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|Characterization of several plasters and one retarder for repository sealing mixtures
|United States. Department of Energy.
Buck, Alan D.
Burkes, J. P.
Reinhold, Ronald E.
|Calcium sulfate hemihydrate
Plaster of Paris
Radioactive waste disposal
|Structures Laboratory (U.S.)
Engineer Research and Development Center (U.S.)
|Miscellaneous paper (U.S. Army Engineer Waterways Experiment Station) ; SL-85-10.
Abstract: Samples of plasters (i.e., calcium sulfate hemihydrate, CaSO4 · 1/2H2O) from six sources and one plaster retarder from a different source were obtained and characterized by a combination of chemical and phys ical tes ting and by petrographic examination. The petrographic examination included X-ray diffraction (XRD), optical microscopy, differential thermal analysis (DTA), and scanning electron microscopy (SEM) procedures. The intent of this work was twofold. One purpose was to determine if plaster per se could be used as an ingredient for cementitious mixtures intended for repository sealing applications. Previous experience had been with use of a proprietary admixture that contained plaster. The second purpose was to determine one or more methods of distinguishing between the alpha and beta forms of calcium sulfate hemihydrate. It was concluded that commercially available plaster of paris rather than a proprietary admixture could be used as an ingredient for the proposed cementitious mixtures. The other finding was that, as expected, the beta form of CaSO4 · 1/2H2O is finer and thus requires more water for mixing so the coarser alpha form is preferable for the described use. While the two forms can be distinguished by physical tests, by DTA, by SEM, and by optical microscopy, the latter procedure is the simplest and the quickest. All that is required is examination of a plaster as an immersion mount with a polarizing microscope to determine particle size and shape. Four of the six plasters were the alpha form while two were the beta form. The retarder contained both forms of plaster along with organic material (thought to be keratin).
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