Valorisation of Industrial Wastes in Magnesium Potassium Phosphate Cements for Extrusion-Based 3D Printing

This study examines magnesium potassium phosphate cements (MKPCs) modified with industrial wastes for extrusion-based 3D concrete printing, evaluating the rheological properties (workability, setting time), mechanical performance and printability of formulations incorporating secondary materials: Mg dross waste (up to 20 wt.%, replacing MgO), calcined sewage sludge (up to 10 wt.%, replacing KH2PO4), alternative fillers such as glass from municipal solid waste glass and from construction and demolition waste and ground blast furnace slag, benchmarked against volcanic ash. The baseline MKPC exhibited initial/final setting times of 34/109 min, good workability and compressive strengths of 29 MPa (1 day)/28 MPa (28 days). Optimal low-waste mixes (e.g., using municipal glass or 20 wt.% Mg dross) shortened the initial setting to 19–25 min (decreasing 24–42%), reduced the slump by 9–18% yet remained printable at laboratory-scale and achieved 1-day strengths >23 MPa/28-day >31 MPa (comparable or superior). Glass from municipal waste proved most promising, due to superior workability, lighter aesthetics and strength gains, supporting circular economy goals while substantially reducing material costs; higher waste levels compromised fluidity and buildability. Mineralogical analyses confirmed K-struvite formation alongside residual periclase, validating these formulations for upscaling sustainable 3D printing.