Some Properties of Contrasting End-Member High Calcium Fly Ashes
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SOME PROPERTIES OF CONTRASTING END-MEMBER HIGH CALCIUM FLY ASHES SIDNEY DIAMOND and JAN OLEK School of Civil Engineering, Purdue University, West Lafayette, IN 47907, U.S.A. Received 1 December, 1986; refereed ABSTRACT High-calcium fly ashes with CaO contents >20% do not necessarily contain the glass type found to be characteristic of such ashes in previous results. Instead of this glass (asymmetrical x-ray maximum at 32 deg. CuK-alpha radiation), the fly ash may contain a lower calcium content glass (symmetrical x-ray maximum near 27 deg.) and a substantial content of crystalline CaO. The properties of a representative fly ash of each type are briefly illustrated, and the characteristics of fly ash pastes and of cement-fly ash pastes produced from the two fly ashes are examined. INTRODUCTION The properties of very high calcium fly ashes (roughly, those of imperfectly established, though they analytical CaO contents >20%) are still have been actively investigated for some years. It is well established that analytically-determined CaO contents of such fly ashes represents the sum of calcium present in the glass phase and calcium present in one or more of several well crystallized substances such as crystalline CaO, anhydrite, tricalcium aluminate, and occasionally even tricalcium silicate or dicalcium silicate. The presence of crystalline calcium-bearing compounds in a given fly ash is readily observed by the usual x-ray diffraction methods, although quantitative analyses may be difficult. As indicated several years ago by Diamond [1] the x-ray diffraction maximum associated with the glass in fly ash shifts in position as a function of the analytical CaO content of the fly ash. The position of the maximum shifts from that characteristic of vitreous silica (about 23 deg. with copper radiation) to about 27 deg. as the analytical CaO content increases to about 20%. Fly ashes with analytical CaO contents more than about 20% were found to maintain an asymmetrical glass maximum with a fixed position at about 32 deg. The original relationship is presented as Figure 1. Recently Hemmings and Berry [2] studied the compositions of size and density fractions of a fly ash of an intermediate CaO content, approximately 10% CaO overall. They indicated that two types of glass coexist in the same ash: a "Glass I" low-calcium glass associated with fly ash particles of low specific gravity, which yields a 23 deg. diffraction maximum, and a second type ("Glass II") richer in CaO and associated with high specific gravity particles. This glass yields a diffraction maximum close to 28 deg. Hemmings and Berry determined glass compositions indirectly, by subtracting the contributions of the amounts of crystalline materials from the overall analysis, and represented them on a triangular CaO - A12 03 - Si0 2 phase diagram. The compositions of the Glass I material fell mostly in the border between the primary phase fields of tridymite (Si0 2 ) and mullite (Al 6 Si 2 0 1 3 ), with CaO contents less than about 10%. The Glass II compositions were
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