Microstructural characterisation of a glass and a glass-ceramic obtained from municipal incinerator fly ash

Scanning electron microscopy (SEM), energy-dispersive X-ray (EDX) spectrosc opy and X-ray diffraction (XRD) analyses were used to characterise the micr ostructure and chemical composition of a glass and a glass-ceramic material obtained from incinerator filter fly ash. Although the as-quenched materia l (vitrified fly ash) was amorphous under the detection limits of XRD, a di spersion of droplets indicating glass-in-glass phase separation was observe d. In the glass-ceramic material (crystallised vitrified fly ash), crystals belonging to the pyroxene group and spinels were identified. The microstru cture of the glass-ceramic consisted of crystals embedded in an amorphous g lassy phase. The crystalline phases contain a higher amount of metallic ele ments (e.g. Al, Cr, Fe, Ni and Zn and most probably also other heavy metals ) than the residual glassy phase. A change of composition of the residual g lass phase in the glass-ceramic product, in comparison with the parent glas s, is considered to explain, in comparative terms, the higher toxic potenti al of the glass-ceramic over the glass. The present results demonstrate tha t for an accurate assessment of the correlation between toxicity, release o f hazardous compounds and microstructure, high-resolution characterisation techniques must be employed. In this context, the effect of crystallisation on the chemical durability of the products remains as an important area fo r further research. (C) 1999 Kluwer Academic Publishers.