## 3 Results and Discussion
### Characterization of the Cement Pastes
#### 3.1.2 Printability and Setting Properties  
Considering the complex equilibria that regulate the extrusion process of cement-based materials, achieving and assessing printability is particularly challenging. Three  
Figure 1: Times of setting of the pastes plotted in function of the concentration of OP and HPMC (**A**) or CMC (**B**) amounts. The setting of the pastes was evaluated every 15 min with a spatula, until the pastes hardened to a non-injectable cement (identified as the time of setting). main requirements must be met: (i) the paste should have the appropriate setting time, allowing for extrusion before hardening since concrete mixtures can be injected from a nozzle and are only printable until they begin to set; (ii) the formulation should be stiff enough to maintain the required shape and prevent collapse; (iii) the formulation should be fluid enough to avoid the formation of voids and incomplete structures. Here, the consistency and fluidity of the cement pastes were initially evaluated by analyzing the aspect of the pastes poured on millimeter grid paper immediately after mixing, using a simplified version of the slump test commonly used to measure the workability of concrete [59]. The appearance of two discarded formulations (too dry on the left, too fluid on the right) is shown in Figure 2A, along with the appearance of the selected pastes (Figure 2B) and the results of the Gillmore test (Figure 2C). According to the results, all of the selected pastes appear fluid enough to be easily workable and extrudable but stiff enough to maintain their shape and prevent deformations (Figure 2B). Parallelly, the setting times, crucial parameters for the 3D printing of toughening materials, were quantitatively evaluated on the pre-selected formulations using the Gillmore test before proceeding with the printability tests. These experiments allowed for correlating the observations reported in Section 3.1.1 with the measured hardening times, t\({}_{1}\) and t\({}_{2}\). According to the results, all pastes displayed similar initial and final setting times (t\({}_{1}\approx 50\) min and t\({}_{2}\approx 240\) min), indicating very long setting periods compatible with the preparation of complex 3D printed shapes that require extended printing durations. Having established that the fluidity of the pastes was suitable for 3D printing, we evaluated their ability to achieve desired outcomes, specifically to avoid structural collapse from excessive workability and the presence of voids from insufficient workability [60]. For  
Figure 2: Pictures of some discarded formulations inappropriate for 3D printing (**A**), and of the selected pastes (**B**) 30 s after mixing; initial and final setting times obtained through the Gillmore test (**C**). this purpose, we used printability parameters [54; 61; 62] to qualitatively and quantitatively describe the geometrical differences between the designed print and the experimental pattern. The samples were printed by extruding the pastes until clogging occurred. Initially, the 2D shapes, prepared at different printing rates, were visually evaluated to assess the presence of voids or incomplete prints. Table 1 reports the roughness parameters calculated from the images, along with a qualitative characterization of their extrudability. In some cases, we achieved a complete 2D structure where the flow rate could be well controlled and regulated by the extrusion force, confirming that the ink was suitable for continuous and controllable extrusion [62]. In other cases, the pastes exhibited non-continuous extrusion, attributable to inadequate flow behavior. The roughness parameter Rp, calculated using ImageJ from the images, was used to evaluate the printing quality of the selected formulations by comparing the designed pattern with the experimental prints (see Figure 3A).