Geoscience Reference
In-Depth Information
pozzolanic mineral additions are pulverized fuel ash (PFA),
ground granulated blastfurnace slag (GGBS), condensed
silica fume (CSF or microsilica), and metakaolin.
Table 21
shows the typical addition rates and advantages and
disadvantages of these four main types. Fly ashes
(including PFA) are byproducts of coal burning power
stations. They largely comprise spherical, 1-150 μm
diameter particles of isotropic aluminosilicate glass, with
some iron-bearing phases and traces of carbonaceous
unburnt fuel. In concrete thin sections, fly ash additions
are readily detected by the presence of unreacted glassy
spheres and cenospheres (hollow spheres) that may vary in
colour from yellow, brown, red, black, or grey, and traces
of opaque unburnt coal (
174
). In hand specimen, concrete
containing a PFA addition may have a darker coloured
cement matrix (medium grey instead of light grey).
Table 21
Generalized proporties of mineral additions for concrete in comparison to concrete made with
Portland cement alone (data mainly from Bamforth, 2004)
Property
Pulverised fuel
Ground
Silica fume (CSF)
Metakaolin
ash (PFA)
granulated
blastfurnace
slag (GGBS)
Typical addition
20-40
40-70
5-10
8-15
rate by % of
cement
Workability
Reduced
Water demand
Increases
Increases
water
may be slightly
water demand,
water demand,
demand
reduced
plasticising
plasticicing
admixture required
admixture required
Heat of hydration
Reduced
Reduced
Increased
Increased
Rate of strength
Rate
Rate reduced
Rate increased
Rate increased
gain
reduced
with higher
with much higher
with higher
final strength
final strength
final strength
Carbonation
Similar
Increases in some
Similar
Similar
situations
Resistance to
3-10 times
3-10 times
Improved
Marked
chloride ingress
better
better
Improvement
Sulfate resistance
Improved
Improved
Improved resistance
Improved
to sodium sulfate
but not magnesium
sulfate
ASR prevention
Improved
Improved
Improved except
Improved
where opaline
aggregate present
Frost resistance
May be
May be more
Insufficient data
Insufficient data
improved at
susceptible
higher addition
to surface
rates
scaling
