As part οf a study of cementitious mixtures for underground sealing and isolation of nuclear wastes, there was concern about the stability of ettrlngite and Letraealcium alum i nat e dichioride-10-hydrate (chlaroaluminate) and the effects of the stability of these phases on such factors as strength and expansion of candidate mixtures. Therefore, a series of paste mixtures using a Portland cement, plaster of paris, and two levels of water content with or without the presence of fly ash, ground granulated iron-blast furnace slag, silica fume, or natural pozzolan were made; four mixtures contained sodiun chloride instead of plaster of paris were also made. Specimens from these mixtures were stored at different temperatures and monitored at different ages up to one year for phase composition by X-ray diffraction (XRD), for compressive strength, and for restrained expansion. The most significant finding was that since loss of ettringite crystallinity at about 100oC, as determined by XRD, did not affect either compressive strength or expansion significantly, the thermal stability of ettringite in this temperature range need not, be a concern for repository sealing use of cementitious mixtures. Other findings were: a. Some ettringite was destroyed at about 75°C while that in other mixtures persisted to about 100°C. The higher stability seemed to relate to impurity, but this could not be definitely proved by XRD. b. If a long-term source of alumina is available and other necessary ingredients are present, new ettringite may continue to form for at least a year. c. Once ettringite had been destroyed by heat, it did not tend to reform under moist storage conditions at room temperature. d. When ettringite was destroyed by heat, tetracaloium aluminate monosulfate-12-hydrate (monosulfoaluminate) did not replace it. Instead there was sometimes formation of hydrogarnet after heating to 100°C. e. Conversion of ettringite to chloroaluminate in the mixtures containing salt ranged from none to complete at 23°C with some coexistence for a year. In no case did ettringite replace chloroaluminate.
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