Stability of Calcium Chloroaluminate and Calcium Sulfoaluminate Phases in Hydraulic-Cement Mixtures.

摘要

Throughout the 1980's expansive behavior was one of the principal design criteria for grouts and concretes to be used in underground nuclear-waste repository sealing. The purpose of this requirement was to make a material that would fit tightly into the host-rock cavity that it is inducing the formation of tetracalcium aluminate trisulfate-32-hydrate (Ettringite) from the reaction between the calcium aluminates or anhydrous calcium sulfoaluminates in the cementitious materials and the calcium sulfates added to the system. This is known to cause expansive behavior under certain conditions (Poole and Walkeley1992). Another design criterion has been that mixtures cannot cause dissolution of the host rock. this is not normally a problem with most host rocks, but can be a substantial problem when placing concrete in rock composed of water-soluble minerals. In the case of rocks composed of halite (NACL), this problem has been solved by saturating the mixing water with sodium chloride. As a result, teracalcium aluminate dichloride-10 hydrate (Chloroaluminate) forms from the calcium aluminates in the cementious materials. This phase is mildly expansive. So, in hydrated systems, at least two forms of hydrated calcium aluminates may coexists, or one may form preferentially, or one may replace the other, depending on the chemical environment.