Properties of concrete - Concrete Mix Design, Quality Control and Specification - page 90

(Type IV) cement also has its tricalcium aluminate content limited for
the different reason that it generates more heat. However, both of these
cements have lower than normal resistance to penetration by chlorides,
so neither should be used in marine situations because seawater contains
both sulfates and chlorides. A better solution is to use fly ash or blast-
furnace slag replacement. The latter is particularly suitable for marine
use especially at higher replacement levels. Fly ash and ground-granulated
blast-furnace slag (GGBS) replacement not only increase resistance to
sulfate and chloride ions but also can significantly reduce temperature
rise. Silica fume is also very effective in reducing penetrability. Surface
carbonation has been found to improve sulfate resistance. In the presence
of magnesium sulfate, the cement paste itself is further weakened as the
calcium silicate hydrate is decalcified. This form of attack is more aggres-
sive and requires greater precautions. BRE Special Digest 1 gives good
advice.
5.1.4 Delayed ettringite formation
Although most durability issues are associated with corrosion of rein-
forcement, delayed ettringite formation (DEF) has resulted in 10 mm
cracks in the unreinforced blocks of a port facility within 4 years of ser-
vice. There is debate about the exact mechanism of DEF. The generally
accepted view is Portland cement concrete, which attains a temperature
of approximately 70°C or more during hydration, appears to inhibit the
formation of preliminary nonexpansive ettringite. The unreacted sulfate
within the concrete is then available to react with tricalcium aluminate in
the cement similar to traditional sulfate attack. Most examples of disrup-
tive DEF have occurred in moist environments such as ports, dams, and
railway sleepers.
There are many factors that have been found to influence the susceptibility
to DEF: sulfate to aluminate ratio, cementitious replacements, cement
fineness, and reactivity. As mentioned in Chapter 6, the authors would
advocate a peak temperature limit of 70°C regardless of other consider-
ations is probably the best way to help ensure that DEF is not a problem.
DEF may not be common, but it can cause severe problems that can be
easily avoided.
5.1.5 Thaumasite
In concrete exposed to sulfate, carbonate, and water at low temperature (less
than approximately 15°C) the calcium silicate hydrate crystals can convert
to thaumasite. Accordingly, the binder within the concrete can be turned
into a weak friable material. The source of carbonate may be calcareous