Kinetics of fly ash leaching in strongly alkaline solutions
We have leached fly ash samples from six power stations in potassium hydroxide solutions at a water-to-solid mass ratio of 40 g/g. A wet chemical method was developed which provides for a detailed characterization of the reactivity of fly ash. The leaching process could be divided into three stages. In stage one, reaction progress measured by the relative mass of fly ash reacted (α) was controlled by the rate of glass network dissolution while very little gel formed (α < 0.1). In stage two, more gel (mainly oxides of Fe, Ca, Mg, and Ti) formed on the glass surface, and the rate of glass dissolution was limited by diffusion (0.1 < α < 0.45). In stage three, zeolite crystallized on top of the gel layer, and an aluminosilicate gel formed in situ, while diffusion continued to control reaction progress (α > 0.45). The data were modeled using a modified Jander equation and rate constants were calculated for each process. The rate constants for stage one reflect an intrinsic glass property, chemical durability, which increased linearly with increasing concentration of network formers in the glass phase of a fly ash.
KeywordsZeolite Geopolymer Relative Mass Reaction Progress Glass Phase
The authors are grateful for financial support of this project from the Vitreous State Laboratory (VSL) of The Catholic University of America (CUA). Chen Chen thanks the Chinese Overseas Fellowship Commission for financial support of his visit to VSL/CUA. The authors thank Dr. Hong Zhao, Dr. Andrew Buechele, and Dr. David McKeown (all VSL) for discussions and experimental support of this study. The authors are grateful to Dr. A. Barkatt (Department of Chemistry, CUA) for his comments and suggestions.
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