The glassy phase and reaction products of Iowa fly ashes

Bergeson, Kenneth
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Civil, Construction, and Environmental Engineering

The crystalline and glassy phases of a series of low calcium to high calcium Iowa fly ashes were investigated. Elemental analyses were conducted by quantitative x-ray fluorescence (QXRF) techniques. Crystalline compounds were determined by quantitative x-ray diffraction (QXRD). Glassy phase compositions of the ashes were estimated from these data. The glassy phase of the high calcium ashes is believed to be in a more highly strained chemical state as compared to the low calcium ashes, and hence, more reactive. It is proposed that the reactivity of the glassy phase can be characterized by a "reactivity index" which is the ratio of glass modifiers to glass formers expressed by (CaO + MgO)/(SiO(,2) + Al(,2)O(,3)) in the glass. It is shown that this relationship is nearly linear to this same ratio based on elemental composition which allows a rapid evaluation of reactivity based on ASTM C-618 analysis data;It was shown that the elemental analysis consistency of Iowa ashes, from a given generating station, is not a valid indicator of its fundamental physical properties. This variability in physical properties (e.g., compressive strength development, setting time, etc.) is believed to be a function of the amount of calcium bearing crystalline compounds present, and correspondingly, the amount of reactive glass present. It was shown that for a high calcium ash, from a given generating station, that the glassy phase is an integral and necessary contributor to early (28 day) strength;The glassy phase of Iowa fly ashes are shown to contain significant amounts of the cementitious building blocks of calcium, aluminum, and silicon. It was postulated that small amounts of secondary chemical activators could be economically utilized to accelerate the attack and dissolution of the glassy phase, thereby releasing calcium, aluminum, and silicon for formation of cementitious calcium aluminate and calcium silicate hydrates. Ammonium nitrate was found effective with a low calcium ash. Dibasic ammonium phosphate was effective with a high calcium and intermediate calcium ash.