Development of Eco-Friendly Construction Materials for 3D Printing Using Fly Ash and Demolition Waste

The study investigates the potential of using Vietnam fly ash (FA) as a substitute for traditional Portland cement to reduce both the volume of landfilled waste and the carbon footprint of concrete mixtures, while maintaining adequate mechanical performance of the produced elements. Additionally, the incorporation of construction and demolition waste, recycled brick aggregate (BR), as a partial aggregate substitute was investigated to enhance the sustainability and resource efficiency of composite formulations. Five mixes, including a reference, were produced by casting and three-dimensional concrete printing (3DCP). Printability (flow table), water absorption (gravimetry and infrared thermography), and flexural/compressive behavior were assessed; printed specimens were tested parallel and perpendicular to the layer plane. Recycled additions reduced flow by 15–22%, yet all mixes remained printable. Printed specimens showed higher capillary uptake than cast ones. In flexure, modified mixtures composition exhibited 50% lower peak stress than the reference. Cast elements outperformed printed ones: the printed reference was 33% weaker than its cast counterpart, and other mixes were 10–15% lower. In compression, printed specimens loaded perpendicular to layers reached 6–7 MPa (35% below cast), whereas parallel loading yielded up to 3.5 MPa with larger scatter. The findings confirm the feasibility of utilizing secondary raw materials in 3DCP formulations to support resource efficiency and carbon footprint reduction in the construction industry.