## 3 Microstructural Analysis Results
### SEM Image Analysis  
Figures 4-8 display the SEM images on the microstructures of the UHP-SHCCs according to both the type of mineral additives included and curing time. Each figure exhibits the difference between the microstructure of the UHP-SHCC based on the curing age. Figure 4 shows the microstructure of specimens A7 and A28, respectively. It is seen that numerous spherically agglomerated particles of varying diameters were still reacting in specimen A7. Thus, the amorphous C-S-H layers did not develop enough to fill and cover the voids, resulting in a loose and irregular fiber-matrix interface. The growing hydrates observed in specimen A7 were not observed on the surface of specimen A28 as the hydrates had grown enough and formed denser amorphous C-S-H layers. The  
Figure 3: Derivative thermogravimetry (DTG) result of (**a**) 7 d and (**b**) 28 d cured specimens. amorphous hydrates also effectively filled the empty spaces between the PE fibers and the matrix, forming a compact interface. Figure 4: Scanning electron microscopy (SEM) images on the microstructure of specimens A7 and A28. Figure 5: SEM images on the microstructure of specimens B7 and B28. Figure 6: SEM images on the microstructure of specimens D7 and D28. Figure 7: SEM images on the microstructure of specimens F7 and F28. Because the silica flour, SF, CKD used in this study had smaller particle sizes compared to OPC, the voids in the microstructure were filled before the additives were fully reacted and dissolved. This effect becomes clearer as the small particles of the reactive additives hydrate and grow. Figure 5 shows the microstructures of specimens B7 and B28 that included the non-reactive silica flour. Unlike in specimen A7, the spherically agglomerated hydrates particles were not present in specimen B7. This can be attributed to the smaller particle size of silica flour compared to OPC that effectively fills the voids in the microstructure and disperse the particles. The microstructure of specimen B7, however, still had a porous microstructure with voids and loose fiber-matrix interfaces. In the case of specimen B28, however, the voids were filled with well-developed hydrates, and the fiber-matrix interface was compact. Figure 6 shows the microstructures of specimens D7 and D28 with GGBS. Specim D7 also had a porous microstructure, but it was denser than specimen A7. Although many voids and unreacted large GGBS particles were present, the microstructure of specimen D7 had a portion of developed amorphous hydrates. This can be attributed to particle size distribution in GGBS and higher specific surface area than that of the OPC. The small GGBS particles filled the voids and reacted faster than the larger ones. A clear difference in the amount of the unreacted large GGBS particles was observed between specimens D7 and D28. It was observed that while the microstructure was denser after 28 d of curing, a few unreacted GGBS particles remained. Figure 7 presents the microstructure of specimens F7 and F28 with SF. It was observed that numbers of reacting SF particles remained in specimen F7, but most of these began to lose their spherical shape and transform into amorphous hydrates, developing the compact microstructure. Thus, the size of the voids in specimen F7 was small. In specimen F28, the amorphous hydrates covered the surface and formed a dense fiber-matrix interface. Figure 8 shows the SEM images of specimens H7 and H28 with CKD. It is observed that the microstructure and the fiber-matrix interface of specimen H7 were dense because CKD is smaller than those of the OPC. Specim H7 had a high portion of developed amorphous  
Figure 8: SEM images on the microstructure of specimens H7 and H28. layers and a more compact microstructure than the other 7 d cured specimens. It was also observed that the hydrate particles of diverse sizes were still dissolving and transforming into the amorphous layer in the microstructure of specimen H7. The hydrates continued to develop after the 7 d age, and the microstructure of specimen H28 became more compact.