Civil Engineering Reference
In-Depth Information
is then analysed for soluble silica and calcium oxide, being the two major
components (expressed as oxides) of Portland cement. The cement content
is determined by simple proportion from the two parameters. Where soluble
components from the aggregate interfere by contributing to the calcium
content (e.g. if a limestone aggregate is present) then the silica value would
be used for the cement content determination. Conversely, if the silica value
was inflated by some soluble component other than the cement, the lime value
would be used, provided the analyst was confident that this was unaffected
by soluble components from the aggregate. In practice, it is normal to analyse
control samples of the aggregate, where these are available, to avoid these
problems. With control samples, an accuracy of better than ±25 kg/m
3
is
readily achievable.
Where cement replacement materials such as pfa (pulverised fuel ash)
and ggbs (ground granulated blastfurnace slag) are present, the situation is
more complex. Nevertheless, accurate results can often be obtained using
total analyses by, for example, X-ray fluorescence methods and applying a
simultaneous equations approach (Grantham, 1994) or by using petrographic
methods combined with scanning electron microscopy (SEM). The topic is
to be revisited in an update of Concrete Society Technical Report No. 32
(Concrete Society, 1989) by a Concrete Society working party.
Sulfate content
Exposure of concretes made with Portland cement to sulfate salts can cause
damage due to an expansive reaction between the tricalcium aluminate
phase of the cement and the sulfate salt to form crystals of ettringite.
Given adequate space to form, the ettringite forms needle-like crystals, but
in confined space causes an expansive reaction as the amorphous product
develops. With good-quality concrete, significant sulfate attack is relatively
rare, and research work suggests that concrete made with a reasonable
cement content (at least 330 kg/m
3
) and a reasonably low water/cement
ratio, is attacked only very slowly. However, the most damaging salts are
the more soluble sulfates based on magnesium or sodium sulfates. Calcium
sulfate (gypsum) is only sparingly soluble and is less likely to cause damage.
The rate of damage is also dependent on the rate of replenishment of the
sulfate salts and hence on groundwater movement.
T
EST
METHODS
Sulfate is usually determined by the method given in BS 1881 : Part 124 :
1988 (BSI, 1988). This involves an acid extraction and precipitation of the
sulfate as barium sulfate with barium chloride solution. The resulting barium
sulfate is filtered and weighed to determine sulfate gravimetrically.
Methods based upon ion selective electrodes and ion chromatography
have also been employed. Petrography is usually the best initial indicator of
a sulfate attack problem.
