Optimization of Direct Ink Writing (DIW) for geopolymers derived from Construction and Demolition Waste (CDW)

This thesis explores the transformation of construction and demolition waste (CDW) into sustainable building materials through geopolymerization and additive manufacturing. With growing emphasis on circular economy principles, the use of CDW presents a viable alternative to traditional cementitious systems by reducing environmental impact and enabling the valorization of industrial by-products. The research focuses on developing a geopolymer binder system using milled CDW as the main precursor, enhanced with metakaolin (MK) and copper slag. The aim was to optimize the binder’s formulation through milling, alkaline activation, and mix design to improve both mechanical properties and printability for extrusion-based 3D printing. The experimental program was conducted in two phases. In the first phase, CDW was milled for durations ranging from 0 to 120 minutes to study the impact of particle size on reactivity and strength. A 60-minute milling duration yielded the highest tensile strength (20.7 MPa), attributed to increased surface area and improved reactivity. However, over-milling beyond 90 minutes led to reduced performance due to particle agglomeration. In the second phase, MK and copper slag were added to the mix to boost aluminosilicate content and mechanical strength. The optimized alkaline activator system consisted of sodium hydroxide, sodium silicate, and potassium silicate. A series of water-to-binder (W/B) ratios were tested, with results showing that lower W/B ratios enhanced strength and reduced porosity. The best performance was observed at W/B = 0.35, where compressive and flexural strengths peaked and microstructure densification was evident. Printability was evaluated using the WASP 40100 LDM printer, focusing on flowability, shape retention, and interlayer adhesion. The optimized mix demonstrated smooth extrusion, strong bonding between layers, and minimal warping, indicating suitability for Direct Ink Writing (DIW) applications. Rheological tests confirmed shear-thinning behavior and sufficient open time—both essential for maintaining filament stability during the layer-by-layer build process. Mechanical tests confirmed that both particle fineness and binder chemistry significantly influenced material performance. The optimized formulation achieved an average tensile strength of 27.6 MPa, with additional improvements in compressive and flexural strengths, indicating its potential for structural and non-structural uses such as pavers, panels, and architectural elements. From a sustainability perspective, incorporating CDW, MK, and copper slag contributes to reducing the carbon footprint associated with construction materials. The use of recycled and industrial waste materials not only diverts waste from landfills but also conserves natural resources, aligning with EU objectives for low-carbon and resource-efficient construction. In summary, this thesis demonstrates that CDW can be effectively processed into a high-performance geopolymer binder suitable for additive manufacturing. The synergy of material optimization, controlled processing, and digital fabrication presents a viable path toward greener, durable, and technically sound alternatives to conventional concrete in construction applications.