Development of Fly Ash and Limestone Calcined Clay-Based Mixtures for Concrete 3D Printing (2025-09)¶

This study explores the formulation and assessment of 3D-printable concrete mixtures that integrate fly ash and limestone calcined clay (LC2) as partial substitutes for Ordinary Portland Cement (OPC), with an emphasis on their flow behaviour and mechanical performance. Rheological tests, including flow table tests, open-time tests, constant shear rate, and flow curve methods, are conducted to evaluate fresh state properties. The results indicate that LC2 enhances fluidity due to its role as a mineral plasticizer, especially when combined with polycarboxylate ether superplasticizer. However, the irregular shape and texture of LC2 and fly ash increase water demand, leading to reduced flowability at lower substitution levels. The printability of the mixtures is assessed through open-time tests, revealing that a 20% LC2 mixture has a reduced printability window of 30 min compared to 60 min for the control mixture. LC2 improved the mixture’s thixotropic behaviour, which enhanced viscosity recovery after shear, making it suitable for layer-by-layer printing. In terms of strength, compressive strength initially decreased with lower LC2 and fly ash substitution but matched the control mixture at 20% LC2 substitution. The anisotropic compressive strength tests indicated that 3D printed specimens have lower compressive strength than mould cast specimens, a result attributed to weak interlayer adhesion during printing.