X-RAY STUDIES ON NANO- AND MICROSCALE ORIENTATION IN COMPACTED BENTONITE AND CALCIUM MONTMORILLONITE

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1 X-RAY STUDIES ON NANO- AND MICROSCALE ORIENTATION IN COMPACTED BENTONITE AND CALCIUM MONTMORILLONITE Jussi-Petteri Suuronen Department of Physics, University of Helsinki

2 Bentonite clay Montmorillonite + accessory minerals (e.g. quartz, feldspar) Lamellar clay composed of aluminosilicate disks and exchangeable interlayer cations Water is absorbed in hydration shells around cations Various applications due to good water retention and swelling properties

3 Microstructural anisotropy of bentonite Tactoids have a high aspect ratio (width in the µm range, thickness in nm range) Preferred orientation of tactoids can result from uniaxial compaction and purification of montmorillonite 3 5 µm 500 nm 90 nm TEM Image: M. Matusewicz et al., Clay Minerals 48: , (2013)

4 What can we learn with x-rays? Incident x-ray beam Sample Microtomography (XMT) Spatial distribution of accessory minerals and porosities Swelling properties 1 µm scale effects Scattered x-rays Small-angle scattering (SAXS) Scattering angle => lamellar period (12-20 Å) Width of reflection => tactoid size Azimuthal intensity variation => preferred orientation 1-10 nm scale effects 400 µm 400 µm

5 Samples and setup Uniaxially compacted clay equilibrated with deionized H 2 O for 4 months Dry density g/cm 3 Both purified Ca-montmorillonite and MX-80 bentonite studied XMT complemented by localized SAXS measurements Use XMT image to aim SAXS beam at a specific sub-volume of sample Humidity control with saturated salt solutions H 2 O Clay Sinter plates 5 Mpix CMOS detector of XMT scanner Pilatus 1M detector for scattering experiments H 2 O CNC sample manipulator stage Microfocus x-ray tube (Mo-anode) plus focusing Montel optics for scattering Nanofocus x-ray tube for XMT system (max. 180 kv/30 W, W target)

6 Wet sample: 100% Relative Humidity (RH) No cracks visible in XMT SAXS measured from two orientations SAXS results show preferred orientation of clay stacks 400 µm Width of arrow = width of SAXS beam ( µm)

7 Sample at ~85% RH for 14 hours, SAXS measurements near a microcrack Rotation of sample µm

8 Sample at room RH (~30%) for > 48 hours Approximately aligned microcracks visible in µct Orientation of tactoids appears to correlate with that of microcracks

9 Quantification of results (1/2) 2. Sum of 2D-autocorrelations of (vertical) slices 1. Segmentation of pore volume within the beam path of SAXS measurement 3. Comparison with corresponding SAXS pattern

10 Quantification of results (2/2) Decay rate of autocorrelation function vs. azimuthal plot of 001-reflection in SAXS pattern Well correlated in calcium montmorillonite, MX-80 not oriented Also increasing salinity reduced orientation Ca-montmorillonite MX-80 bentonite Azimuthal angle (º) Azimuthal angle (º) BOA Seminar / Otaniemi

11 SAXS results combined Sum of SAXS intensities of all measurements (no background correction => some peaks due to PCR hood) Loss of interlamellar water results in smaller d-spacing Splitting of 001-peak can also be seen under some conditions => heterogeneous stacking structure Intensity (a.u.) Ca-montmorillonite RH 100% RH 85% RH 30% Intensity (a.u.) MX-80 bentonite RH 85% RH 32% RH 100% q-vector magnitude (1/Å) q-vector magnitude (1/Å)

12 Re-wetting of Ca-montmorillonite sampled parallel or perpendicular to axis of compaction Initial situation Video: 1 min to 3 hrs after start of the experiment Left: perpendicular sampling Right: parallel sampling

13 Conclusions Multiscale analysis with XMT and SAXS reveals that: Microcrack and tactoid orientation are correlated in purified, uniaxially compacted Ca-montmorillonite The presence of accessory minerals and lack of purification reduces orientation in MX-80 How does this influence the buffer properties of bentonite? Anisotropy should be considered when using purified montmorillonite as an analogue for bentonite Significant anisotropy observed in drying but not swelling Anisotropic diffusion in the buffer/backfill could be a design consideration in some waste disposal scenarios

14 Acknowledgements Samples prepared by Michal Matusewicz, VTT Discussions and analysis of results: Kari Pirkkalainen (now Oxford Instruments) Ritva Serimaa (UH) Markus Olin (VTT) Special thanks for help with the experiments: Ville Liljeström (presently Aalto University) Aki Kallonen (UH) Financial support: BOA project, NGSMP and Väisälä fund