X-ray absorption fine structure spectroscopy and X-ray diffraction study of cementitious materials derived from coal combustion by-products |
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Authors: | S Pattanaik G.P Huffman R.J Lee |
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Affiliation: | a Illinois Institute of Technology, Bailey Hall 703, 3101 South Wabash Avenue, Chicago, IL 60616, USA b Consortium for Fossil Fuel Science, University of Kentucky, 111 Whalen Building, 533 South Limestone Street, Lexington, KY 40506, USA c R.J. Lee Group, 350 Hochberg Road, Monroeville, PA 15146, USA |
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Abstract: | Cementitious materials derived from coal combustion by-products have been investigated by means of X-ray diffraction (XRD) and S and Ca K-edge X-ray absorption fine structure (XAFS) spectroscopy. The XRD analysis revealed that these materials are a complex mixture of a small amount of quartz [SiO2] and three calcium-bearing compounds: hannebachite [CaSO3·1/2H2O], gypsum [CaSO4·2H2O] and ettringite [(Ca6(Al(OH)6)2(SO4)3·26H2O)]. Analysis of the S XAFS data focused on deconvolution of the X-ray absorption near-edge structure (XANES) regions of the spectra. This analysis established that sulfate and sulfite are the two major sulfur forms, with a minor thiophenic component contained in unburned carbon in the fly ash. Increasing sulfate and decreasing sulfite correlated well with increasing gypsum and ettringite and decreasing hannebachite content in the samples. Different calcium compounds were identified primarily through simple comparison of the Ca K-edge XANES and radial structure functions (RSFs) of the cementitious samples with those of reference compounds. Because of the complex coordination chemistry of calcium in these materials, it was difficult to obtain detailed local atomic environment information around calcium beyond the first CaO peak. Analysis of the extended X-ray absorption fine structure (EXAFS) and the RSF gave average CaO distances in the range 2.44-2.5 Å, with each calcium atom surrounded roughly by eight oxygen atoms. In certain samples, the average CaO distances were close to that in ettringite (2.51 Å), suggesting that these samples have higher ettringite content. The results of S and Ca K-edges XAFS and the XRD data were in reasonable agreement. |
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Keywords: | X-ray diffraction X-ray absorption fine structure (XAFS) spectroscopy Filler Ettringite Coal combustion by-products |
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