Study on structure changes of molten blast furnace slag (BFS) during cooling process

摘要

In the present study, blast furnace slag（BFS）re-melted respectively at 1350℃, 1450℃ and 1550℃ was water-quenched to prepare samples, along with an air-quenched sample re-melted at 1450℃ as reference. The samples were investigated by using XRD, DTA and NMR techniques. The results show that during cooling process, two structure transformation regions exist in the melts. One is high temperature transformation region (HTT region). In this region, main structures with high polymeric Q3(3Al) and melilite units have formed from the molten slag. Under very slow or very fast cooling speed, the molten slag would fall into the region and precipitate melilite crystals, or be frozen into homogeneous glass phase. The other is a low temperature transformation region (LTT region). In this region, coexistence structure of high polymeric Q4(2Al) or melilite units with low polymeric Q0 units has occurred from the molten slag. Under slow or fast cooling speed, the structure features of molten slag would change from HTT to LTT region. As the result, mervinite would be crystallized with a slow colling speed, or the molten slag be frozen to generate phase-separated glass with a slow colling speed. If the molten slag falls into LTT region, the Mg2+ in the molten slag also changes its coordination number from 4 to 6. The change is one of driving forces for transformation from structure of akermanite (Ca2MgSi2O7) to mervinite (Ca3MgSi2O8) and accordingly increases the amounts of Q0 units, contributing to the formation of phase separation structure in the molten slag.