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Stumm, W.; Morgan, J. J. Aquatic Chemistry; Wiley: New Uork, 1981; pp 297-299. Nassau, K.; Miller, A. E.; Graedel, T. E. Corros. Sci. 1987, 27, 703. Hoffmann, M. R.; Jacob, D. J. SOz,N O and NOz Oxidation Mechanisms: Atmospheric Considerations; Calvert, J., Ed.; Butterworth Publishers: Boston, MA, 1984;pp 101-173. Opila, R. L. Corros. Sci. 1987, 27, 685. NOAA, Local Climatological Data-Central Park, N e w York, N Y ; National Climatic Center: Asheville, NC, 1981. Osborn, D. H. Mater. Des. Eng. 1963, June, 80. Nassau, K.; Gallagher, K. P.; Miller, A. E.; Graedel, T. E. Corros. Sci. 1987, 27, 639. Muller, A. J.; McCrory-Joy, C. Corros. Sci. 1987,27,695.
(25) Blade, E.; Ferrand, E. F. J. Air Pollut. Control Assoc. 1969, 19, 973. (26) Department of Environmental Conservation. N e w York State Air Quality Report, Continuous and Manual Air Monitoring Systems; New York State DEC: Albany, NY,
1981; p 25.
(27) Franey, J. P.; Davis, M. E. Corros. Sci. 1987, 27, 659.
Received for review June 19,1990. Revised manuscript received April 8,1991. Accepted April 15,1991. This work was funded by the North Atlantic Historic Preservation Center of the N a tional Park Service through a grant f r o m the A n d y Warhol Foundation.
Long-Term Processes in a Stabilized Coal-Waste Block Exposed to Seawater Daryl E. Hockley” and Hans A. van der Sloot Netherlands Energy Research Foundation, (ECN), P.O. Box 1, 1755 ZG Petten, The Netherlands
A stabilized waste block, formed by cementation of coal combustion wastes with portland cement and lime, was retrieved after 8-years exposure to a marine environment, The block was analyzed to obtain concentration profiles for major and minor elements. Combination of the major element profiles with semiquantitative X-ray diffraction results allowed identification of diffusion-limited precipitation and dissolution processes. The precipitation and dissolution processes moved as a sharp boundary from the surface toward the center of the block, penetrating about 10-20 mm in the 8 years. Leaching of minor elements was limited to the region between the surface and the boundary. Concentration profiles for sea salts suggest that precipitation in the matrix pores of the boundary region restricted diffusion further into the block. This process, known as “pore refinement” in concrete durability literature, may also restrict degradation of the waste block matrix and the leaching of contaminants. Introduction
Background. According to recently compiled statistics, approximately 280 million tonnes of fly ash is produced each year by the combustion of coal. Less than 10% of this volume is utilized, primarily as R cement replacement (1). Disposal of the remainder is complicated by the fact that many fly ashes contain appreciable amounts of contaminants, especially metals and oxy anions. In the United States, coal combustion wastes have historically been disposed in landfills near the combustion facility. Disposal in the ocean has been a more common practice in the United Kingdom (2). A more recent alternative is to combine coal wastes with other cementing agents to produce hardened blocks of ”stabilized waste”. In principle, these stabilized forms are better able to retain contaminants than the bulk wastes and are therefore more acceptable for utilization or disposal. The Coal Waste Artificial Reef Program (CWARP) studied the environmental consequences of utilizing stabilized coal combustion wastes as construction material for artificial fishing reefs. On September 12, 1980, some 16 000 blocks of stabilized waste were released from a hopper barge to form a 500-tonne artificial reef in the New York Bight. The blocks consisted of coal fly ash and flue gas desulfurization residues stabilized with lime and portland cement additives. Results of the CWARP re1408
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search, which ended in 1984, were generally encouraging. The coal-waste blocks showed little deterioration and no decrease in compressive strength. Elements of environmental concern remained inside the blocks or were leached at very slow rates. Biological communities developed on the coal-waste reef and showed no evidence of contaminant uptake (3-8). Divers from the State University of New York returned to the coal-waste reef in the summer of 1988. The biological communities growing on the coal-waste blocks were surveyed and found to be identical with those observed on nearby blocks of uncontaminated concrete, suggesting that the contaminants in the waste blocks continued to have little or no adverse impact on their immediate surroundings. Subsequent strength testing of blocks retrieved during the 1988 dives showed no decrease in compressive strength, indicating that the blocks had not been adversely affected by further exposure to the marine environment. One of the retrieved blocks was forwarded to the Netherlands Energy Research Foundation (ECN) for detailed chemical and mineralogical profiling. The results of these analyses are reported herein and the chemical and mineralogical processes responsible for the apparently favorable behavior of the coal-waste reef are identified. E x p e r i m e n t a l Section
Retrieved Block. Details of the mixing, forming, and curing of the coal-waste blocks were reported previously in the CWARP publications (3, 6 ) . The reef was constructed on sandy sediments at a water depth of 40 m. The block forwarded to ECN was retrieved in July 1988, after 409 weeks in the sea. It was one of the type referred to as “Conesville” blocks in the earlier reports. It measured approximately 20 x 20 X 40 cm and weighed approximately 25 kg. The block was found lying on the seabed and was overgrown by an encrusting biological community on all exposed surfaces. The underside was partially buried and remained clean of biological growth. The pattern and extent of the biological growth suggests that the block had remained in the same orientation on the seabed for several years. Chemical Analyses. Samples measuring 4 X 4 X 10 cm were cut from selected locations in the block with a wet-cutting diamond blade stone saw. The samples were subsequently sliced in a dry-cutting device fitted with a high-volume dust sampler. Starting from the surface of
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0 1991 American Chemical Society
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Flgure 1. Concentration profiles for major elements in the coal-waste reef block. Filled shapes indicate proflies from the block retrieved in 1988 after 8-years exposure. Open circles show profiles measured in a block retrieved after 80-weeks exposure (7).
the block, thin slices were removed by the diamond blade saw and the dust was collected on a microsorban filter mounted in the dust sampler. A new filter was inserted for each slice. The dust was removed from the filters and analyzed for major, minor, and trace elements. Ca, Mg, and Mo were measured by inductively coupled plasma emission spectrometry after dissolution of the dust in a pressure bomb containing a HC1-HC104-HN03 mixture. Na was measured by atomic absorption spectrometry. Fe, Mn, Br, As, Sb, Se, W, La, and Sc were measured by neutron activation analysis. C1, SO4, and F were measured by ion chromatography after dissolution of the sample in NaOH melt. This procedure measures both SO3 and SO4 as sulfate. Carbonate was measured by acidification, volatilization as COz, trapping in a solution of barium perchlorate, tertbutyl alcohol, and glycerine, and titration by NaOH. All concentrations were reported as weight percentages and subsequently normalized by Sc to account for porosity changes. The reference NBS coal fly ash 1633a was analyzed along with the other samples as a check on the accuracy of the analytical results. To obtain pH profiles indicative of porewater conditions in the block, portions of the dust from the microsorban filters were rewetted with demineralized water at a liquid/solid ratio of 2 mL/g. Measurements were taken with a pH electrode after a 5-min contact time. Morphological Examination. The retrieved block was inspected visually and photographed. The profile samples were also inspected and a dark layer was noted about 10-20 mm from the surface. A scanning electron microscope
(SEM) was used to examine the dark layer and to search for crystalline precipitates. Energy dispersive X-ray (EDX) mapping was used to identify regions of increased Ca and Mg concentrations. Mineralogical Analysis. Samples were taken from depths of 1,4.0, 6.4, 9.7, 13.6, 17.4, 30.5, and 78 mm in a single profile and analyzed by powder X-ray diffraction The diffractograms were examined and mineralogically distinct regions were identified. Diffractograms from samples representative of each region were then used for semiquantitative analysis.
Results and Discussion Concentration Profiles. As mentioned above, all concentrations were normalized by Sc so that the resdting Sc profile is, of course, flat. Before the normalization, the Sc profiles showed a slight (
