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a CNRSSP, 930 Boulevard Lahure, 59505 Douai cedex, France
b Ohio State Univ., School of Natural Resources, 2021 Coffey Road, Columbus, OH 43210
* Corresponding author (laperche{at}cnrssp.org)
Received for publication April 18, 2001.
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMATERIALS AND METHODSRESULTSCONCLUSIONSREFERENCES |
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Abbreviations: FGD, flue gas desulfurization • FGDG, flue gas desulfurization grout • CCBs, coal combustion by-products • FA, fly ash • FC, filter cake • XRD, x-ray diffraction • SEM, scanning electron microscopy • EDS, energy dispersive x-ray analysis • IR, infrared • FTIR, Fourier transform infrared • TGA, thermogravimetric analysis • DTA, differential thermal analysis • GFAA, graphic furnace atomic absorption
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSCONCLUSIONSREFERENCES |
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 | [1] |
The Ca-sulfite so produced may be partially or fully oxidized to some form of Ca-sulfate as per Reaction [2]:
 | [2] |
In nonrecovery systems, the reaction product(s) is partially dewatered to produce a filter cake (FC) that is blended with other coal combustion by-products (CCBs), such as fly ash (FA), for codisposal. In 1996, approximately 21.7 million Mg of FGD by-products were produced (Am. Coal Ash Assoc., 1997), and this amount was expected to increase through the end of the century (Dick et al., 2000). Unfortunately, landfill sites are becoming scarcer and tipping fees are constantly increasing. Numerous studies have therefore been conducted to find beneficial uses with minimal environmental impacts for these materials. Possible high volume applications include utilization as agricultural lime substitutes (Stehouwer et al., 1995; Korcak, 1995; Ritchey et al., 2000); as amendments for improving soil physical properties (Norton, 1995); as construction materials for road embankments, pond liners, and livestock platforms (Dick et al., 2000); and as grouts for sealing abandoned mines and neutralizing acid mine drainage (Laperche and Traina, 1999).
Permits for beneficial reuse of FGD by-products are often based on bulk chemical analyses without considering the fact that such materials are usually mixtures of CCBs, excess sorbent, and reaction products. Thus, contaminant species may not be uniformly distributed or mobile. The components of these complex mixtures may also undergo further reactions that ultimately affect their physical and chemical properties. For example, ettringite [Ca6Al2(SO4)3(OH)12·26H2O]–type minerals that degrade the strength of cementitious CCBs may form over time (Day, 1992). These minerals may also influence the mobility of hazardous components through solid solution (McCarthy et al., 1992) or sorption reactions. Myneni et al., (1997) recently demonstrated that ettringite can sequester arsenate, and the solubility of As(V) in equilibrium with this mineral was much lower than with other Ca arsenate phases in high pH (>10.5) environments. Under neutral to acid conditions, ettringite dissolves incongruently to form gypsum, Al-hydroxides, and/or Al-hydroxy sulfates (Myneni et al., 1998), which may undergo secondary reactions with trace elements originally contained in the ettringite matrix.
At present, our ability to predict the future behavior of FGD by-products is usually limited by an incomplete knowledge of the chemistry and mineralogy of their component phases. The objective of this study was to demonstrate that simple fractionation and selective dissolution techniques can be coupled with standard chemical and mineralogical analyses to provide a detailed characterization of FGD materials. A FGD grout (FGDG) and its starting components (FA, FC, and hydrated lime) were analyzed to evaluate the distribution of mineral and chemical constituents before and after formulating the grout. This particular FGDG was prepared from a 1:1 mixture (wt./wt.) of FC and FA to which 50 g kg-1 hydrated lime was added before use as an abandoned mine sealant. Previous unpublished studies had shown this composition would yield a product with maximum strength after curing.
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MATERIALS AND METHODS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSCONCLUSIONSREFERENCES |
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Total Chemical Analyses
The samples were dried and ground to a fine powder (<100 µm) with a synthetic sapphire mortar and pestle and then digested in 950-W microwave digestion bombs (MD 2100, CEM Corp., Mathews, NC) using the coal ash method recommended by CEM Corporation. The resulting HF–HCl–HNO3 digests were analyzed in triplicate using optical emission, inductively coupled plasma spectroscopy (ICP-OES, Perkin Elmer, Norwalk, CT). All elements reported in this study were measured in the digests except B and C. Total B analyses were performed by XRAL Laboratories (Ontario, Canada) using ICP-OES after fusion of the samples with Na2CO3. Total C was determined by dry combustion at 950°C (Nelson and Sommers, 1982), and CaCO3 equivalent was measured by using a gasometric method (Dreimanis, 1962).
Mineralogical Analyses
X-Ray Diffraction (XRD). Samples were ground with a synthetic sapphire mortar and pestle to 
250 µm, and randomly oriented, back-filled sample mounts were prepared. All XRD analyses were conducted with a Philips x-ray diffractometer (Philips Analytical, Natick, MA) using CuK
radiation at 35kV and 20 mA. Step-scanned data were collected from 8 to 70°2
with a fixed time of 4 s per 0.05°2 for routine analyses and 10 s per 0.01°2 for more detailed analyses. All data were analyzed using a semiquantitative data reduction software (JADE, version 2.0) from Materials Data Inc. (Livermore, CA, 1992). Crystalline phase assignments were based on published literature, searches of the ICDD international database, and comparative analyses of reference mineral samples.
Scanning Electron Microscopy (SEM) and Energy Dispersive X-Ray Analysis (EDS). Samples analyzed by XRD were also examined with a scanning electron microscope (Philips XL-30 or JEOL JSM-A20) capable of energy dispersive chemical analyses. Samples were mounted on stainless steel stubs using double-stick tape and then C or Au coated. The chemical compositions of various mineral phases were obtained from the EDS spectra. The peak area of each element was taken as proportional to the quantity of the element present in the sample (Wenk, 1976). Data were obtained both from pellets (total chemical analysis) and from single particles. An average of 10 chemical analyses was performed on each pellet; whereas, the number of chemical analyses for single crystals depended on the number of crystals present in the sample.
Infrared Spectrometry. Diffuse reflectance IR spectra of powdered samples were collected on a Mattson Polaris Fourier Transform Spectrometer (FTIR) equipped with a broad band mercury cadmium telluride (MTC) detector, a KCl beam splitter and a Harrick Praying Mantis diffuse reflectance cell. Two to 5 mg of sample were mixed with 200 mg of KBr (infrared grade). All infrared spectra were recorded between 4000 and 450 cm-1 as an average of 200 scans at 2 cm-1 resolution.
Thermogravimetric Analysis. Thermal analyses of all materials were conducted using a Seiko SS5000 instrument that permitted the simultaneous collection of thermogravimetric (TGA) and differential thermal (DTA) data. Thirty-mg samples were heated from 25 to 900°C at a rate of 25°C min-1 under a continuous flow of dry N2.
Fly Ash Fractionation and Analysis
Magnetic separations of the FA were performed to facilitate chemical and mineralogical analyses. One hundred g of FA were added to 1 L of deionized water and stirred for 5 min. A strong hand-magnet was applied to the base of the beaker while the surpernatant was poured into another beaker. This operation was repeated until only nonmagnetic particles were left in the supernatant.
Hematite (–Fe2O3) was dissolved using the method of Mehra and Jackson (1960). Forty mL of citrate buffer were added to 1 g of whole FA and each of the subfractions (nonmagnetic and magnetic). Samples were placed in a water bath and the temperature was brought to 75 to 80°C. Two, 1-g portions of Na-dithionite were then added to the samples while stirring occasionally for 5 min. Additional Na-dithionite was added if all the hematite was not dissolved as determined by the sample color. The solutions were filtered, and the Fe contents were determined by atomic absorption spectrometry (AAS Perkin Elmer, 3030B, Norwalk, CT). The whole FA and its subfractions were also subjected to a HF etching treatment to selectively dissolve the glass phase (Hulett and Weinberger, 1980). One hundred mg of each sample were placed in a plastic tube containing 10 mL of 1%-HF and were shaken slowly for 2 to 144 h. The solutions were filtered and the mineral residues were washed several times with deionized water. The solids were then examined by XRD and SEM-EDS, and the solutions were analyzed by ICP-OES for all elements except As, which was determined by graphite furnace atomic absorption (GFAA) spectrometry (Perkin-Elmer 4100ZL, Norwalk, CT).
Composition of Flue Gas Desulfurization Grout
The chemical and mineralogical composition of the FGDG was determined as described previously with the following modification. The FA fraction was isolated and quantified by mixing preweighed samples (20 g) with a Na acetate–acetic acid buffer (pH 5.0) to form a thick slurry. The slurry was treated with 30% H2O2 to convert hannebachite (CaSO3·0.5 H2O) to more soluble gypsum (CaSO4·2H2O). Following oxidation and heating at 60°C to decompose excess H2O2, the samples were quantitatively transferred to cellulose dialysis bags and submerged in Na acetate–acetic acid buffer (pH 5.0) for 4 to 6 d to remove acid-soluble materials such as calcite (CaCO3). The samples were then dialyzed against deionized water to remove excess salt and to dissolve residual gypsum. Removal of gypsum was judged to be complete when a negative test for dissolved SO2-4 was obtained with BaCl2. The samples were then quantitatively transferred to tared beakers, dried at 105°C, and weighed to determine the residual ash content. The soluble fraction was determined by difference.
The glass content of the FGDG was determined by placing 1 g of each sample in a plastic tube containing 100 mL of 1%-HF. The tubes were shaken slowly for 16 h, filtered, and washed a few times with deionized water before being quantitatively transferred to tared beakers. The samples were then dried at 105°C and weighed to determine the crystalline phase content and, by difference, the glass content.
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RESULTS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSCONCLUSIONSREFERENCES |
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 | [3] |
 | [4] |
As expected from these reactions, calcite (CaCO3) and portlandite [Ca(OH)2] were the major phases detected by XRD analysis along with smaller quantities of brucite [Mg(OH)2], quartz (SiO2), and lime (Fig. 1a) . The FTIR analysis confirmed the presence of calcite, portlandite, and brucite based on reference data reported by Farmer (1974). Absorption bands for calcite were detected between 1458 and 1410 cm-1, and at 873 and 713 cm-1. The hydroxyl bands of portlandite and brucite occurred at 3643 and 3696 cm-1, respectively (Fig. 2a) . Quantitative measures of brucite (35 g kg-1) and portlandite (321 g kg-1) were obtained by TGA using the weight losses associated with dehydroxylation over a temperature range of 330 to 490°C. The content of calcite (495 g kg-1) was likewise determined by the weight loss from decarbonation over a temperature range of 665 to 826°C. Calcite measured by TGA agreed with that calculated from total C by dry combustion (64 g kg-1 C equivalent to 530 g kg-1 calcite) and with that determined by the gasometric method of Dreimanis (1962) (515 g kg-1). The quartz content was estimated to be 19 g kg-1 from chemical analysis for Si. By difference, the lime content was estimated to be 55 g kg-1 (Fig. 3) .
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OH: 3549 cm-1 and 3: 1140 and 1115 cm-1), but no carbonates were identified. A OH band at 3696 cm-1 suggested the presence of brucite. From the chemical composition (6.2 g kg-1 Mg), a brucite content of 15 g kg-1 was estimated (Fig. 3). The Si concentration was 15.4 g kg-1, which is equivalent to 42 g kg-1 SiO2. However, no quartz was identified by XRD, probably because Si and other elements were present as glass rather than as crystalline material.
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Chemical and Mineralogical Characterization of Fly Ash
Bulk Fly Ash. The bulk FA was composed of mullite (Al6Si2O13), quartz, glass, hematite, and magnetite (Fe3O4) (Fig. 1c). Most of the FA particles were spheres with either smooth or rough surfaces and diameters ranging from <1 to 100 µm (Fig. 4b). The chemical composition (determined by EDS) of the spheres was variable. Some were rich in Fe (550 g kg-1), whereas others (Fig. 4c) contained primarily Si (290–500 g kg-1) and Al (270–420 g kg-1) with much less Fe (22–290 g kg-1). Nonspherical particles were generally poor in Fe (29–38 g kg-1) but rich in Si (400–680 g kg-1). A few nonspherical particles had high S contents (250 g kg-1). Some spheres had broken surfaces (Fig. 4d) and contained smaller spheres. The broken spheres as well as the smaller, internal spheres were usually rich in Fe (>550 g kg-1). Bulk chemical analysis showed the FA to contain 452 g kg-1 SiO2, 214 g kg-1 Al2O3, 219 g kg-1 Fe2O3, 22 g kg-1 CaO, 18 g kg-1 K2O, 8 g kg-1 MgO, and 5 g kg-1 Na2O. These contents are typical for FA from the combustion of bituminous coal (Mattigod et al., 1990) and would cause the FA to be classified as a low Ca or class F material by the American Society for Testing and Materials.
The trace element composition of fly ash depends on the type of coal used. In general, the concentrations of minor elements are much higher in FA than in coal due to enrichment from the combustion process (Lindsay, 1979). The concentrations of most trace elements in the FA examined in this study (Table 1) were lower than reported mean concentrations in FA derived from bituminous coal (Ainsworth and Rai, 1987; Eary et al., 1990). Tungsten and Sb concentrations were below detection limits for all samples. Uranium concentrations were significant (9.1 mg kg-1), but the cost and difficulty in analyzing this element did not permit further study.
Magnetic and Nonmagnetic Fractions. The magnetic fraction comprised 22.2% of the total mass of the FA and was mostly composed of smooth surfaced spheres. By contrast, the nonmagnetic fraction (77.8% of the total mass of the whole FA) contained mostly irregular particles and rough surfaced spheres. X-ray diffraction patterns of the nonmagnetic fraction (data not presented) showed that it was composed mostly of quartz, mullite, hematite, and glass; the diffuse scattering maximum between 17 and 29°2 that is typical of glass (Diamond, 1983; McCarthy et al., 1988) was more intense in the nonmagnetic fraction than in the whole FA. The magnetic fraction contained magnetite, hematite, glass, and traces of quartz and mullite (Fig. 1e).
As expected, Fe was highly concentrated in the magnetic fraction along with some enrichment of Co, Cr, Mo, and Mn (Table 1, Fig. 5) . Some elements such as Cd, Cu, Ni, and V were equally distributed between the two fractions, but most elements were enriched in the nonmagnetic fraction (Fig. 5). This distribution is similar to that reported by Hulett et al. (1980) for the first-row transition elements. Hulett et al. (1980) also observed that V, Cr, Mn, Co, Ni, Zn, and Cu were more concentrated in the magnetic phase and suggested that these elements were probably incorporated into spinel structures like magnetite. There is no known spinel with Mo substitution, so Mo probably occurs as the free oxide (Theis et al., 1982).
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Dissolution of the magnetic fraction with 1% HF was less rapid than the other samples and required 48 to 96 h to reach a plateau with 
40% of the original weight remaining. The XRD patterns after etching (Fig. 1f) showed glass was not the only material dissolved. Magnetite was completely removed from the sample after 96 h contact, leaving a residue composed mostly of hematite, quartz, and traces of mullite. The XRD peak positions of the hematite (Fig. 1e) matched almost perfectly (
< 0.003°2) with those of reference hematite (ICDD no. 33-0664), and the absorption bands observed with FTIR (Fig. 2e) also agreed within a few cm-1 of those reported by Russell and Fraser (1994) for well crystalline hematite.
The extraction efficiency of the 1% HF for trace elements from the bulk FA varied by element. Boron (99.3%), As (88.2%), Mo (90%), and Li (76.6%) were almost completely dissolved after only 2 to 4 h contact and were brought completely into solution after 16 to 48 h. The amounts of Ba (3.1%), Ca (6.7%), Ni (39.2%), K (44.9%), Co (48.1%), Ca (48.6%), Cu (54.7%), and Zn (54.7%) in solution after 2 h varied, but complete extractions were obtained after 48 to 144 h. The extraction of Mg, Mn, Sr, and V was 80 to 90% complete after 144 h. The amounts of Al (68.2%), Be (50.8%), Cd (37.5%), Cr (60.1%), Fe (65.4%), Pb (52.4%), Si (67.9%), and Ti (71.3%) in solution 144 h after the start of the extraction also varied. When the glass was completely removed after 16 h (leaving a residue of mullite, quartz, hematite, and magnetite), 60 to 100% of the following trace elements had been released: As (90%), B (100%), Co (71%), Cu (86%), Mn (65%), Mo (90%), Ni (72%), Pb (60%), V (69%), and Zn (85%). Lower proportions were extracted for Cd (15%), Cr (34%), and Be (44%). These data show clearly that the more reactive glass phase also contained the highest concentrations of most trace elements. Previous studies (Eary et al., 1990) have demonstrated many of the more volatile and most of the hydride-forming elements actually concentrate on the surface of ash particles. This distribution may account for the mobility of many trace elements when fly ash is leached under environmental conditions. For example, Wasay (1992) reported that about 40% of the total Cr, As, and Hg were readily leached from fly ash in landfills.
After 8 to 15 extractions with Na-dithionite solution, all hematite was removed from FA and its subfractions as shown in Fig. 1g. The magnetic fraction after 15 CBD extractions contained only magnetite, quartz, and traces of mullite. All magnetite XRD peaks showed asymmetry on the high angle side, and some of the peaks had a shoulder on the low angle side. The peak asymmetry suggests the presence of other minerals with a spinel structure, especially ones rich in Mg, Al, Cr, Mn, Ni, or Zn. However, only a manganese-rich spinel with composition (Fe, Mn)3O4 was directly observed by SEM (Fig. 4f). The octahedral crystals contained more Mn (1.5–2 atoms per unit cell) than Fe (1–1.5 atoms per unit cell). No such crystals were identified for Cr, Ni, Zn, Cu, V, and Co, but the concentrations of these trace elements were two to seven times less than the concentration of Mn (Table 1). Even if all the Mn were contained in a spinel structure (as FeMn2O4, or MnFe2O4), this phase would represent <0.2% of the total mineralogical composition of the magnetic fraction of FA and <0.05% of the whole FA.
The elemental composition of the solutions after 16 h etching was used to calculate the amount of glass in the FA (Fig. 3), and the Al content of the residue was used as a measure of mullite. The amount of Si not in glass or mullite was used to calculate the quartz content. The Fe content in the solutions reached a plateau between 48 and 96 h, and this value was used to calculate the amount of magnetite. The Fe content in solution after Na-citrate dithionite treatment was used to determine hematite. Based on these computations, the bulk FA and its nonmagnetic and magnetic fractions were composed of 363, 383, and 130 g kg-1 crystalline silicates (mullite and quartz); 422, 515, and 287 g kg-1 glass; and 159, 55, and 565 g kg-1 iron oxides (hematite and magnetite), respectively (Fig. 3).
Chemical and Mineralogical Characterization of Flue Gas Desulfurization Grout
The FGDG examined in this study was prepared as a 1:1 mixture of FA and FC with 50 g kg-1 added lime. Analyses of the FGDG showed that this mixture was not mineralogically stable. No lime or bassanite was detected, and a new mineral phase in the form of ettringite (Ca6Al2(SO4)3(OH)12·26H2O) was identified by XRD (Fig. 1h) and SEM (Fig. 4g). Ettringite formation occurs at high pH in the presence of soluble Ca, SO4, and Al (McCarthy and Solem-Tishmack, 1994) as per Reaction [5]:
 | [5] |
The other major mineral phases detected by XRD were hannebachite, mullite, quartz, hematite, and magnetite (Fig. 1h). Analysis of the specimens by IR spectroscopy did not provide any additional diagnostic information; the FGDG spectrum was very similar to that of the FC (Fig. 2b).
The content of hannebachite, as determined by TGA, was 314 g kg-1 (Fig. 3). No suitable method was found or developed to directly quantify the amount of ettringite; however, ettringite could be dissolved along with hannebachite–gypsum in Na acetate–acetic acid buffer (pH 5.0) and water. Collectively, these minerals represented 46.1% of the sample (Fig. 3). Considering the known amount of hannebachite, this sample contained 147 g kg-1 ettringite. This amount of ettringite is compatible with the JADE results. The JADE program was run on six FGDG spectra and the average ettringite content was 12.6 ± 1.8%. Selective dissolution of the remaining glass phase gave 225 g kg-1 glass and 314 g kg-1 insoluble residues (mullite, quartz, and iron oxides). The ratio of insoluble to glass phases in the FGDG was identical to that in the FA (1.40 and 1.39, respectively). From this ratio, we calculated the amounts of hematite, magnetite, mullite, quartz, and glass in the FGDG (Fig. 3).
The chemical composition of FGDG can obviously vary depending on the type of coal ash, absorbent (lime, calcite, dolomite, etc.), scrubber process (dry or wet), and the ratio of FC/FA used in its preparation. The FGDG examined in this study contained 287 g kg-1 SiO2, 121 g kg-1 Al2O3, 118 g kg-1 Fe2O3, 200 g kg-1 CaO, 200 g kg-1 SO3, 16 g kg-1 K2O, 11 g kg-1 MgO, and 2 g kg-1 Na2O. These values are in the major element ranges reported for other FGD by-products (Summers et al., 1983; Ainsworth and Rai, 1987). The concentrations of minor elements were also comparable to those of similar materials (Summers et al., 1983; Ainsworth and Rai, 1987) except that As was slightly higher than the usual range ([As] = 63 mg kg-1; 0.8–53 mg kg-1). The fact that FA was mixed with FC to form the FGDG did not decrease the concentration of some elements (B, Cu, Mn, Se, and Sr) because they were also present in the FC at relatively high levels. The Pb concentration in the FA (65.3 mg kg-1) was similar to the Pb concentration in the FGDG (59.9 mg kg-1), and no Pb was detected in the FC. This result can only be due to contamination by Pb during preparation of the FGDG.
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CONCLUSIONS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSCONCLUSIONSREFERENCES |
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The FGDG examined in this study contained 461 g kg-1 soluble solids in the form of hannebachite and ettringite, and 539 g kg-1 FA (including 225 g kg-1 glass). Our data showed that most trace elements were concentrated in the FA component, but the mobility of these elements was highly variable. Etching with 1% HF for 16 h gave quantitative dissolution of the glass phase and released 60 to 100% of most trace elements to solution. Contrary to the results of Hansen and Fisher (1980), Fe in our samples was not associated with the aluminosilicate matrix but occurred primarily as hematite and magnetite. Hematite was insoluble in 1% HF for up to 120 h but was readily soluble in buffered Na-dithionite. By contrast, magnetite was dissolved in <48 h contact with 1% HF. Magnetite dissolution was presumably related to oxidation of structural Fe2+ in the presence of HF. The XRD and chemical results also showed that trace element substitutions (Co, Cu, Mn, and Ni) were common in the FA magnetite. These elements were almost entirely (>90%) released by dissolution of glass and magnetite in the FA.
The FA component of the FGDG was also the primary source of Al for the formation of ettringite as per Reaction [5]. This study demonstrated that semiquantitative analyses for ettringite in FGD by-products are possible. More quantitative studies of this type are needed to understand how the composition of FGD by-products influences the rate and extent of ettringite formation. Such knowledge is important because ettringite formation causes undesirable expansion of FGD materials used for construction purposes (Odler and Gasser, 1988; Day, 1992). The mineral may also represent a transient sink for many trace elements, especially oxyanions such as borate, selenate, selenite, chromate, and arsenate (Poellmann et al., 1990; Kumarathasan et al., 1990; McCarthy and Solem-Tishmack, 1994). Because ettringite is unstable at low pH, it may dissolve in contact with neutral to acidic solutions, thereby providing a latent source of pollution (Laperche and Traina, 1999) and further weakening structures prepared from FGD materials.
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ACKNOWLEDGMENTS |
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REFERENCES |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSCONCLUSIONSREFERENCES |
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