Cementitious materials - Concrete Mix Design, Quality Control and Specification - page 12

where chloride resistance is as important, or even more important,
than sulfate resistance, such as in marine structures. What should be
used in these circumstances is blast-furnace cement, fly-ash substitu-
tion, or silica fume incorporation. Where none of these are available,
an OPC concrete should be used with a low w/c ratio and possibly a
corrosion inhibitor.
2. Low-heat cement is generally almost as sulfate resisting as sulfate-
resisting cement (since C 3 A is also limited to reduce heat generation),
however, sulfate-resisting cement is not necessarily low-heat generat-
ing. This is because most of the heat generation comes from the C 3 S
component (of which there is always much more than the C 3 A) and
the proportion of this is not necessarily limited in sulfate resisting
cement. Low-heat Portland cement is not readily available and has
been effectively replaced by the use of fly ash or ground-granulated
blast-furnace slag (GGBS) in massive elements.
It is now coming to be recognised that suitability for different purposes
is often better attained by the use of variable proportions of fly ash, GGBS,
natural pozzolans, or silica fume than by the use of different types of
Portland cement. These alternative materials, being essentially by-prod-
ucts of other manufacture, used to be thought of as inferior substitutes for
cement and used only to reduce cost. The reaction of concrete specifiers
to these supplementary cementitious materials (SCMs) was that they were
often prohibited or strictly limited in proportion.
Fortunately, the situation has changed and the benefits of SCMs are now
widely accepted. For example, in the Middle East where the environment is
particularly severe, many specifications require the use of SCMs to reduce heat
of hydration and chloride diffusion. SCMs are not locally produced and so are
imported and usually considerably more expensive than Portland cement.
2.2 FLY ASH (OR PULVERISED FUEL ASH [PFA])
2.2.1 General characteristics
Fly ash, otherwise known as pulverised fuel ash (PFA), is a pozzolanic
material. This means essentially that it is capable of combining with lime
(in a suitably reactive form) in the presence of water to form cementitious
compounds. As lime is liberated in substantial quantities when normal
cement reacts with water and is present as reactive calcium hydroxide, there
is a distinct attraction in adding fly ash to concrete.
Fly ash looks like cement to the naked eye but will not set at all when
mixed with water (unless a class C ash, which is a type of ash that contains
substantial calcareous material). Fly ash is sometimes even finer than
cement; generally spherical particle shape, including some larger hollow