51st U.S. Rock Mechanics/Geomechanics Symposium,
San Francisco, California, USA
2017. American Rock Mechanics Association
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ABSTRACT: When anhydrite dissolves upon contact with water, the sulphate and calcium ions in the pore water can lead to precipitation of gypsum. This anhydrite to gypsum transformation (AGT) results in an increase in the solid volume by 61% and in a decrease or increase in the pore volume. On a macroscopic scale the so far insufficiently understood phenomenon “swelling of anhydritic rock” is observed, which is known to cause massive damage to underground constructions, such as tunnels. This contribution focuses on one specific aspect of AGT, the precipitation of gypsum directly on the surface of the dissolving anhydrite, which creates a diffusion barrier for the dissolving ions and slows further anhydrite dissolution down (hereafter referred to as “self-sealing”). The development of such gypsum layers on anhydrite is investigated experimentally by storing natural rock specimens consisting of 96%-99% anhydrite in water and observing the thickness of the developing gypsum layer optically via photography during the tests and via microscopic analysis after drying of the samples. The measured thicknesses corresponded well to the amount of precipitated gypsum determined post test via thermogravimetric analysis and to predicted values which were calculated with an existing kinetic model. The plausibility of this model could thus be verified so far.
1. INTRODUCTION AND CONCEPTUAL MODEL
As anhydrite comes into contact with water it will begin to dissolve under most circumstances below 40°C (at atmospheric pressure). As the saturation concentration with respect to anhydrite (ceq,A) is higher than that of gypsum (ceq,G), anhydrite dissolution results in oversaturation with respect to gypsum, thus triggering growth of gypsum crystals, whereby the solid volume of gypsum is 61% higher than that of anhydrite due to the additional water molecules bound in the crystals. The chemical reaction from anhydrite (CaSO4) to gypsum (CaSO4 • 2H2O) will be referred to here as AGT (Anhydrite – Gypsum Transformation) and can be formulated in short as CaSO4 + 2H2O → CaSO4 • 2H2O.
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