## 4 Conclusions  
In this study, the characteristics of high-strength concrete according to the C/S molar ratio were analyzed by adjusting the mixing quantities of silica fume and quartz powder, which are silica-based materials, under high-temperature/high-pressure conditions to create a microstructure with the minimum possible porosity in concrete. The experimental results can be summarized by the following conclusions:  
1. In the quartz powder-centered specimens with a C/S molar ratio ranging from 0.6 to 0.95, compressive strength increased and absorption rate decreased as the C/S molar ratio increased. In terms of the quantity of hydrates according to ignition loss, it was confirmed that the higher the C/S molar ratio, the higher the quantity of C-S-H hydrates.  
2. In the silica fume-centered specimens with a C/S molar ratio ranging from 0.6 to 0.95, the highest compressive strength and the lowest absorption rate were shown at the C/S molar ratio of 0.8, and the concrete performance decreased at other C/S molar ratios. Despite high reactivity of silica fume, its optimum mixing ratio is judged to be around 10% of the cement weight in terms of optimum particle size composition.  
3. Pore size and volume analysis revealed that the pore size and volume decreased as the C/S molar ratio increased, whereby meso-pores increased and capillary pores decreased. In all specimens, the pore volume distribution ratio tended to coincide with the result of the compressive strength test, which implies that the threshold pore diameter decreases with an increase in the C/S molar ratio due to the pore-filling effect of the hydrates generated in the specimens of hardened concrete.  
4. In high-temperature/high-pressure curing conditions, the optimum C/S molar ratio according to the use of silica fume and quartz powder ranged from 0.8 to 0.9, and an optimal mix ratio of powder power was identified. The mixing design adjusted by quartz powder showed excellent results in terms of absorption rate and compressive strength, which allows the conclusion that it is appropriate to derive and utilize the optimum amount of silica fume taking into account the increase in pore volume according to the increase in specific surface area.  
Overall, this study confirmed that the C/S molar ratio is a major factor that affects the compressive strength and porosity, and that the optimal molar ratio exists. However, the reactivity varies depending on the material used and curing condition employed. Therefore, the hydration products obtained using the curing conditions should be investigated further.  
Conceptualization, J.K.; methodology, J.K. and J.R.; software, J.R.; validation, J.K. and S.S.; formal analysis, J.R.; investigation, J.K.; resources, J.K. and S.S.; data curation, J.R. and S.S.; writing--original draft preparation, J.K. and J.R.; writing--review and editing, J.K. and J.R. and S.S.; visualization, J.R.; supervision, J.K.; project administration, J.K.; funding acquisition, J.K. All authors have read and agreed to the published version of the manuscript.  
This research received no external funding.  
Not applicable.  
This work was supported by the National Research Foundation of Korea(NRF) grant funded by the Korea government(MSIT) (No. NRF-2020R1A2C2013161).  
\begin{table}
\begin{tabular}{c c c c c c c} \hline \hline  & \multicolumn{2}{c}{**Plain**} & \multicolumn{2}{c}{**QP0.9**} & \multicolumn{2}{c}{**SF0.9**} \\ \cline{2-7}  & **Ca** & **Si** & **Ca** & **Si** & **Ca** & **Si** \\ \hline Point 1 & 32.2 & 17.91 & 21.07 & 25.55 & 16.31 & 29.72 \\ \hline Point 2 & 12.48 & 34.02 & 12.54 & 14.13 & 20.15 & 31.15 \\ \hline Point 3 & 18.8 & 12.56 & 17.7 & 18.33 & 7.17 & 61.11 \\ \hline \hline \end{tabular}
\end{table}
Table 5: EDS point analysis (at.%) of the specimen in Figure 15.