Production of zero-carbon steel is highly anticipated as a sustainable solution for net-zero. Hydrogen reduction of the iron-ore sinter in the blast furnace operation is considered a promising candidate. Here, delayed molten phase formation would be problematic. In the iron ore sinter, silico-ferrite of calcium and aluminum (SFCA) is a major bonding phase, in which Fe2+ prefers to place at octahedral sites, while Fe3+ at tetrahedral sites. During the reduction in the ironmaking process, however, the phase stability and crystalline structure are modified, yielding drastic changes in the primary molten slag formation. In this study, in-situ observation of phase transformation was carried out using a confocal laser scanning microscope(CLSM). The phase transformation before and after reduction was examined using differential scanning calorimetry(DSC), separately. An SFCA sample was placed in a platinum crucible and heated under an Ar-5%H2 gas atmosphere while investigating with CSLM. The sample was melted at 1170~1190℃, which was similar to the result of DSC. The liquid phase region gradually expanded, but at 1330~1400℃, the dendrite phase suddenly appeared, and the liquid phase transformed into a solid phase. When the temperature increases to 1500℃, a liquid phase is formed again overall. The sudden solidification of slag formed in the early stages adversely affects the fluidity of the slag and the carburization of iron. Consequently, the phenomenon is investigated by comparing the reduction reaction rates of SFCA and Fe2O3. The present result suggests an understanding of the molten slag formation route in hydrogen-enriched blast furnace operation. (This work was supported by the Industrial Strategic Technology Development Program (20212010100040, Development of hybrid ironmaking processes for lower CO2 emissions), funded by the Ministry of Trade, Industry & Energy in the Republic of Korea.)