Characterization of air-void systems in 3D printed cementitious materials using optical image scanning and X-ray computed tomography

For many 3D printed cementitious materials, air voids may play a dominant role in the interlayer bond strength. However, to date, far too little attention has been paid to reveal the air void characteristics in 3D printed cementitious materials. Therefore, to fill this gap, this study attempts to provide an example of systematically characterizing the typical air void system of 3D printed cementitious materials via different image acquisition and analysis techniques. Two printable limestone and calcined clay-based mixtures were employed to prepare the printed samples. The micrographs were acquired by using optical image scanning and X-ray computed tomography. Afterwards, air void metrics in printed cementitious materials were determined, i.e., content, distribution, size, and shape. The results revealed that most of the air voids with the diameter in the range of 10-1000 mu m were distributed evenly in the layer region of printed samples. Large air voids (1000-6000 mu m) were enclosed mainly between the printed filaments (interface region), which resulted in the relatively higher local porosity than that of layer region. Additionally, the majority of air voids displayed irregular and elongated shapes, which could be attributed to the extrusion and layer-wise manufacturing processes in 3D printing. Finally, a comparison between optical image scanning and X-ray computed tomography was given.