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.