The Microstructure of National Bureau of Standards Reference Fly Ashes
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THE MICROSTRUCTURE OF NATIONAL BUREAU OF STANDARDS REFERENCE FLY ASHES J.C. QIAN, E.E. LACHOWSKI and F.P. GLASSER Department of Chemistry, University of Aberdeen, Meston Walk, Old Aberdeen, AB9 2UE, Scotland. Received 17 November, 1988; refereed ABSTRACT A suite of three fly ashes, National Bureau of Standards* Standard Reference Materials (SRMs) 2689, 2690 and 2691, was studied by transmission electron microscopy and by analytical electron microscopy (aem): sample preparation techniques are described. The chemical compositions of the glassy phase, analyzed by aem, showed a wide dispersion of values although the mean composition of the glass remained close to that of the whole fly ash. The microstructure of these materials is complex: besides crystalline inclusions, the aluminasilica glasses have undergone phase separation on a nanometer scale, and consist of two glassy phases. Since liquid-liquid phase separation has also been found in British fly ashes, it appears to be a characteristic feature of the glassy phase in Class F ashes and is also encountered in occasional Al-Si rich particles in Class C ashes. High iron content glass was found in fly ash SRM 2689, and its phase separation and crystallization were investigated. INTRODUCTION A recent review of fly ashes summarizes the state of knowledge [1]. It is shown that the structure and mineralogy of fly ashes reflects the coal source and combustion conditions. Fly ashes are normally internally inhomogeneous, containing variable proportions of glass and crystalline materials. Until recently, it has been possible to examine and analyze microstructures of fly ash by optical and electron microscopy only on a relatively coarse scale; typically, scanning electron microscopy has a spatial resolution in its analytical mode of 5-10 gim3. However, transmission electron microscopy is capable of much higher spatial resolution. Application of the technique to a range of mainly British fly ashes has revealed a number of hitherto undisclosed features. Major, but highly localized, fluctuations in chemical compositions of the glassy phases were noted between different particles, as well as from point to point within any one particle [2]. Superimposed on this apparently random dispersion of compositions about the mean value was another widespread feature, common to the low-line glasses; the occurrence of liquid immiscibility [3]. Most glasses rich in A120 3 and Si0 2 were found to exhibit amorphous phase separation, typically on a nanometer scale. The incidence and mode of phase separation were correlated with observations on its occurrence in laboratory experiments on synthetic A12 0 3-SIO 2 compositions [4,5]. Very similar microstructures were also observed in low-lime aluminosilicate laboratory melts, prepared to simulate real fly ash glasses. Of course, real fly ashes are chemically complex, containing other oxides which may modify the glass structure. But their effect on phase separation was conjectured to be slight, at least when present at low concentrations. In view of th
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