Influence of different waste-derived fillers in extruded geopolymeric matrix for 3D printing

Abstract

One of the key challenges for the development of 3D printing in construction is the optimization of proper mix design in the fresh and hardened state. It is well known that the incorporation of nano- or micro-sized fillers generally improves thixotropic properties of the mixtures, having a great influence on the performance of extruded geopolymers by 3D printing.This work reports the preliminary results related to the optimization of geopolymer mix design for extrusion purpose. The effect of different fillers on fresh and hardened properties was assessed by manually extrusion of different geopolymeric mixes. Geopolymers and alkali activated materials are innovative binders with comparable and sometimes superior properties compared to traditional cementitious materials. The setting time of these materials is generally quite fast and usually occurs at room or at low (less than100 °C) temperature. In addition, they exhibit satisfying mechanical properties, hydraulicity and mesoporous microstructure.In this study, micro-sized fillers, sourced from different industrial wastes, such as fly ash and ceramic waste from porcelain tile grinding and lapping processes, were tested. For comparisons’ purpose, calcium carbonate was also tested as reference filler. The microstructure of the investigated materials as fillers was analyzed by particle size distribution, X-ray diffraction (XRD) and scanning electron microscopy (SEM). Then, geopolymers were prepared by mixing metakaolin and the selected fillers with sodium hydroxide and sodium silicate solutions, considering the following molar ratios: SiO2/Al2O3 = 2.24, Na2O/Al2O3 = 0.64, Na2O/SiO2 = 0.28. All the samples were cured at room temperature in sealed conditions. The optimized filler concentration was selected by investigating the workability of the fresh mixes using a mini-cone slump tester. Preliminary attempts of extrusion process were manually carried out using a 5 mm-conical nozzle in air at room temperature. Moreover, the setting time was evaluated by Vicat test, while physical properties of hardened samples were analyzed by hydrostatic weighing. The preliminary results exhibited that the particle size and shape of the fillers strongly influence the fresh properties of the mix and thus the suitability of the selected mixes for extrusion in the final 3D printing application.