Hydration products, pore structure, and compressive strength of extrusion-based 3D printed cement pastes containing nano calcium carbonate

While interest in extrusion-based 3D concrete printing technology has been growing in recent years, only limited literature investigates the hydration products and pore structure of 3D printed cementitious materials (3DPC). The microstructure and compressive strength results of 3D printed cement paste with and without nano calcium carbonate (NC) were investigated and compared to conventionally cast cement pastes. Herein, all mixes possess approximately the same static yield stress, which was adjusted by an additional polycarboxylate superplasticizer (SP). The compressive strength test was employed to quantify the mechanical anisotropy of samples. The hydration products and microstructure of samples were evaluated at 7 days using X-ray diffrac-tion (XRD), scanning electron microscopy (SEM), and backscattered electron image (BSE) anal-ysis. The results demonstrate that 3D printed cement paste showed a lower compressive strength than cast cement pastes in all directions. The intense and sharp calcium hydroxide (CH) peaks were seen in the XRD analysis of the 3D printed sample, which suggested its high crystallinity. The large size of plate CH can be observed in SEM images. These results indicate that CH grains in 3D printed samples have enough space to grow due to the relatively loose microstructure. This observation was confirmed by the porosity of the 3D printed sample analyzed by BSE images. The BSE imaging analysis also showed that the pore size distribution of the 3D printed sample was different from that of the cast sample. Additionally, the interfacial transition zone (ITZ) in the 3D printed sample presented a looser microstructure than the cast sample. Therefore, the reduced compressive strength of 3D printed cement pastes is the relatively loose microstructure, high porosity, and weak ITZ between unhydrated clinkers and hydration products.