Rheological and mechanical properties of carbon-negative 3D printed mortar using functionalized biochar

Abstract

Biochar offers a lucrative pathway for reducing the carbon footprint of cement-based composites. However, printability is a critical constraint for incorporating biochar into extrusion-based 3D printing, due to high-water absorption and weak interfacial bonding. This limitation can be overcome by systematically altering the physicochemical properties of biochar. This study introduces a novel approach to developing carbon-negative 3D printable mortar by using functionalized biochar up to 26% (by weight of binder) with calcined clay (CC). The influence of biochar and CC on fresh-state behavior, including extrudability, open time, and buildability, was systematically evaluated, followed by assessments of mechanical properties and life cycle assessment. Experimental results revealed that the incorporation of biochar significantly improved the workability and extrudability of 3D printable mortars, offering a wide-open time up to 90 min. The buildability performance was strongly influenced by CC dosage, which improved thixotropic recovery and shape retention of the extruded filament. Mechanical performance revealed that the 24% biochar-containing mold-cast sample achieved a compressive strength of 54 MPa at 28 days of sealed curing, whereas the control batch (without biochar and CC) exhibited 51 MPa. Moreover, the 3D-printed samples with 24% biochar exhibited the highest directional strengths of 49, 45, and 35 MPa in the x, y, and z axes, along with maximum flexural and interlayer strengths of 8.5 MPa and 1.6 MPa, respectively. The life cycle assessment revealed that all functionalized biochar-CC batches achieved carbon-negative footprints, with permanent sequestration of up to 92 kg CO₂ eq/m³ after incorporating all the emissions associated with raw material production.