ABSTRACT

ABSTRACT

Deformational behavior of a rock mass can be modeled with a constitutive law based on theories of elasticity, and plastic and viscous deformation. For most rock types, such constitutive models may be used with confidence because there is no geochemical interaction between the rock-forming minerals and water. However, if volume changes in the rock due to geochemical phase transitions are predicted, then these volume changes must be converted into mechanical parameters and must be included into the constitutive laws. In this paper, geochemical phase transitions between anhydrite (CaSO4) and gypsum (CaSO4.2H20) will be brought to the attention of researchers in rock mechanics. Hydration of anhydrite to gypsum may yield volume increases up to 62.6 percent and dehydration of gypsum to anhydrite may cause volume decreases up to 38.5 percent. If one were to convert the volume increases in hydrating anhydrite into strains and calculates the required stresses to restrict the expansions, the magnitude of stresses will be found at GPa levels. Such stresses on swelling anhydrite layers cannot be provided by geologic media. Therefore, the host rocks will deform under these high stresses. On the other hand, under increasing stresses, geochemical transition of anhydrite to gypsum may stop after some hydration of anhydrite, and anhydrite and gypsum systems may become stable. Changes in temperature and solution compositions in the anhydrite/ gypsum system also control the stability of the geochemical system, and therefore, the extent of vol, me changes. A similar conceptual model can be drawn for volume decreases due to dehydration of gypsum to anhydrite. In this case, stress reduction on the gypsum layer may cause extensive fracturing and changes in the state of stress in the host rock mass.

INTRODUCTION

Evaporites have been reported from all continents, and approximately 25 percent of the continental areas are underlain by evaporitic rocks. Gypsum, anhydrite and halite are the most prominent minerals in evaporitic deposits. The original source of these evaporitic minerals is seawater. Precipitation of evaporitic minerals is usually generated either by direct evaporation of brine or by an indirect manner involving dissolution, transportation and reprecipitation of primary evaporitic deposits in waters circulating in the upper crust. Anhydrite and gypsum deposits generally occur in the vicinity of bedded and domal salt deposits. Phase transitions in calcium sulfate minerals are controlled by pressure, temperature and the composition of coexisting aqueous solutions. Phase diagrams showing the stability ranges of gypsum and anhydrite as a function of these parameters can be used to predict whether a hydration or a dehydration reaction could occur when environmental conditions in a rock system are changed. The geochemical stability of various anhydrite/gypsum systems and conceptual models for mechanical behavior of calcium sulfate bearing rock masses for their potential applications to tunnels, foundations and nuclear waste repositories in evaporitic rocks are discussed in the following paragraphs.

GEOCHEMICAL ASPECTS OF HYDRATION/DEHYDRATION OF ANHYDRITE AND GYPSUM

Solutions to the troublesome engineering problems associated with phase transformations of gypsum and anhydrite require basic information on the factors controlling the transition and on the geologic environment in which the transitions are likely to occur.

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