It’s limestone, look clearly. During blast furnace production, iron ore, coke, and slagging flux (limestone) are loaded from the top of the furnace, and preheated air is blown in from the tuyere located at the bottom of the furnace along the furnace circumference. At high temperatures, the carbon in the coke (some blast furnaces also inject auxiliary fuels such as pulverized coal, heavy oil, natural gas, etc.) burns with the oxygen blown into the air to produce carbon monoxide and hydrogen. During the process of rising in the furnace, the carbon monoxide and hydrogen in the iron ore are removed. Oxygen is reduced to iron. The smelted molten iron is released from the iron mouth. The non-reducible impurities in the iron ore combine with fluxes such as limestone to form slag, which is discharged from the slag outlet. The generated gas is led out from the top of the furnace, and after dust removal, is used as fuel for hot blast furnaces, heating furnaces, coke ovens, boilers, etc. Limestone is used for slag removal! The main raw materials of glass are: quartz sand, limestone, feldspar, soda ash, boric acid cement. Limestone, clay, and iron ore powder are ground and mixed in proportion. The mixture at this time is called raw meal. Then it is calcined, usually at a temperature of about 1450 degrees, and the calcined product is called clinker. Then the clinker and gypsum are ground together and mixed in proportion to become cement. The cement at this time is called ordinary Portland cement.