Evaluation of Heterogeneity in Steam Aging Process of Slag Subbase Course Materials

Steelmaking slag, a by-product of the steelmaking process, contains free CaO (f-CaO), which hydrates to form Ca(OH)2, causing significant volume expansion and disintegration. Basic Oxygen Furnace (BOF) slag with its high f-CaO content exhibits particularly severe expansion. At JFE Steel, steelmaking slag intended for use as subbase course material is treated in a steam aging facility to stabilize expansion and slag expansion after aging is evaluated using the expansion stability test specified in the JIS standard. However, nonuniform steam distribution during aging often prolongs stabilization and increases steam consumption. In addition, some aged slags are not sufficiently stabilized and fail to meet the criteria, requiring re-aging and reducing productivity. This study aims to investigate the mechanisms and causes of delayed temperature rise and steam flow heterogeneity in steam aging facilities. Steam flow heterogeneity was analyzed through laboratory-scale experiments, on-site sampling, and numerical simulations based on actual slag properties. The results revealed that the steam flow heterogeneity arises from ruts formed by heavy machinery and particle size segregation during the slag piling process, with ruts having a greater influence than segregation. These findings indicate that optimizing piling methods is essential for improving steam distribution, reducing energy consumption, and enhancing productivity in steam aging facilities.