In-Situ Interaction between Cement and Clay: Implications for Geological Disposal
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In-Situ Interaction between Cement and Clay: Implications for Geological Disposal A. Sneyers1, M. Paul2, M. Tyrer3, F.P. Glasser2, J. Fays1 and P. Van Iseghem1 1 Belgian Nuclear Research Center SCK•CEN, Boeretang 200, B-2400 Mol, Belgium 2 University of Aberdeen, Department of Chemistry, Meston Walk, Aberdeen AB9 2UE, United Kingdom 3 Imperial College, London, United Kingdom ABSTRACT The extent and the consequences of interactions between cementitious materials used in radioactive waste management and clay host rock are described. In-situ tests were performed on seven cement formulations representing materials applied in repository construction, for backfilling or for solidification of radioactive waste. Samples were exposed to realistic repository conditions of the Boom Clay Formation in the HADES underground laboratory. Chemical, physical and mineralogical changes across the cement-clay interface were identified by combined observations from Electron Probe Microanalysis, Infrared microscopy and X-Ray powder diffraction. Significant interactions in both the cement and the clay part were found in a zone extending up to several hundreds of microns. The most prominent features are (1) leaching of cement with loss of calcium and/or silicon; (2) development of a calcium-rich zone in Boom Clay close to or at contact; (3) the formation of a contact zone marked by the precipitation of a (hydrated) magnesium aluminate phase; (4) reduction in apparent porosity of initially porous/permeable materials and (5) precipitation of calcite within the cement. This elemental exchange tends to diminish pH and reduce the buffering capacity of the cement. Although hydroxide will diffuse into the clay, the development of an extensive alkaline halo in the surrounding clay is unlikely owing to the buffering capacity of the Boom Clay pore water. INTRODUCTION Cementitious materials are widely applied for the solidification of low and intermediate level radioactive waste. In addition, these materials are used for construction purposes or as backfill materials in deep geological repositories. In general, cementitious materials provide an effective physical and chemical barrier restricting the release of radionuclides into the environment. In a deep geological repository, cementitious materials may come in contact with repository groundwater leading to the alteration and the degradation of the cement matrix. Besides cement matrix degradation processes, cementitious materials possibly will interact with the surrounding host rock, which results in physical, chemical and mineralogical changes of the near field. Until now, few experimental data under real repository conditions exist on the nature and the extent of cement-host rock interactions and their consequences for the long-term safety of disposal. This study reports results from in-situ experiments on the interaction between various cementitious materials and the Boom Clay Formation, which is the Belgian candidate host rock for the deep geological disposal of medium and high level waste. The m
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