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Sulphur content of recycled aggregates applied in concrete production 501

The elements present in the specimens and the elemental composition can be deter-
mined. This technique also allows the microscopic observation of specimens, so it
is possible to discern morphological details.
Ettringite can be identified by SEM and EDS. The SEM usually provides two
different images: the secondary electron (SE) image and the backscattered electron
image (BSD). SE supply a lot of information about the surface topography of the
specimen. This image is very useful for identifying likely ettringite crystals which
have an acicular shape and which will probably be found in pours, cracks or gaps
around the aggregate particles.
It is also possible to recognise ettringite crystals in a backscattered image which
provides a grey level map depending on the atomic number of the elements present.
The higher the atomic number of the element, the more brightness it shows.
Although the grey level associated to ettringite is similar to other hydration pro-
ducts present in the hardened concrete, it is possible to identify the acicular form in
BSD image, but not with the same detail as in the SE image.
The last powerful technique, currently, which can reveal the presence of ettrin-
gite is energy-dispersive X-ray microanalysis. Ettringite shows strong peaks due to
calcium, sulphur and aluminium which should appear aligned as shown in
Figs. 17.1 and 17.2.
The presence of sulphur from the capping can be detected by chemical meth-
ods, but X-ray diffraction is the appropriate method for verifying in which crystal-
line structure it is present. In general, the sulphur from the capping of control
specimens presents the stable crystalline structure S 8 . It is interesting to combine
the chemical analysis with the XRD to determine with reliability the amount of
sulphur in the form of sulphates and the amount of sulphur which is not soluble in
the crystalline state.

Figure 17.1 Formation of ettringite inside a pore of a recycled aggregate.