Tempering and modification of basic oxygen furnace slag with blast furnace slag: Mineral phase transformations and cementitious material enrichment mechanisms

This study investigates the phase evolution and enrichment mechanism of the cementitious materials in a blend of blast furnace (BF) and basic oxygen furnace (BOF) slags. Theoretical calculations reveal that there is an optimal modification temperature range of 1400–1550 °C, which is crucial for the transformation of the slag constituents. At lower temperatures (500–850 °C), the addition of BF slag reduces the proportion of calcium iron oxide (Ca2Fe2O5, C2F) and promotes the formation of merwinite (Ca3MgSi2O8, C3MS2). At higher temperatures (850–1450 °C), the addition of BF slag decreases the free CaO content, enhancing the stability of the modified slag. The presence of the dicalcium silicate and tricalcium phosphate complex (α-Ca2SiO4–Ca3(PO4)2, α-C2S-C3P), dicalcium ferrite (C2F), and magnesioferrite (MgFe2O4) in the modified slag facilitates the creation of dicalcium silicate (Ca2SiO4, C2S) and C3MS2 phases, which improve the stability of the slag and increase the amount of cementitious active materials, indirectly reducing crushing costs. Experimental results indicate that excessive BF slag content results in the over formation of gehlenite (Ca2Al2SiO7, C2AS) and C3MS2, potentially affecting the cementitious active material proportion. Insufficient BF slag does not fully utilize its CaO, SiO2, and Al2O3 for enhancing the BOF slag stability and increasing the cementitious material content. A BF slag content of approximately 30 % optimally consumes free CaO, promotes C2S formation, and enhances the stability of the modified slag. This research highlights the crucial role of the temperature and BF slag content in the phase evolution and cementitious potential of BF-modified BOF slags, providing guidelines for optimized utilization in construction materials.