The Inside Story: Compositional Tomography

The Inside Story

II. Compositional Tomography

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Compositional Tomography of Mineral and Engineering Objects

10 to 15 years ago the prospect of examining large dense objects was considered to be feasible only with neutrons; synchrotron sources have changed all that. In the Diffraction II section we noted that whereas some common materials could only be penetrated by distances of around 50 µm in the case of laboratory X-rays (e.g. copper Kα 1.54 Å), such penetration increases to the millimetre level at shorter wavelengths (e.g. 0.56 Å). However third generation synchrotron multipole magnet devices produce intense X-rays with energies more than an order of magnitude greater than the laboratory copper source. This has enabled many experiments to be performed on the internal analysis of bulk objects such as rock minerals, building stone and concrete, ceramic/metal objects and so on. There is space to describe just one technique with one example: that is, looking at the interior of a large (77 mm × 78 mm × 42 mm) block of concrete using synchrotron energy-dispersive diffraction. The technique used in this case is called TEDDI (Tomographic Energy-Dispersive Diffraction Imaging, by Hall et al., Nucl. Instr. & Meth. in Phys. Res. B140, 253 (1998)) and the basic idea is illustrated below:

A key feature again is that the region of interest inside the concrete is defined by the collimated incident and diffracted beams. The concrete block is traversed in a raster fashion (in this particular case in two dimensions; i.e. along the x- and z-directions) so that the diffracting region covers a region of interest; in this case a 13 × 6 mm area covered in 13 × 13 traverse steps. The beauty of energy-dispersive diffraction is that the whole diffraction pattern is collected by a stationary energy-dispersive detector. Another twist in this case is that 3 detectors were used (see previous diagram) so 3 diffraction patterns, which complemented each other, were collected each time from the same region. Eventually diffraction patterns were collected from all points over the traversed volume and so one could then construct maps of various phases according to the intensity of their characteristic diffraction peaks. In the following figure we show 4 such maps for the traversed 13 × 6 mm area.

Calcite	Dolomite


Portlandite	Ettringite

From the calcite and dolomite maps one can perceive the outline of part of two aggregate regions - the left half and the top right corner - i.e. from the stones used in the concrete. If then one looks at the portlandite (i.e. calcium hydroxide) and ettringite (a portland cement hydrate) maps it is immediately obvious that these hydrates populate the regions in between the aggregates; this is just, of course, what we would expect as the function of the cement is to bind together the aggregate material. The key point however is that this interior region has been examined without breaking open the concrete block (as would have happened within microscopy analysis) and one can return again and again to the same region to see how this region develops as the cement hardens or as it is attacked by environmental exposure.

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Author(s): Paul Barnes
Simon Jacques
Martin Vickers