more sensitive to microstructural continuity remaining similar to CTRL at 2.5 vol.%, while 5 and 10 vol.% retained ≈6.5% and ≈8% deficits. Microstructural analysis indicated a denser interfacial transition zone at 2.5–5 vol.% and pore discontinuities at 10 vol.%. All RHB mixes showed higher CO₂ uptake than CTRL, with a maximum at 5 vol.% (≈17–18 wt.% of binder). Incorporation of 10 vol.% RHB reduced total CO₂ emissions by 6.7%. The results identify a practical operating window (2.5–5 vol.%) balancing print performance, strength retention, and carbon uptake; higher dosages may target buildability or sequestration. Overall, this study advances sustainable 3D-printed materials toward low-carbon and potentially carbon-negative construction.