## 3 Results and Discussion
### Microstructure Analysis  
In general, in the case of cement composites, the average thickness of the interface transition zone (ITZ) between the cement mortar and aggregate is known to be about 10 to 50 \(\upmu\)m, and the thickness of this ITZ is reported to affect the strength and durability of the cement composites [61; 62]. Figure 13 shows the photos taken for the surface shape of the developed conductive CFRCM and SFRCM fracture surfaces through SEM image observation. As shown in Figure 13a, a carbon fiber has a round and circular shape, and the fiber surface is very clean and fairly smooth in shape. This seems to be because the interfacial adhesion force between the fibers and the matrix is low as carbon fibers are difficult to crystalize cement particles and hydration products and have low affinity. As featured in Figure 13b, however, large and small cement particles and hydration products are formed and attached on and around the surface area of the steel fibers. This indicates that the interfacial adhesion force between the fibers and matrix was significantly improved due to the formation and increase of cement particles and hydration products. As a consequence, we can judge that the steel fiber reacts with calcium hydroxide (Ca(OH\({}_{2}\))), a primary hydration product, and soluble components such as calcium (Ca), silicon (Si), and aluminum (Al) in the cement mortar to form calcium silicate hydrate (C-S-H) gels, etc., inducing the formation of cement hydration products.  
Figure 14 shows the results of elemental analysis by EDS spectra of carbon and steel fibers taken from the fracture surfaces of the development conductive CFRCM and SFRCM specimens developed after the flexural test. For elemental analysis of hydration products, the amount of element components (C, O, Ca, Fe, etc.) on the fiber surface was measured through EDS attached to SEM. As a measuring method to accurately determine the amount of component elements, the amount of elements was measured by obtaining the average value of the area of the two points at an arbitrary location at the same distance from each fiber. As seen in Figure 14, from the elemental analysis by EDS spectra of a developed conductive CFRCM specimen containing carbon fibers detected 86.65% of carbon (C), 12.83% of oxygen (O) and a small amount of calcium (Ca) 0.52%, respectively, in the red position (1). In addition, it was analyzed that the components indicated in the red position (2) contain 90.57% of carbon (C), 8.61% of oxygen (O), and a small amount of calcium (Ca) 0.82%. Besides, in the developed conductive SFRCM specimen containing steel fibers, iron (Fe) 64.76%, carbon (C) 12.47%, oxygen (O) 10.32%, calcium (Ca) 10.3%, and a small amount of silicon (Si) 1.63% and aluminum (Al) 0.51% were detected from the components indicated in the blue position (1). At the same time, it was analyzed that the components indicated in the blue position (2) contain 75.25% of iron (Fe), 18.26% of carbon (C), 5.74% of oxygen (O), and 0.74% of calcium (Ca) in a small amount. Accordingly, the carbon fibers in the developed conductive CFRCM specimen showed that a large amount of carbon (C) and a small amount of calcium (Ca) were present, while the hydration products were confirmed in the steel fiber in the developed conductive SFRCM specimen, and a small amount of  
Figure 13: SEM images of conductive CFRCM and SFRCM; (**a**) carbon fiber (500\(\times\)), and (**b**) steel fiber (150\(\times\)).  
silicon (Si) and aluminum (Al) as well as calcium (Ca) were detected, indicating that the hydration reaction is actively proceeding. Such results of the elemental analysis by the EDS spectrum are almost in agreement with the results of SEM images shown in Figure 13.