Optimal Mix Design of Cementitious Materials for 3D Printing

Abstract

Recently, 3D printing of cementitious materials has been investigated as a novel construction method. The automated aspect of this technology enables waste reduction, reduced labor requirements, and complex geometries fabrication. However, the use of cementitious materials for this application is challenging. The printed mix shall be flowable for simplicity of extrusion. Also, the 3D printed layers shall sustain their shape and gain strength quickly to support their weight and the succeeding layers. Nevertheless, there is a lack of guidance and instruction on designing and developing cementitious material mixtures for 3D printing. This research aims to develop a 3D printing system with a newly fabricated extruder capable of printing cement-based mixes. The extensive printing and testing of various cementitious-based materials confirm and assure the extruder's efficiency. Also, the aim is to develop a cementitious material mixture using local materials and to evaluate it in terms of several tests and methods. In general, compared with traditional cast-in-mold cementitious composites structures, the weak mechanical properties of 3D printed parts may limit the technology's capabilities. However, this study investigated the addition of CNTs and silica fume in a cementitious mix to enhance the mechanical performance and optimize the printing quality and buildability. The experimental program addressed fresh state properties through buildability, printability, open time, extrudability, printing quality tests, and shape retention. In this study, the printing quality test is described using measures of surface quality and dimensions of printed layers. Compressive strength and flexural strength have been evaluated as mechanical properties in this research by testing 10 several mixes. Based on observation, quantitative measurements, and mechanical properties, one mix was chosen as the optimum. It was observed that the inclusion of Carbon nanotubes (CNTs) and silica fume enhances printing quality and strength characteristics. It was found that adding 20% silica fume with 0.2% CNTs leads to continuous printing without blockage of 40 minutes on average and a maximum height of 263 mm on average as well. In addition, 20% silica fume and 0.2% CNTs in a printable mix with a w/c ratio of 0.38 exhibited a compressive strength of 29 MPa, 67 MPa, 79 MPa, and 85 MPa at 3 days, 7 days, 14 days, and 28 days respectively. Finally, flexural strength was increased due to the incorporation of 20% silica fume and 0.2% CNTs and was evaluated to be 5.4 MPa, 10.7 MPa, 12 MPa, and 19 MPa at 3 days, 7 days, 14 days, and 28 days correspondingly. Flexural strength results demonstrated that CNTs are better oriented through the print process.

Date of Award	Jul 2022
Original language	American English