Crystallisation behaviour of blast furnace slag modified by adding fly ash

Abstract

Crystallisation of molten blast furnace (BF) slag can increase its viscosity, which can in turn affect the quality of slag fibres. Fly ash was added to BF slag to control its crystallisation and modify its chemical composition. FactSage simulation and analyses using X-ray diffraction (XRD), scanning electron microscope-backscattered electrons (SEM-BSE) coupled to an energy dispersive spectrometer (EDS), and single hot thermocouple technique (SHTT) were performed to explore the crystallisation behaviour of the modified BF slag. The relationship between temperature, mineral precipitation, and added fly ash content was investigated. The minerals contained in the modified BF were melilite, anorthite, clinopyroxene, and spinel. Variation in the fly ash content did not change the composition of the precipitate, but changed its content and the crystallisation temperature of the minerals, which affects the initial crystallisation temperature of the modified BF slag. It decreased as fly ash content increased, and was influenced by the crystallisation of melilite when the added fly ash content was between 5% and 20%. When the added fly ash content increased to 25%, the initial crystallisation temperature was influenced by the precipitation of anorthite. The initial crystallisation temperatures obtained by FactSage simulation, XRD analysis, and SHTT experiments differed due to kinetic effects. The modified BF slag with a fly ash content of 15% is considered suitable for manufacturing of slag fibres due to its low initial crystallisation temperature and cost.