Hydration Reactions and Etrringite Formation in Selected Cementitious Coal Conversion By-Products
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HYDRATION REACTIONS AND ETrRINGITE FORMATION IN SELECTED CEMENTITIOUS COAL CONVERSION BY-PRODUCTS
JODY K. SOLEM and GREGORY J. McCARTHY
Department of Chemistry, North Dakota State University, Fargo, ND 58105 U.S.A.
ABSTRACT
Cementitious reactions and ettringite formation, which occur when water is added to high-calcium fly ash, to many dry flue gas desulfurization solids, and to two of the residues from the emerging "clean coal" technologies, fluidized bed combustion and limestone injection multiphase burner, have been studied. The parameters varied included water to solid ratio and curing time. Crystalline reactants and products were monitored by X-ray powder diffraction. The amount of ettringite, the principal crystalline cementitious reaction product, was determined after three months of curing. Results are discussed in terms of available Ca, Al and S in each by-product and w/s ratio. INTRODUCTION
The knowledge base of chemical, mineralogical, and physical characteristics of North American fly ashes, needed for a technical understanding of utilization and disposal, has improved considerably during the last decade [1-3]. However, with the advent of new environmental regulations and "clean coal" technologies, the proportion of high-calcium and high-sulfur (sulfate or sulfite) solid residues among coal conversion by-products is growing, and may some day surpass that of fly ash in North America. These materials include the residues of flue gas desulfurization (FGD) scrubbers, and "advanced" coal combustion technologies such as fluidized bed combustion (FBC), limestone injection multistage burner (LIMB) and other sorbent injection technologies. Many of these dry residues are cementitious, as they harden when moistened. (However, as Berry et al. [4] point out, not all such cementitious residues may be hydraulic in the conventional sense.) Of particular interest in understanding the cementitious properties as they pertain to possible utilization and to their behavior on disposal, and in evaluating these residues for possible use in hazardous waste immobilization [5,61, is
the formation of ettringite (AFt), nominally Ca 6A]2(S04)3(OH) 12-26H 20. Ettringite provides much of the initial strength upon hydration of cementitious residues. High-calcium fly ashes are likewise cementitious, and include ettringite among their initial hydration products. The objective of the work presented here was to use X-ray diffraction (XRD) to screen typical high-calcium (ASTM Class C) fly ashes and advanced coal combustion residues for their ability to harden and form ettringite, and any other crystalline hydration products, as a function of water/solid ratio and curing time. An intermediate-calcium fly ash and a low-calcium fly ash (both ASTM Class F) were included in the study to establish baseline behavior where (presumably) no ettringite would form. EXPERIMENTAL Materials
Table 1 lists the fly ashes and residues used in the study. The fly ashes (FA) are classed according to the U.S. State of origin and type of coal burned (e.g. W
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