Calcium Leaching of 3D-Printed Cement Paste Exposed to Ammonium Chloride Solutions

Understanding the degradation of 3D-printed cement paste (3DPC) is essential to evaluate changes in the long-term durability of concrete structures subjected to aggressive water. However, the degradation mechanism of 3DPC has yet to be reported, as the microstructure and pore characteristics of 3DPC are very different from those of its cast counterparts. This work studies the performance anisotropy of 3DPC due to calcium leaching to investigate the degradation mechanism. Samples with aggregate micro fines (AMF) and fly ash (FA) were prepared. A 6 mol/L NH4Cl solution was used in the accelerated experiment. At specific leaching durations, performances were tested on the samples in different leaching directions. The performance anisotropy of 3DPC exposed to aggressive water was investigated by comparing the changes in bulk density, water absorption, leaching depths, and compressive strength in different leaching directions. X-ray diffraction (XRD), differential thermal gravity-thermogravimetric (DTG-TG), and mercury intrusion porosimetry (MIP) were used to characterize the changes in hydration products and pore structure in different leaching directions. The results show that the performances of 3DPC in aggressive water have a significant anisotropic behavior. The evolution of pore defects and hydration products mainly governs the performance anisotropy of degraded 3DPC. The remaining hydration products of the surface of 3DPC leached in the Z direction are fewer than the other two directions because calcium ions leached in the Z direction are difficult to compensate through weak interfaces between layers. The test results clearly show that the calcium leaching mechanism of 3DPC in aggressive water is directly influenced by the hydration products, such as portlandite (CH) and C-S-H, and the pores. The current study may help us understand the degradation mechanism of 3DPC to assess its durability performance anisotropy.