## 5 Conclusions  
Compressive strength and direct tensile tests were performed on nine mixes of UHP-SHCCs. The nine mixes consisted of OPC and reactive and non-reactive mineral additives, namely, silica flour, GGBS, SF, and CKD that substituted a portion of the OPC in each specimen. The microstructural characteristics, resulting from the type of the mineral additives and curing conditions were investigated with TG-DTG and SEM inspections. The mechanical test results were analyzed to specifically examine the influence of each type of mineral additive, and the following conclusions were derived:  
1. At the curing age of 7 d, only the CKD-included specimens exhibited higher compressive strength than the mix with only OPC as a binding material. After 28 d of curing, the reactive GGBS, SF, and CKD have strengthened the compressive strength of the specimens in comparison to that of the specimen with only OPC, except for the specimen incorporating 30% of SF. On the other hand, in the case of specimens containing the non-reactive silica flour, the compressive strength decreased;
2. The strain capacity of the 7 d cured specimens was improved by substituting the OPC with the reactive mineral additives. In particular, specimen H7, containing the 15% CKD, demonstrated the highest tensile performance with a tensile strength of 11.6 MPa and strain capacity of 7.5%;
3. Most of the 28 d cured specimens with OPC, silica flour, GGBS, or SF, exhibited considerably improved tensile behavior compared to the 7 d cured specimens, except for specimens H28 and I28, with CKD. The strain-hardening behavior of specimens A28 to E28 was extended by over 6.0%, and the tensile strength ranged between 8.2 MPa and 11.6 MPa;
4. At the curing age of 7 d, specimens with OPC and silica flour, absorbed lower energies than the specimens with the GGBS, SF, and CKD. Moreover, the 28 d cured specimens, exhibited significantly increased energy absorption capacities compared to the 7 d cured specimens, reaching 558.7 kJ/m3;
5. For practical applications of UHP-SHCCs, the use of 15% of CKD is recommended when securing the early age mechanical performance is a priority, and the use of 50-100% of GGBS and 20% of SF is recommended to optimize mid- to long-term performance.  
Conceptualization, M.-J.K.; methodology, M.-J.K., B.C., and H.-J.C.; formal analysis, M.-J.K. and W.S.; investigation, M.-J.K.; data curation, M.-J.K. and H.-J.C.; writing--original draft preparation, M.-J.K.; writing--review and editing, D.-Y.Y.; visualization, M.-J.K.; supervision, D.-Y.Y.; funding acquisition, D.-Y.Y. All authors have read and agreed to the published version of the manuscript.  
This research was supported by a Grant (21 CTAP-C152069-03) from the Technology Advancement Research Program funded by the Ministry of Land, Infrastructure, and Transport of the Korean government.  
\begin{table}
\begin{tabular}{c c c c c c c c c c} \hline \hline
**Specimen** & **A7** & **B7** & **C7** & **D7** & **E7** & **F7** & **G7** & **H7** & **I7** \\ \hline \#Cracks (ea) & 10 & 4 & 9 & 6 & 13 & 29 & 17 & 25 & 21 \\ \#Drops (time) & 11 & 10 & 18 & 17 & 19 & 40 & 29 & 48 & 33 \\ Strain capacity (\%) & 1.7 & 1.1 & 2.2 & 2.9 & 2.6 & 4.6 & 2.8 & 7.5 & 5.2 \\ Energy absorption capacity (kJ/m3) & 144.6 & 93.3 & 206.9 & 458.1 & 182.8 & 281.1 & 153.7 & 667.1 & 434.5 \\ \hline
**Specimen** & **A28** & **B28** & **C28** & **D28** & **E28** & **F28** & **G28** & **H28** & **I28** \\ \hline \#Cracks (ea) & 27 & 28 & 39 & 39 & 35 & 36 & 36 & 34 & 33 \\ \#Drops (time) & 57 & 43 & 59 & 52 & 64 & 65 & 64 & 78 & 66 \\ Strain capacity (\%) & 6.2 & 6.6 & 7.3 & 6.9 & 6.6 & 7.2 & 6.0 & 6.8 & 6.9 \\ Energy absorption capacity (kJ/m3) & 537.2 & 446.1 & 558.7 & 560.7 & 532.2 & 442.0 & 419.3 & 475.3 & 493.2 \\ \hline \hline \end{tabular}  
*Cracks = number of cracks and #Drops = number of drops in tensile stress.  
\end{table}
Table 4: Test indexes for tensile and cracking behaviors of UHP–SHCCs.