Geoscience Reference
In-Depth Information
174
Blastfurnace slag (including GGBS) is a byproduct of
the iron-making process that is rapidly cooled (during
either granulation or pelletization) to form slag with a
glassy disordered structure. In hand specimen, concrete
containing a GGBS addition exhibits a distinctive dark
green cement matrix, which with time changes to a
cream colour as the sulfides in the GGBS oxidize. In thin
section, the GGBS is readily detected by the presence of
isotropic glassy shards of unreacted GGBS that are
typically 1-100 μm in size (
175
). The shards are usually
transparent and colourless, or light brown/green (or
sometimes white or dark brown) and sporadic, opaque
grains or inclusions of iron may be present.
It may be necessary to determine the GGBS or PFA
content of a blended cement product. If the individual
ingredients are available separately this can be achieved
by point-counting of highly polished specimens in
reflected light. A number of reference specimens should
be made up with a range of known GGBS/PFA contents
and these should be point-counted along with the blended
cement sample in order to estimate the relative accuracy
of the determination. It will be necessary to etch the
specimens (e.g. with 1% nitric acid solution) to distinguish
the mineral addition from the cement clinker.
Microscopically determining the quantity of GGBS or PFA
in hardened concrete is much more difficult as much of
the mineral addition will be too small to resolve optically
or will have been consumed by pozzolanic or hydration
reactions. The most reliable techniques involve using
scanning electron microscopy and EPM of highly polished
specimens, as described by French (1991b, 1992).
Silica fume (CSF or microsilica) is a byproduct of the
electric arc furnace manufacture of silicon or ferrosilicon
alloys from high purity quartz and coal. The process gives
off fumes consisting of extremely small spheres (<0.1-
1 μm diameter) of very pure amorphous silica that is a very
reactive pozzolana. The extreme fineness presents
handling difficulties so it is usually supplied in the form of
pellets or slurry. Detecting silica fume additions by
microscopy is difficult owing to its extreme fineness, low
addition rate (typically 5-10%), and the fact that almost all
particles are consumed by the pozzolanic reaction.
However, its presence may be disclosed by agglomerations
of silica fume if dispersion during concrete mixing has
been incomplete (
176
). In addition, the cement matrix may
appear optically dense and portlandite depleted.
Metakaolin is another highly reactive pozzolanic mineral
addition. It is manufactured from deposits of naturally
occurring kaolin clay by heating in a kiln at 700-800°C. As
with silica fume it is unlikely to be directly detected by
microscopy, for the same reasons.
Ternary (triple blend) cements comprise Portland
174
Close view of concrete containing PFA. The
cement matrix exhibits unreacted glassy spheres
(white, brown) and cenospheres (centre) of PFA and
traces of unburnt coal (black); PPT, ×300.
175
175
Close view of concrete containing GGBS. The
cement matrix exhibits angular, glassy 'shards' of
GGBS (transparent) and traces of iron (black); PPT,
×300.
176
176
Close view of concrete exhibiting agglomerations
of incompletely dispersed silica fume (light brown,
centre); XPT, ×200. (Courtesy of Mike Eden.)
