## 3 Results and Discussion
### Compressive Strength  
Compressive strength tests were performed on the 3 DPC specimens in three loading directions, i.e., perpendicular, longitudinal, and lateral directions [45]. The compressive strength of 50F5, SF25, and S5F0 in the X, Y, and Z directions at different ages are shown in Figures 9-14. It can be seen that the compressive strength of all specimens increased with age. Figures 9-11 show that the compressive strength values of different mix designs follow the order of S5F0 > SF25 > S0F5. This indicates that compressive strength increases with the increase in the amount of GGBFS cement replacement. The reason is that GGBFS significantly improves the early  
The literature reveals that the direction of loading has a strong influence on the compressive strength of 3 DPC due to anisotropy [49]. For each group of specimens (SOF5, SF25, and SSF0), the compressive strength values in different directions followed the order of X direction > Z direction > Y direction, as shown in Figures 12-14. [PERSON] et al. pointed out that the printing direction may have a significant influence on the overall load bearing capacity of 3 DPC [50]. The load in the X direction of the printed specimen is parallel to the interface between strips and layers. When the specimen is damaged, cracks generally extend in the printing direction, making the specimens behave as multiple columns bound  
Figure 14: Compressive strength of 3 DPC in F0S5. Figure 12: Compressive strength of 3 DPC in FSS0. Figure 13: Compressive strength of 3 DPC in FS25. together under a load. The load in the Y direction of the printed specimen is parallel to the interface between layers but perpendicular to the interface between strips. During failure, the crack extension direction often exhibits a conical fracture pattern. The load in the Z direction of the printed specimen is parallel to the interface between strips but perpendicular to the interface between layers, resulting in a similar conical fracture pattern during failure to that observed in the Y direction. Therefore, the bonding between layers in the X and Z directions is better, while the interlayer bonding in the Y direction is weaker. In addition, the material deposition in the X and Z directions is more uniform, resulting in higher density. Additionally, stress concentration and crack propagation are more significant in the Y direction, leading to lower strength. These results are consistent with most of the previous works [49; 50; 51; 52]. However, according to [PERSON] and [PERSON] [46] and [PERSON] et al. [53], there is a significant reduction in compressive strength when measured in the longitudinal direction compared with the perpendicular direction. In addition, findings in [PERSON] et al. [16] reveal that printed samples exhibit their lowest strength along the longitudinal direction and their highest strength in the lateral direction. These conflicting results highlight the necessity for further research and more comprehensive experimental data, as well as a better understanding of the mechanisms influencing compressive behavior across various testing directions. Figure 15 shows the compressive strength of the 3 DPC and cast specimens at the age of 28 days. Figure 16 shows the failure mode of the compressive strength test 3 DPC and cast specimens at the age of 28 days. The compressive strength of the cast specimens is a key indicator of the material's structural integrity and its suitability for applications in construction, including 3D printing. The inclusion of FA generally resulted in a decrease in compressive strength at early ages due to its delayed pozzolanic reactivity. However, GGBFS, with its finely ground particles and latent hydraulic properties, contributed to higher early-age strength than FA. The combination of FA and GGBFS showed balanced performance, with moderate reductions in early compressive strength but overall enhancements in long-term durability and strength.