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A normally sound dolostone aggregate was found to be unstable in concrete during sustained heating at 150°C. Petrographic and other related investigations of the rock have indicated that the primary cause of deterioration of particles is the oxidation of iron sulfide. In this aggregate, fine pyrite is intergranular with dolomite, calcite, and clastic material, particularly in shaly sections. Analyses of disintegrated particles revealed that the granular powders consisted of unaltered carbonate and clastic grains, relict pyrite, and sub-micron material containing hygroscopic, iron-rich sulfate. It was concluded that the pyrite had oxidized to iron sulfate hydrate under these particular conditions; the resulting volume change disintegrated the permeable, often weakly bonded particles, and stresses arising from aggregate expansion ruptured the concrete. Tests revealed also that the aggregate was relatively stable at 75 and 300°C and in dry oxygen, which suggests that water vapor in a particular pressure-temperature range is critical for the reaction. Water released from the aggregate by sustained heating is in part responsible.
Research scientist, Mineralogy Section, Mineral Processing Laboratories, Mineral Sciences Laboratories, CANMET, Ottawa,
Stock #: CCA10235J