Mineralogical Transformations and Microstructure After Disposal of Cementitious Advanced Coal Technology by-Product
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G.J. McCARTHY*, R.D. BUTLER**, D.W. BREKKE**, S.D. ADAMEK*, J.A. PARKS*, H.J. FOSTER**, and J. SOLC** * Department of Chemistry, North Dakota State University, Fargo, ND 58105 ** Energy and Environmental Research Center, University of North Dakota, Grand Forks, ND 58202 ABSTRACT
By-products from two advanced coal technologies, Fluidized Bed Combustion (FBC) and Limestone Injection Multistage Burner (LIMB), were found to be cementitious when mixed with water and compacted. However, exposure to natural conditions in test cells resulted in losses of strength and increases in permeability over a period of years. Changes in mineralogy and microstructure with time in recovered core samples have been characterized by powder X-ray diffraction (XRD) and scanning electron microscopy (SEM). Up to 80 wt% of the core materials had converted to crystalline hydrate phases by the end of two years. Ettringite, gypsum and portlandite were the initial hydration products. In the LIMB materials, formation of thaumasite in microfractures, channels and voids was detected after one year, and in the FBC material after two years. Thaumasite formation was accompanied by reductions in gypsum and portlandite; it did not appear to be forming at the expense of ettringite. EDS examination of many ettringite and thaumasite crystals showed that the former always contained some Si and the latter some Al, which is evidence for ettringite-thaumasite solid solution. Thaumasite formation accompanied marked losses in strength and increases in permeability. INTRODUCTION
In the United States, various laws, such as the 1991 Clean Air Act, require reduced emissions of sulfur dioxide and nitrogen oxides. The advanced coal ("clean coal") technologies necessary to meet these regulations result in solid by-products that differ greatly from conventional combustion fly ash and bottom ash, especially in their high contents of calcium sulfate (or sulfite) and free lime. Utilization or proper disposal of these by-products requires an understanding of their long-term behavior in nature. To begin to obtain such an understanding, by-products from two advanced coal technologies were emplaced in instrumented test cells at three sites in the United States. Results from two sites will be discussed in this paper: "* Limestone Injection Multistage Burner (LIMB), a sorbent duct injection technology combined with low-NOx multistage burner technology; site near the Ohio Edison Edgewater Plant in Lorain, Ohio; "* Fluid Bed Combustion (FBC) by-product; Midwest Grain Products, Inc. bubbling bed unit; test cell site at the Freeman United Buckheart Mine in central Illinois. The objectives of the overall project were to: monitor field performance of advanced coal technology by-products; develop a physical-chemical model of by-product behavior; and develop management strategies for by-products [1,21. The objective of this paper is to summarize the mineralogy of the hydration reactions and microstructure development with time that figure in development of the physical-chemical model.
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