Andreas Jenni

Granular flow in polymineralic rocks bearing sheet silicates: new evidence from natural examples

Abstract Natural deformation in carbonate mylonites bearing sheet silicates occurs via a complex interaction of granular flow and solution transfer processes and involves continuous cycles of dissolution, grain boundary diffusion, nucleation and growth. In this way, new sheet silicates (a) nucleate within voids formed by grain boundary sliding of calcite grains, (b) grow, and (c) rotate towards the shear plane. As a consequence, small mica grains show a wide range of orientations with respect to the shear plane, but moderate to large grains are subparallel both to each other and to the shear plane. Increases of average grain sizes with increasing temperature of sheet silicates in mica-rich layers is more pronounced than in mica-poor layers. In the calcitic matrix, however, sheet silicates can only grow via solution–precipitation and mass transfer processes. Therefore, the observed grain size variability indicates drastic differences in mass transfer behavior between the individual layers, which might be related to differences in the fluid flux. Based on these observations, a conceptual model for the microfabric evolution in sheet silicate bearing mylonites is presented.

Quantitative microstructure analysis of polymer-modified mortars

Digital light, fluorescence and electron microscopy in combination with wavelength‐dispersive spectroscopy were used to visualize individual polymers, air voids, cement phases and filler minerals in a polymer‐modified cementitious tile adhesive. In order to investigate the evolution and processes involved in formation of the mortar microstructure, quantifications of the phase distribution in the mortar were performed including phase‐specific imaging and digital image analysis. The required sample preparation techniques and imaging related topics are discussed. As a form of case study, the different techniques were applied to obtain a quantitative characterization of a specific mortar mixture. The results indicate that the mortar fractionates during different stages ranging from the early fresh mortar until the final hardened mortar stage. This induces process‐dependent enrichments of the phases at specific locations in the mortar. The approach presented provides important information for a comprehensive understanding of the functionality of polymer‐modified mortars.

Benchmark reactive transport simulations of a column experiment in compacted bentonite with multispecies diffusion and explicit treatment of electrostatic effects

Bentonite clay is considered as a potential buffer and backfill material in subsurface repositories for high-level nuclear waste. As a result of its low permeability, transport of water and solutes in compacted bentonite is driven primarily by diffusion. Developing models for species transport in bentonite is complicated, because of the interaction of charged species and the negative surface charge of clay mineral surfaces. The effective diffusion coefficient of an ion in bentonite depends on the ion’s polarity and valence, on the ionic strength of the solution, and on the bulk dry density of the bentonite. These dependencies need to be understood and incorporated into models if one wants to predict the effectiveness of bentonite as a barrier to radionuclides in a nuclear repository. In this work, we present a benchmark problem for reactive transport simulators based on a flow-through experiment carried out on a saturated bentonite core. The measured effluent composition shows the complex interplay of species transport in a charged medium in combination with sorption and mineral precipitation/dissolution reactions. The codes compared in this study are PHREEQC, CrunchFlow, FLOTRAN, and MIN3P. The benchmark problem is divided into four component problems of increasing complexity, leading up to the main problem which addresses the effects of advective and diffusive transport of ions through bentonite with explicit treatment of electrostatic effects. All codes show excellent agreement between results provided that the activity model, Debye-Hückel parameters, and thermodynamic data used in the simulations are consistent. A comparison of results using species-specific diffusion and uniform species diffusion reveals that simulated species concentrations in the effluent differ by less than 8 %, and that these differences vanish as the system approaches steady state.

INTERACTION OF CORRODING IRON WITH BENTONITE IN THE ABM1 EXPERIMENT AT ÄSPÖ, SWEDEN: A MICROSCOPIC APPROACH

Bentonite and iron metals are common materials proposed for use in deep-seated geological repositories for radioactive waste. The inevitable corrosion of iron leads to interaction processes with the clay which may affect the sealing properties of the bentonite backfill. The objective of the present study was to improve our understanding of this process by studying the interface between iron and compacted bentonite in a geological repository-type setting. Samples of MX-80 bentonite samples which had been exposed to an iron source and elevated temperatures (up to 115°C) for 2.5 y in an in situ experiment (termed ABM1) at the Äspö Hard Rock Laboratory, Sweden, were investigated by microscopic means, including scanning electron microscopy, μ-Raman spectroscopy, spatially resolved X-ray diffraction, and X-ray fluorescence.The corrosion process led to the formation of a ~100 mm thick corrosion layer containing siderite, magnetite, some goethite, and lepidocrocite mixed with the montmorillonitic clay. Most of the corroded Fe occurred within a 10 mm-thick clay layer adjacent to the corrosion layer. An average corrosion depth of the steel of 22–35 μm and an average Fe2+ diffusivity of 1–2 × 10−13 m2/s were estimated based on the properties of the Fe-enriched clay layer. In that layer, the corrosion-derived Fe occurred predominantly in the clay matrix. The nature of this Fe could not be identified. No indications of clay transformation or newly formed clay phases were found. A slight enrichment of Mg close to the Fe—clay contact was observed. The formation of anhydrite and gypsum, and the dissolution of some SiO2 resulting from the temperature gradient in the in situ test, were also identified.

Ceramic formulation and processing design for plutonium disposition

The focus of this research programme is to develop a single phase ceramic wasteform for waste PuO2 that is unsuitable for fuel manufacture. A suite of synthetic mineral systems have been considered including titanate, zirconate, phosphate and silicate based matrices. Although a wealth of information on plutonium disposition in some of the systems exists in the literature, the data is not always directly comparable which hinders comparison between different ceramic hosts. The crux of this research has been to compile a database of information on the proposed hosts to allow impartial comparison of the relative merits of each system.

Alteration of MX-80 bentonite backfill material by high-pH cementitious fluids under lithostatic conditions – an experimental approach using core infiltration techniques

Abstract We characterize and quantify processes at a cement–bentonite interface spatially and temporally during a long-term core infiltration experiment. A young ordinary Portland cement pore fluid (K+–Na+–OH−: pH 13.4) was infiltrated into a MX-80 bentonite core with an initial saturated density of 1920 kg m−3 that shifted to 1890–1930 kg m−3 after 761 days. A hydrostatic external pressure of 4.1 MPa and an infiltration pressure of 2.1 MPa were applied in a triaxial-type apparatus. A decrease in hydraulic conductivity from approximately 2.2×10−13 to approximately 4.2×10−15 m s−1 was observed passing from advection-dominated flow to a diffusion-dominated regime. Sulphate replaced chloride in the outflow during the high-pH infiltration period controlled by the dissolution of gypsum, the uptake of K+ by ion exchange, and complex mineral reactions occurred near the inlet. X-ray computed tomography (CT) scans performed repeatedly during the experiment tracked a progressing hemispherical reaction plume in the first millimetres of the bentonite, revealing a zone of bulk density increase. This zone consisted of two distinct, but overlapping, zones of Mg- and Ca-enrichment related to precipitation of saponite and calcite. The experiment attested an effective chemical buffering capacity for bentonite, a progressing coupled hydraulic–chemical sealing process and also the preservation of the physical integrity of the interface region in this set-up with a total pressure boundary condition on the core sample.

Experimental Characterization and Quantification of Cement-Bentonite Interaction using Core Infiltration Techniques Coupled with X-ray Tomography

Deep geological storage of radioactive waste foresees cementitious materials as reinforcement of tunnels and as backfill. Bentonite is proposed to enclose spent fuel canisters and as drift seals. Sand/bentonite (s/b) is foreseen as backfill material of access galleries or as drift seals. The emplacement of cementitious material next to clay material generates an enormous chemical gradient in pore-water composition that drives diffusive solute transport. Laboratory studies and reactive transport modeling predicted significant mineral alteration at and near interfaces, mainly resulting in a decrease of porosity in bentonite. The goal of this thesis was to characterize and quantify the cement/bentonite interactions both spatially and temporally in laboratory experiments. A newly developed mobile X-ray transparent core infiltration device was used to perform X-ray computed tomography (CT) scans without interruption of running experiments. CT scans allowed tracking the evolution of the reaction plume and changes in core volume/diameter/density during the experiments. In total 4 core infiltration experiments were carried out for this study with the compacted and saturated cores consisting of MX-80 bentonite and sand/MX-80 bentonite mixture (s/b; 65/35%). Two different high-pH cementitious pore-fluids were infiltrated: a young (early) ordinary Portland cement pore-fluid (APWOPC; K+–Na+–OH-; pH 13.4; ionic strength 0.28 mol/kg) and a young ‘low-pH’ ESDRED shotcrete pore-fluid (APWESDRED; Ca2+–Na+–K+–formate; pH 11.4; ionic strength 0.11 mol/kg). The experiments lasted between 1 and 2 years. In both bentonite experiments, the hydraulic conductivity was strongly reduced after switching to high-pH fluids, changing eventually from an advective to a diffusion-dominated transport regime. The reduction was mainly induced by mineral precipitation and possibly partly also by high ionic strength pore-fluids. Both bentonite cores showed a volume reduction and a resulting transient flow in which pore-water was squeezed out during high-pH infiltration. The outflow chemistry was characterized by a high ionic strength, while chloride in the initial pore water got replaced as main anionic charge carrier by sulfate, originating from gypsum dissolution. The chemistry of the high-pH fluids got strongly buffered by the bentonite, consuming hydroxide and in case of APWESDRED also formate. Hydroxide got consumed by mineral reactions (saponite and possibly talc and brucite precipitation), while formate being affected by bacterial degradation. Post-mortem analysis showed reaction zones near the inlet of the bentonite core, characterized by calcium and magnesium enrichment, consisting predominately of calcite and saponite, respectively. Silica got enriched in the outflow, indicating dissolution of silicate-minerals, identified as preferentially cristobalite. In s/b, infiltration of APWOPC reduced the hydraulic conductivity strongly, while APWESDRED infiltration had no effect. The reduction was mainly induced by mineral precipitation and probably partly also by high ionic strength pore-fluids. Not clear is why the observed mineral precipitates in the APWESDRED experiment had no effect on the fluid flow. Both s/b cores showed a volume expansion along with decreasing ionic strengths of the outflow, due to mineral reactions or in case of APWESDRED infiltration also mediated by microbiological activity, consuming hydroxide and formate, respectively. The chemistry of the high-pH fluids got strongly buffered by the s/b. In the case of APWESDRED infiltration, formate reached the outflow only for a short time, followed by enrichment in acetate, indicating most likely biological activity. This was in agreement to post-mortem analysis of the core, observing black spots on the inflow surface, while the sample had a rotten-egg smell indicative of some sulfate reduction. Post-mortem analysis showed further in both cores a Ca-enrichment in the first 10 mm of the core due to calcite precipitation. Mg-enrichment was only observed in the APWOPC experiment, originating from newly formed saponite. Silica got enriched in the outflow of both experiments, indicating dissolution of silicate-minerals, identified in the OPC experiment as cristobalite. The experiments attested an effective buffering capacity for bentonite and s/b, a progressing coupled hydraulic-chemical sealing process and also the preservation of the physical integrity of the interface region in this setup with a total pressure boundary condition on the core sample. No complete pore-clogging was observed but the hydraulic conductivity got rather strongly reduced in 3 experiments, explained by clogging of the intergranular porosity (macroporosity). Such a drop in hydraulic conductivity may impact the saturation time of the buffer in a nuclear waste repository, although the processes and geometry will be more complex in repository situation.

Tracking high-pH reaction fronts in MX-80 bentonite using infiltration techniques and 4D CT

Geological storage of radioactive waste foresees bentonite as backfill material enclosing spent fuel drums. Concrete is proposed as building material for tunnel reinforcement or as backfill. The emplacement of high-pH cementitious material next to clay generates a chemical gradient in pore water chemistry that drives diffusive transport. Laboratory studies and reactive transport modeling predict significant mineral alteration near interfaces [1]. We aim to characterize and quantify the cement/bentonite skin effects spatially and temporally, focusing on the advectivediffusive transport domain resolved at intermediate spatial scales. Equipment made of carbon fiber and plastics holds a cylindrical sample under confining pressure and imposes a constant hydraulic gradient to drive a small advective flux. Compacted and saturated MX-80 bentonite and sand/bentonite mixtures are used. Infiltration of high-pH pore-fluids into the bentonite plug alters the mineral assemblage over time. The related change in phase densities, porosity and local bulk density is tracked by CT scans during ongoing infiltration. The resulting micrographs describe the electron density distribution in 3D and as a function of time. Electron densities are transformed into material densities using reference materials and are calculated on the basis of the data of the NIST [2]. After 1-2 years the experiment is stopped and the samples subjected to post-mortem analysis. In a first experiment running for >6 months, MX-80 bentonite (ρwet=1.875 g/cm3) is used as starting material, and a synthetic cement pore fluid of pH 13.7. The reaction front is tracked in the CT and appears as precipitations in the thin filter separating bentonite and infiltration fluid, and as an evolving zone of higher density in the bentonite next to it. Simultaneously, hydraulic conductivity is decreasing by 58% over 6 months.

Stability of Cs-Ionsiv in Portland cement blends for radioactive waste disposal

The suitability of Portland cement blends for encapsulation of Cs-Ionsiv in a monolithic wasteform was investigated. No evidence of reaction or dissolution of the Cs-Ionsiv in the cementitious environment was found by scanning electron microscopy and X-ray diffraction. However, a small fraction (≤1.6 wt%) of the Cs inventory was released from the encapsulated Ionsiv during leaching experiments carried out on hydrated samples. Cs release was enhanced by exchange of K and Na present in the cementitious pore water. Cement systems lower in K and Na, such as slag based blends, showed lower Cs release than the fly ash based analogues.

Micro-scale chemical speciation of highly heterogeneous cementitious materials using synchrotron-based X-ray absorption spectroscopy

Note: Times Cited: 1 Reference EPFL-ARTICLE-166466 URL: ://WOS:000236669700005 Record created on 2011-06-06, modified on 2017-05-10