Utilizing silica-enriched filter cake as a cement replacement and rheology modifier in 3D-printable mortar containing coffee husk and palm leaf ashes

This study evaluates a novel combination of coffee husk ash (CHA), palm leaf ash (PLA), and silica-enriched filter cake (SEFC) for extrusion-based 3D concrete printing (3DCP), where CHA and PLA serve as waste-derived supplementary cementitious materials (SCMs) and SEFC functions both SCM and as a rheology-modifying additive to enhance fresh and hardened performance. Physicochemical characterization revealed high silica contents in SEFC (80.6%), CHA (39.3%), and PLA (42.4%), with elevated specific surface areas compared to ordinary Portland cement (OPC), indicating potential pozzolanic characteristics. X-ray diffraction analysis confirmed the presence of amorphous phases favorable for hydration reactions. SEFC significantly improved rheological behavior, enhancing extrudability, shape retention, and buildability, enabling stable layer-by-layer printing. Fresh-state results indicated controlled workability loss and suitable setting behavior for layer-by-layer deposition. In the hardened state, SEFC-modified mortar (optimized mix) exhibited compressive strength improvements of 19.3% and 12.8% at 7 and 28 days, respectively, compared to the CHA–PLA reference mixture (M11). Microstructural analysis showed a denser matrix with reduced porosity and increased calcium silicate hydrate formation, explaining the improved mechanical performance. Overall, CHA, PLA, and SEFC demonstrate strong potential as SCMs and functional additives for 3DCP, contributing to reduced cement consumption and circular waste valorization.