Pore evolution in extrusion-based 3D printed cementitious materials due to calcium leaching

Extrusion-based 3D printed cementitious materials (3DPC) require specific parameters to be met that will substantially impact printed part durability, particularly when exposed to aggressive external conditions. Due to the specificity of the spatial distribution of the pores, 3DPC are reported to exhibit performance anisotropy compared to cast materials. A fundamental understanding of the discrepancy in the pore evolution between the printing direction, lateral direction, and top direction is essential for calcium leaching from 3DPC in aggressive water. This research investigates pore structure evolution in 3DPC with the three directions in a 6 mol/L ammonium chloride solution. The leaching depths of the samples in the three directions were tested and compared. In addition, their porosity, maximum, median, and average pore diameters, pore size distribution, and pore morphology are investigated using optical microscope (OM) and mercury intrusion porosimetry (MIP) analysis. Significant differences were found between the leaching depths in the different directions of the 3DPC. Samples leached in the top direction show less leaching depth than in the other two directions, indicating the anisotropic nature of 3DPC. This was attributed to the different pore structures of the sample in the top direction compared to the other directions, as confirmed by OM and MIP analysis. Furthermore, the pore morphology is more irregular and elongated in the printing and lateral directions than in the top direction. In turn, the calcium leaching increases the porosity and coarsens the pore structure of 3DPC. The results of this study can be used as a reference for the application of 3DPC in aggressive water environments.