Concrete - Geomaterials: Under the Microscope - page 79

shows a normal Portland cement grain with the calcium
silicates mainly consisting of C 3 S (alite). Figure 169 shows
a normal Portland cement grain with the calcium silicates
mainly consisting of C 2 S (belite). Figure 170 shows an
SRPC grain with the interstitial phases predominantly
consisting of C 4 AF (ferrite). A comprehensive illustrated
reference for the examination of cement clinker is
provided by Campbell (1999).
It is important to note that owing to its low C 3 A
content, SRPC is more susceptible to chloride ion ingress
as C 3 A binds with chloride ions to form calcium
chloroaluminate, making them unavailable to initiate
reinforcement corrosion. This increased chloride ingress
of SRPC is a major disadvantage for reinforced concrete
that is exposed to chlorides, such as in coastal or
highways structures.
White or coloured concrete is sometimes required for
architectural purposes. White Portland cement is used to
provide white or light-coloured concrete, in combination
with suitably coloured aggregates. White Portland
cement is manufactured from very pure white limestone
and china clay, which have a very low iron content
(<0.3%) and may be burned at temperatures in excess of
1600°C (Bye, 1999). In thin section, white Portland
cement is readily identified as the unhydrated clinker has
a near absence (<1%) of ferrite (C 4 AF) and a high content
of alite (C 3 S). Figure 171 shows a concrete made with
white Portland cement with the lack of brown ferrite in
the interstitial phases being apparent. The content of
aluminate (C 3 A) is similar or slightly higher than in
normal Portland cement clinker. White Portland cement
is more expensive, has a lower alkali content, and

typically lower strength than normal (grey) Portland
cement. Coloured concrete is produced using coloured
cement that usually comprises white Portland cement
with an added pigment. Red, yellow, brown, or black
cements can be produced by intergrinding 5-10% (by
weight) iron oxide pigment of the corresponding colour.
Green cements can be made by using chromium oxide
and blue-coloured cements are made using cobalt blue
pigment. An example of a mortar incorporating yellow-
coloured cement is shown in Figure 318 .
In thin section ( 172 ), the hardened Portland cement
matrix of uncarbonated concrete is seen to consist
mainly (approximately 70%) of essentially amorphous

171

171 White Portland cement concrete exhibiting a
frequent abundance of unhydrated and partially
hydrated (with hydration rims visible) residual white
Portland cement grains (white) in the cement matrix
(brown). Quartz fine aggregate particles appear
white/rounded; PPT, ×150.

172

172 Close view of Portland cement concrete showing
the cement matrix (black) which contains brightly
coloured portlandite crystallites (white/yellow) and
relict cement grains (dark brown). Flint (upper/grey)
and quartz (grey/white) fine aggregate particles are
also visible; XPT, ×150.