## 3 Results and Discussion  
### X-CT Test Results  
Compared to cast concrete, 3 DPC exhibits significant spatial anisotropy in pore morphology. While pores in cast concrete are randomly distributed and predominantly spherical [39], the extrusion-deposition process of 3 DPC imparts distinct directional characteristics to pore geometry and distribution [40]. In the X direction (printing path), pores are elongated into aligned ellipsoids due to extrusion shear forces and nozzle dragging effects. The Z direction pores become flattened under layer-by-layer compaction from self-weight pressure. For the Y direction, interfacial compression between filaments results in intermediate pore sizes (X \(>\) Y \(>\) Z), collectively forming spatially anisotropic ellipsoidal pore configurations. This anisotropic pore architecture (X \(>\) Y \(>\) Z) provides critical microstructural insights into the mechanistic origins of 3 DPC's mechanical anisotropy.  
As shown in Figure 3, the average porosity of 3 DPC decreased with increasing flowability, exhibiting a steeper gradient in the low- and medium-flowability ranges, while  
Figure 2: Cutting diagram.  
the decline moderated and stabilized in the high-flowability range. This indicates that flowability exerts a more pronounced regulatory effect on porosity at low to medium levels, whereas porosity approaches the densification limit of the material under high-flowability conditions. The connected pores at the intersections of the interlayer and intralayer zones decreased substantially with increasing flowability. Owing to the unique extrusion-deposition mechanism of 3 DPC, the pore morphology and spatial distribution demonstrate distinct directional characteristics. Specifically, the pores were observed to be significantly elongated along the printing direction (X-axis).  
The evaluation of pore defect elongation ratios was conducted across four representative flowability groups (F = 165, 173, 180, and 191 mm), as illustrated in Figure 4, where the elongation ratio was defined as the ratio of the median to maximum axis lengths in pore projections, with values approaching 0 indicating more elongated pore geometries. The stable region (elongation ratio > 0.5) and elongation region (elongation ratio \(\leq\) 0.5) were divided to characterize the elongated state of the pore geometry according to the elongation distribution. As the pore size increases, the number of elongated pores in 3 DPC progressively rises while stable pores decrease, demonstrating that pore geometry becomes more elongated and approaches an ellipsoidal shape with increasing pore dimensions. As shown in Figure 5, the majority of pores reside within the stable region (cumulative frequency \(\geq\) 72.55%), while a smaller fraction occupies the elongated region (cumulative frequency \(\leq\) 27.45%). Notably, specimens with low flowability exhibit the highest relative frequency of elongated pores (27.45% at 165 mm flowability), indicating pronounced pore elongation. As flowability increases, the proportion of elongated pores progressively decreases, reaching 12.76% at 191 mm flowability. This 14.69% reduction demonstrates that enhanced flowability effectively reduces ellipsoidal pore populations and improves pore characteristics.  
Figure 3: Average porosity and three-dimensional pore distribution with different flowabilities.  
Figure 4: Elongation ratio with different flowabilities: (**a**) 165 mm; (**b**) 173 mm; (**c**) 180 mm; (**d**) 191 mm.  
Figure 5: Frequency distribution histogram of elongation at different flowabilities: (**a**) 165 mm; (**b**) 173 mm; (**c**) 180 mm; (**d**) 191 mm.