## 3 Microstructural Analysis Results  
### Thermogravimetric Analysis  
The secondary reactions of the GGBS and SF require or consume calcium hydroxide (CH) in the hydrating mortar to form additional calcium silicate hydrates (C-S-H) in the composite. These additional hydrates enhance the microstructure of the composite by filling the voids and increasing the bond strength between the fiber and the matrix. The rates of the secondary reactions, however, are typically slower than that of the direct hydration of the OPC with similar particle size. The GGBS was found to reduce the rate of development of the early age strength of the cementitious composites as it requires the alkaline mortar to dissolve its glassy surface. The SF, on the contrary, accelerates the production and consumption of CH from the beginning of the hydration due to its large contact area with water and high reactivity [12; 24]. The CKD is known to contain a high concentration of sulfate, chloride, and alkaline substances that accelerates the reaction of the cementitious composite at the initial stage but consequently deteriorates the microstructure [13]. The CKD used in this study, however, contained a low sulfate content, lower than that of the OPC, and an alkali content of 0.41%, as described in Table 2. Additionally, the CKD had a specific surface area of 8200 cm\({}^{2}\)/g, over twice that of the OPC (3550 cm\({}^{2}\)/g). Figure 3 shows the result of the differential thermogravimetric (DTG) inspection for the five specimens of each mineral additive used in this study. It is noted in the DTG  
Figure 2: Japan Society of Civil Engineers (JSCE) direct tensile test: (**a**) test setup and (**b**) specimen detail. curves of the first peak increased, and that of the second peak decreased as the curing time increases from 7 d to 28 d. The notable decrease in the second peak is attributed to the simultaneous reduction in the amount of CH due to the continuous formation of C-S-H after hydration and pozzolanic reactions of the matrix and the continued curing at 40 \({}^{\circ}\)C after the initial 7 d of curing. Subsequently, from the first peak to before the second peak, the DTG curve is influenced by dehydration of a few types of hydrates, including C-S-H, ettrinigite, and mono-sulfate. However, a clear increase in the amount of C-S-H is observed during the additional curing phase after 7 d to 28 d when the notable difference between the DTG curves of the 7 d and 28 d cured specimens is observed in the dehydration section. The type of mineral additives also discernably affected the DTG curves, as shown in Figure 3. The DTG curves show that specimen B with the non-reactive silica flour did not notably differ from specimen A at both ages of 7 d and 28 d. However, specimens D and F, with GGBS and SF, showed a notable decrease in the second peak at both ages of 7 d and 28 d, compared to specimen A. Although the GGBS requires a certain degree of CH concentration to activate its hydration process, the process itself does not consume a considerable amount of CH. Thus, the decrease in the amount of CH, caused by the inclusion of the GGBS, was because a large dosage of OPC was substituted by the GGBS. The SF consumes the CH in the pozzolanic reaction forming the C-S-H and accelerates the production of the CH from the hydrating matrix [12; 24]. Thus, its first peak was extremely high compared to that of the other specimens, while the second peak was significantly lower at both ages of 7 d and 28 d. The second peak of specimen H7, with the CKD, was higher than that of specimen A7, and that of specimen H28 was lower than that of specimen A28, similar to that of the other 28 d cured specimens. Considering that CKD is composed of approximately 38% of calcium carbonate, it is clear that the 15% OPC substitution by the CKD decreases the total amount of the C-S-H and CH.