Microstructure and Chemical Variation in Class F Fly Ash Glass
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MICROSTRUCTURE AND CHEMICAL VARIATION IN CLASS F FLY ASH GLASS J.C. QIAN, E.E. LACHOWSKI and F.P. GLASSER Department of Chemistry, University of Aberdeen, Aberdeen, AB9 2UE, Scotland Received 28 September, 1987; refereed
ABSTRACT Fly ash consists of mixtures of crystalline substances in a glassy matrix. This matrix is itself inhomogeneous. The combustion process gives rise to compositional fluctuations typically on a micrometer scale; these fluctuations are preserved in the glass and are gradational. However, high-resolution transmission electron microscopy of Class F ash also reveals the existence of interfaces on a nanometer scale. These arise as a consequence of phase separation. Textures and interfaces typical of spinodal decomposition and possibility also suggestive of classical immiscibility have been observed. It is believed that the occurrence of phase separation resulting in nanometer-scale inhomogeneities will be a feature common to most Class F glasses. The consequences of this complex microstructure to reactivity are not as yet known, but some speculations are presented. INTRODUCTION Previous Symposia in this series [1-31 have added substantially to our knowledge of coal combustion products and their potential uses. The present paper is concerned with one particular type of coal combustion product -- Class F fly ash -- and one particular application -- its use as a pozzolanic additive to cement systems although, of course, its conclusion may be applicable elsewhere. The mineralogy and microstructure of Class F fly ashes have been studied extensively. It is known that the pozzolanic activity of the ash arises from the glassy phase; its crystalline constituents, principally quartz, mullite, spinel and hematite, contribute little, if at all, to pozzolanic activity. Electron microscopy has extended our knowledge of the physical form of fly ash particles. These may exhibit varied morphological development occurring, for example, as single or multiple hollow spheres, as well as fused, solid particles. The crystalline constituents are wet by the high-temperature melt phase and the rapidly-cooled product preserves this texture, so that the cooled product consists of a continuous glassy phase enclosing the crystals. The inhomogeneity of the mineral content of coal, as well as the relatively brief high temperature excursion occurring during combustion, suggest the possibility that the glass will have insufficient time to homogenize. However, direct analyses have been lacking until recently when Uchikawa reported analyses of the average bulk composition of the glassy phase [4]. We have reported additional bulk analyses, we well as the variability in A12 0 3 contents within particles [5]. The dispersion of analytical results for A12 0 3 discloses that substantial local compositional fluctuations occur, typically on a micrometer scale. However, these fluctuations do not in general generate new interfaces. The purpose of this paper is to show that interfaces do, however, develop spontaneously within the glass and th
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