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|Title:||Individual and combined effects of chloride, sulfate, and magnesium ions on hydrated portland-cement paste|
|Authors:||United States. Department of Energy.|
Sandia National Laboratories.
Poole, Toy S.
Wakeley, Lillian D.
Young, Cameron L.
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-94-12.|
Abstract: Ground water with a high concentration of magnesium ion is known to cause deterioration to portland cement concretes. A proposed mechanism for this deterioration process published previously involves an approximate 1:1 replacement of Ca ions by Mg ions in the crystalline phases of hydrated cement. The current study was undertaken to determine which ions, among magnesium, chloride, and sulfate, cause deterioration; whether their deleterious action is individual or interdependent; and to relate this mechanism of deterioration to the outlook for a 100-yr service life of concretes used in mass placements at the Waste Isolation Pilot Plant. Loss of Ca ion by cement pastes was found to be strongly related to the concentration of Mg ion in simulated ground-water solutions in which the paste samples were aged. This was true of both salt-containing and conventional cement pastes. No other ion in the solutions exerted a strong effect on Ca loss. Mg ion did not accumulate in the chemically altered cement paste at the same rate that Ca was lost. Ca-ion loss was more than six times the Mg-ion gain. No crystalline Mg-bearing phases were detected in the deteriorated pastes. Ca ion left first from calcium hydroxide in the pastes, depleting all calcium hydroxide by 60 days. Some calcium silicate hydrate remained even after 90 days in the solutions with the highest concentration of Mg ion, while the paste samples deteriorated noticeably. Softening of the samples occurred without complete destruction of calcium silicate hydrate, and with no apparent formation of magnesium silicate hydrate. The results indicated a mechanism that involves dissolution of Ca phases and transport of Ca ions to the surface of the sample, followed by formation of Mg-bearing phases at this reaction surface rather than directly by substitution within the microstructure of hydrated cement. Given that calcium hydroxide and calcium silicate hydrate are the principal strength-giving phases of hydrated cement, this mechanism indicates the likelihood of significant loss of integrity of a concrete exposed to Mg-bearing ground water at the WIPP. The rate of deterioration ultimately will depend on Mg-ion concentration, the microstructure materials of the concrete exposed to that groundwater, and the availability of brine.
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