Preparation and Hydration Kinetics of Pure CaAl 2 O 4
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PREPARATION AND HYDRATION KINETICS OF PURE CaA1204
M. A. Gulgun, 0. 0. Popoola, I. Nettleship, W. M. Kriven and J. F. Young, Department of Materials Science and Engineering, University of Illinois at Urbana-Champaign, Urbana, IL 61801.
ABSTRACT Single phase, pure monocalcium aluminate (CaA1204) powders are chemically synthesized at temperatures as low as 900"C. The powders have a specific surface area of approximately 10 m2 /g. The hydration kinetics of CaA120 4 and the morphology of the hydrates are analyzed using electron microscopy techniques. INTRODUCTION Monocalcium aluminate (CaA120 4 , CA) and calcium dialuminate (CaAl 4 0 7 , CA 2) are the two major phases present in high alumina cements which are used in macro-defect free cement composites. However, monocalcium aluminate is primarily responsible for the characteristic hydration behavior (i.e. early strength development) of this kind of chemically bonded ceramic. In addition, CaA120 4 has important refractory and infrared optical applications depending on its crystallinity. In general, monocalcium aluminate powders are produced by high temperature solid state reactions between calcia, or calcium carbonate, and alumina powder. The extent of the reaction depends on the particle size, surface area and the mixing of the reactant powders, and usually requires temperatures in excess of 1400' C. Typically, powders produced by this method have a surface area of 0.235m 2 /g to 0.56 m2 /g [1,2]. X-ray amorphous, high specific surface area calcium aluminate powder has been chemically prepared by Uberoi and Risbud [3] using aluminum di-sec-butoxide acetoacetate esterchelate and calcium nitrate precursors. Roy et al.[4] reported an evaporative decomposition of solution technique to produce CA powders at 900 0C from nitrate precursors of calcium and aluminum. The hydration behavior of calcium aluminate powders prepared by conventional high temperature synthesis techniques have been studied in the temperature range 4°C to 40'C using calorimetric analysis [1,2], solution chemistry [1,5], X-ray diffraction [2,6], and analytical electron microscopy [6]. At temperatures up to 30'C, an Al(OH) 3 gel and a so-called " intrusion hydrated layer " of thickness 12 nm were observed to form on CA particles [1]. Upon the destruction of this hydration layer, nuclei of crystalline products form and hydration proceeds by a dissolution-precipitation mechanism. Percival et al. [51 studied the precipitation of CAH 10 (CaO-A1203.10H20) from supersaturated calcium aluminate solution at 21'C, and found that the amounts of calcia and alumina in solution determine which of the hydrate phases will precipitate. High concentrations of ions in the solution yielded C 2AH 8 (2CaO-A12O3.8H20), CAH 10 and AI(OH) 3, whereas lower concentrations precluded CAH 10 precipitation. The hydration behavior of Ca 3 A12 0 6 (C3 A) in aqueous suspension (w/c = -50) has been studied by Breval [6]. The products formed from the hydration of C 3 A are the same as those formed during the hydration of CA, except that th
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