Performance of 3D-Printed Cementitious Composites With Recycled Waste Materials

3D concrete printing (3DCP) has developed rapidly over the last decade and the interest in sustainable cementitious composites by incorporating recycled waste materials has also increased in 3DCP. Regarding the research works on 3DCP, the main focus has been put on the rheological and hardened mechanical properties, in contrast to relatively limited research outputs on the correlation between mechanical and microstructural properties of 3D-printed cementitious composites. Furthermore, unidirectional printing patterns have been widely utilised for the printing process. However, voids due to underfilling between extruded filaments may occur depending on the fresh properties of materials, leading to negative impacts on the overall mechanical performance. Therefore, it is also expected to modify the unidirectional pattern to enhance the mechanical properties of 3D-printed cementitious composites potentially. This work aims to develop a solid understanding of the relationship between the mechanical properties and microstructural features of 3D-printed cementitious composites containing different categories of recycled waste materials. Three kinds of recycled waste materials are considered for this work, including recycled glass particles, recycled crumb rubber and recycled carbon fibres. The research scope and experiment plan are determined in the first section of this work. A laboratory-scaled extrusion-based printer is used to fabricate 3D-printed cementitious composite specimens. The fresh properties of mortar materials, including slump flow and buildability, are evaluated prior to mechanical tests to ensure the materials' printability. The mechanical properties of 3D-printed specimens to be evaluated mainly consist of compressive and flexural strengths. The microstructural features include the void structures inside printed specimens and the microstructural morphology of cement matrix, which are investigated via X-ray micro-computed tomography (μCT) and scanning electron microscopy, respectively. The second section of this work focuses on the flexural properties of 3D-printed cementitious mortar specimens containing 50 wt.% recycled glass particles as the partial replacement of sand. Two grades of recycled glass are used, including the coarse and fine grades with the median size of 796 and 367 μm, respectively. The mortar specimens are 3D printed with the conventional unidirectional pattern. As for the three-point test, the mid-point load is always along the layer deposition direction. When the beam span is parallel to the printing direction, the flexural strengths experienced a decrease with the addition of recycled glass, which is correlated to the μCT analysis results that the proportion and morphology of voids affect the crack propagation. When the beam span is perpendicular to the printing direction, the addition of recycled glass positively influences the flexural strength. Such an opposite trend can be correlated to the analysis results of μCT and SEM, showing the strength improvement and the way the crack propagates depends on the glass particles situated alongside the crack path. After that, the third section of this work moves on to the compressive properties of 3D-printed cementitious mortar specimens containing 15 wt.% recycled crumb rubber as the partial replacement of sand. The crumb rubber is pre-coated with cement paste, with three cement-to-rubber ratios (C/R) considered (0.25, 0.4 and 0.55). The 3D-printed mortar specimens are fabricated with the unidirectional pattern. The anisotropic behaviour in compression is more obvious for the medium-to-high C/Rs compared to the low C/R. According to μCT analysis, the mechanical anisotropy of printed mortar with medium-to-high C/Rs could be correlated with two factors: void morphology and void orientation relative to the direction of external compressive loads. As for the low C/R, the insufficient bonding between the rubber surface and cement matrix, rather than the void structure, is considered more critical to influencing the mechanical properties of printed mortar. The fourth section investigates the compressive and flexural properties of 3D-printed mortar specimens with recycled carbon fibres (up to 2 vol.%). Unlike the previous two studies, this study further modifies the unidirectional pattern by introducing the layer offset strategy and explores the effects of layer offset on the mechanical and microstructural properties of 3D-printed specimens. It is found that when the degree of layer offset increases, the compressive and flexural strengths along all testing directions are improved. Meanwhile, the anisotropic level of compression in 3D-printed specimens also decreases as the layer offset degree increases. μCT analysis shows that the internal void structures comprise channel and micro voids. The geometry regularity of channel voids is lower than that of micro voids. Varying the layer offset degree can change the total porosity and volumetric proportions of channel voids, thus affecting the level of stress concentration around the voids themselves and the mechanical properties. The experiment results indicate that layer offset can be considered a potential method to enhance the mechanical performance of 3D-printed cement mortar specimens containing recycled waste materials. Considering the importance of μCT analysis on the void structure analysis of 3D-printed concrete, the fifth section further provides an extended review of applying X-ray μCT analysis to investigate different categories of concrete, followed by the machine learning application on improving the phase segmentation accuracy from the μCT sliced images of concrete. Based on the review outcomes, the machine learning models are recommended to be integrated with the X-ray μCT analysis of 3D-printed cementitious materials in future studies to accurately capture different phases of concrete materials and understand the inherent properties of 3D-printed materials. In summary, this research provides new insights into the mechanical performance of 3D-printed cementitious composites with recycled waste materials from the perspective of microstructural characteristics. Through this doctoral research, the mechanical properties of 3D-printed specimens are found to be closely related to the internal void structures and microstructural morphology of cement matrix, providing fundamental suggestions and guidelines for potentially enhancing the mechanical properties of 3D-printed cementitious specimens in future research. The experiment results also demonstrate that incorporating recycled waste materials into cementitious materials and improving the mechanical properties of printed specimens via modifying the printing pattern are practical solutions for 3D concrete printing.