Characterization of Alite in BOF Slag by Polarized Light Microscopy

Abstract

Basic oxygen furnace (BOF) slags can approach C ₃ S saturation during converter processing and therefore contain alite as a high‐temperature phase. However, alite in BOF slags has rarely been described in detail. In this study, polished thin sections of BOF slags were investigated by polarized light microscopy and compared with Portland cement clinker.

Two distinct microstructural domains were observed in the slag: a homogenized melt‐derived slag matrix and relict flux domains interpreted as incompletely assimilated lime‐rich regions. Both domains contain large idiomorphic alite crystals differing significantly from clinker alite. The crystals show sharp boundaries and different inclusions, indicating crystallization from a low‐viscosity Fe‐rich melt rather than dissolution–precipitation processes characteristic of Portland cement clinker. Optical investigations on alite revealed consistent extinction behavior and no pronounced internal domain structures. The observations suggest high symmetry alite modifications, although the optical methods alone do not allow an unambiguous polymorph identification.