###### Abstract  
In this experimental study, tests were conducted to obtain the CaO/SiO\({}_{2}\) (C/S) molar ratio that minimizes the micropores of hardened concrete. To this end, the compressive strength, thermogravimetric analysis (TGA), and microporous structure of hardened concrete were investigated using concrete specimens made with silica fume and quartz powder under high-temperature/high-pressure conditions. The tests yielded the following results: (1) the highest compressive strength (200 MP) was exhibited in the C/S molar ratio range of 0.7-0.9, and lower compressive strength was exhibited in the C/S molar ratio ranges of \(\leq\) 0.6 and \(\geq\) 0.95; (2) the productivity of calcium silicate hydrate (C-S-H) tended to increase in proportion to the C/S molar ratio in all specimens; (3) the microstructure was measured using mercury intrusion porosimetry (MIP), and the maximum total porosity of specimens was calculated to be 10%. As the C/S molar ratio increased, the total porosity decreased, as did the pore diameter and threshold pore diameter. Thus, C/S molar ratio was found to be a major factor affecting the compressive strength and microporous structure of autoclave-cured cement and the optimal mechanical properties were exhibited in the C/S molar ratio of 0.8-0.9. However, the reactivity varies depending on the material used and curing conditions employed. Therefore, the hydration products obtained using the curing conditions need to be investigated further.  
Keywords:high-strength concrete; calcium silicate hydrate; C/S molar ratio; autoclave curing; micro structure +
Footnote †: journal: Medical  
to minimize the pores in concrete to improve its durability and surface characteristics. [PERSON] et al. reported that a concrete's low flexural strength was attributable to the pores in that concrete [8]. They proposed macro-defect-free (MDF) cement, with large pores (\(\geq\)100 \(\upmu\)m) removed, and reported the research result that high flexural strength could be obtained. However, MDF mixed with organic polymers shows a rapid decrease in strength when contacted with water, and has poor chemical and thermal resistance [9; 10].  
Methods to reduce pores in concrete include making aggregates of optimum particle size, filling the pores with fine particles, and enhancing reactivity [11]. Fine powders used in concrete are mostly materials with high silica content. Silica raw materials are used as fillers rather than reactive materials at room temperature, but can be used as reactive materials under high temperature/high pressure conditions [12; 13]. Different types of silica raw materials tend to have different reactivity depending on oxide content and mineral form, whereby the contribution of the calcium silicate hydrate (C-S-H) reaction is of crucial importance. C-S-H minerals are compounds made by the reaction between the Ca and Si ions in cement and Si ions in the silica raw materials [14; 15]. It is, therefore, crucial to investigate the ionization and reaction of these ions under specific conditions [16]. In particular, the properties of hardened concrete change under high-temperature/high-pressure conditions according to the CaO/SiO\({}_{2}\) (C/S) molar ratio.  
This study experimentally investigated how to minimize pores in high-strength concrete in order to enhance its utility. Specifically, in order to make the structure of high-strength concrete into a microstructure with minimized porosity, the compressive strength and pore structure characteristics were measured, varying the C/S molar ratio in the concrete specimens by adjusting the amount of the silica-based materials in the cement (silica fume and quartz powder) under high-temperature/high-pressure conditions. In addition, the relationship between pore structure and compressive strength was examined by performing thermogravimetry/differential thermal analysis (TG-DTA), X-ray diffraction (XRD) analysis, and scanning electron microscopy (SEM), while varying the C/S molar ratio in the concrete specimens.