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Title: Thermal stability of certain hydrated phases in systems made using Portland cement
Authors: United States. Department of Energy.
Buck, Alan D.
Burkes, J. P.
Poole, Toy S.
Keywords: Chloroaluminate
Portland concrete cement
Thermal stability
Freidel's salt
X-ray diffraction
Publisher: Structures Laboratory (U.S.)
Engineer Research and Development Center (U.S.)
Series/Report no.: Miscellaneous paper (U.S. Army Engineer Waterways Experiment Station) ; SL-85-7.
Description: Miscellaneous Paper
Abstract: As part of the study of hydraulic-cement systems for use in possible underground isolation of nuclear wastes, this study was made to determine the temperature stability of ettringite and chloroaluminate. Either or both of these phases may be expected in a hydraulic cement system depending on the presence of salt (NaCl). The study of ettringite was made using 15 mixtures that contained portland cement, plaster, 2 levels of water, and in some mixtures, 1 of 6 pozzolans (3 fly ashes, 1 slag, 1 silica fume, 1 natural pozzolan), plus a 16th mixture with anhydrous sodium sulfate replacing plaster (CaSO4 · 1/2H2O). Specimens were made and stored at 23°, 50°, and 75° Cor 23°, 75°, and 100° C (all four temperatures in one case) for periodic examination by X-ray diffraction for phase composition and ettringite stability, and testing for compressive strength and restrained expansion. A more limited study of the stability of chloroaluminate was made along the same lines using fewer mixtures, salt instead of plaster, and higher temperatures plus some pressure. It was found that while some ettringite was decomposed at 75° C, depending on the composition of the mixture, all ettringite was undetectable by X-ray diffraction at 100° C, usually within a few days. The evidence indicates that the ettringite became amorphous and no significant new phases formed in its place. Since there was no corresponding loss in strength or reduction in volume, this loss of ettringite crystallinity was considered to be non-damaging. Based on much more limited data, chloroaluminate was found to decompose between 130° C at 25 psi and 170° C at 100 psi; no significant phases replaced it.
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