Crystallisation kinetics of fly ash modified blast furnace slag for slag fibre production

Blast furnace slag (BFS) is a by-product generated during iron-making process. The recovery of BFS for slag fibre production can efficiently utilise its latent heat. The crystallisation of BFS during cooling process would reduce fibre strength and induce fibre fracture. In this study, fly ash was employed to modified BFS. The effect of fly ash addition content on the crystallisation behaviour of BFS during cooling process was investigated using single hot thermocouple technique (SHTT). Time-temperature-transformation (TTT) and continuous-cooling-transformation (CCT) curves were constructed based on SHTT analysis. The obtained results indicated that the increase in fly ash content from 5 wt-% to 25 wt-% enhanced slag viscosity, decreased crystallisation onset temperature from 1300 °C to below 1000 °C, and reduced critical cooling rate from 180 °C/min to below 5 °C/min. The activation energies for the crystallisation of melilite and anorthite phase were 369 kJ/mol and 489 kJ/mol, respectively, in the modified BFS with fly ash content of 5 wt-%. Increasing fly ash content to 10 wt-% enhanced activation energy for anorthite phase crystallisation to 585 kJ/mol. Adding fly ash inhibits the crystallisation of BFS and makes the amorphisation of BFS is easier to control, which is beneficial to slag fibre production. The 15 wt-% fly ash modified BFS with the highest temperature window of 131 °C, crystallisation onset temperature of 1100 °C, and a critical cooling rate of 30 °C/min is most suitable for slag fibre production.