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|Title:||Durability of concrete materials in high-magnesium brine|
|Authors:||United States. Department of Energy.|
Sandia National Laboratories.
Wakeley, Lillian D.
Poole, Toy S.
Burkes, J. P.
Portland cement concrete
Waste Isolation Pilot Plant
Salado mass concrete
|Publisher:||Structures Laboratory (U.S.)|
Engineer Research and Development Center (U.S.)
|Series/Report no.:||Technical report (U.S. Army Engineer Waterways Experiment Station) ; SL-94-11.|
Abstract: Cement pastes and mortars representing 11 combinations of candidate concrete materials were cast in the laboratory and monitored for susceptibility to chemical deterioration in high-magnesium brine. Mixtures were selected to include materials included in the current leading candidate concrete for seals at the Waste Isolation Pilot Plant (WIPP). Some materials were included in the experimental matrix to answer questions that had arisen during study of the concrete used for construction of the liner of the WIPP waste-handling shaft. Mixture combinations compared Class C and Class F fly ashes, presence or absence of an expansive component, and presence or absence of salt as a mixture component. Experimental conditions exposed the pastes and mortars to extreme conditions, those being very high levels of Mg ion and an effectively unlimited supply of brine. All pastes and mortars showed deterioration with brine exposure. In general, mortars deteriorated more extensively than the corresponding pastes. Two-inch cube specimens of mortar were not uniformly deteriorated, but showed obvious zoning even after a year in the brine, with a relatively unreacted zone remaining at the center of each cube. Loss of calcium from the calcium hydroxide of paste/aggregate interfaces caused measurable strength loss in the reacted zone comprising the outer portion of every mortar spectmen. Comparisons of the microstructure of deteriorated specimens and control specimens revealed the destruction of the bond-forming calcium hydroxide in reacted zones, and formation of gypsum at these interfaces. Changes in phase composition as determined by X-ray diffraction, and in total chemical composition as determined by energy-dispersive X-ray analysis, showed that calcium loss began immediately with initiation of exposure. Calcium hydroxide was destroyed rapidly and calcium silicate hydrate more slowly, with the deterioration product magnesium silicate hydrate appearing only after more than 6 months of brine exposure. Strength loss is related to loss of calcium rather than formation of weak phases. Of the variables analyzed statistically, presence or absence of salt as a mixture component had the strongest influence on rate of deterioration. Salt mitigated magnesium-related deterioration of mixtures that had an expansive component. Rates of deterioration were estimated from strength loss. The deterioration rate estimated for mixtures formulated with an expansive component but without salt was about 0.04 mm/day. For other mixtures, the rate was estimated to be between 0.015 and 0.03 mm/day. The current candidate mass concrete for WIPP seals includes salt as an initial component, and has a relatively closed initial microstructure. Both of these features contribute to its suitability for use in large placements within the Salado Formation.
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