Degradation of Portland Cements Exposed to Evaporite Brine at Hydrothermal Temperatures.
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DEGRADATION OF PORTLAND CEMENTS EXPOSED TO EVAPORITE BRINE AT HYDROTHERMAL TEMPERATURES. James L. Krumhansl and Steven J. Lambert, Laboratories, Albuquerque, New Mexico.
Sandia National
ABSTRACT The long-term integrity of cementitious materials is of interest where cement is used as part of the isolation system in underground workings sited in evaporite rocks. However, the slow rates of cement/fluid interactions at room temperature provide minimal guidance for predicting long-term cement behavior. To accelerate reactions that degrade cement, hydrothermal brine/cement interaction experiments were carried out at temperatures between 100° and 250° C. The pastes used in these experiments were two batches containing 65 (weight) % Class H cement and 35 % fly ash (one with Class C and one Class F, both with 10 mass equivalent % plaster), and one batch of Type K expansive cement, all with NaCl-saturated mix water. The brine used in testing was based on a composite representation of groundwaters from the Rustler formation of southeastern New Mexico, a mixed evaporite/clastic rock sequence. This brine was sufficiently concentrated that its use afforded an opportunity to study several mechanisms likely to degrade cement performance. At the conclusion of each experiment solid products were characterized and the coexisting fluids analyzed. In general, the cement was extensively altered by the brine at all temperatures assessed. In addition to the expected sulfate attack, it was found that magnesium effectively displaced calcium from the CSH gels that impart strength to cementitious materials. Sodium and potassium did not play a role in altering the cement.
INTRODUCTION The Delaware Basin of southeast New Mexico is a mineralrich region that produces commercial amounts of potash and hydrocarbons. In addition, the nation's first low level nuclear waste repository is slated to open in the salt beds of the Salado formation near Carlsbad. In each of these activities, cementitious materials are employed in various capacities to serve as sealants and structural members. It is often important to predict the condition of cements used in such applications for many decades into the future. This problem is made more acute by the chemically aggressive groundwaters of the region. In addition to being saturated with sodium chloride, these fluids typically contain many parts per thousand of dissolved sulfate, potassium, and magnesium [1,2]. Because of the large potential impact of cement failure, there are good reasons for developing a predictive capability for cements in this environment. This paper explores the use of elevated temperatures as a guide to identifying deleterious cement-brine interactions. This work was sponsored by the U.S. Contract No. DE-AC04-76-DP00789.
Department of Energy under
Mat. Res. Soc. Symp. Proc. Vol. 245. @1992 Materials Research Society
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EXPERIMENTAL METHOD Three grouts formulated for use in evaporite rocks were obtained from the US Army Corps of Engineers Waterways Experimental Station (WES). Table I
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