Civil Engineering Reference
In-Depth Information
clay was needed to produce mortars with a high performance, so their use
was not conditioned by the availability of natural pozzolans (Hazra and
Krishnaswamy, 1987).
The use of pozzolans is justified from a number of perspectives. Technically,
depending on their nature, pozzolans afford cement special characteristics, in
particular by enhancing its durability, understood here to mean mortar and
concrete resistance to chemical attack by aggressive external agents. This
is because pozzolanic additions set off a chemical reaction in cement paste
in which the lime hydrolysed from portlandite, a calcium silicate present in
clinker, combines with active acid components in the pozzolan, silica and
alumina to form silicates and aluminates. This raises the tobermorite content
in the hydrated cement paste, with beneficial results. The various forms and
structures of tobermorite (calcium silicate hydrate - CSH) are primarily
responsible for cement paste (and consequently mortar and concrete) bonding
and strength. Therefore, any increase in the (secondary) tobermorite formed
by pozzolanic action over the (primary) amount formed directly during
hydration of the silicates present in clinker enhances the mechanical strength
of the binder. This effect is more significant in the medium and long term,
since pozzolanic reactions tend to be slow, although that depends on the
reactivity of the pozzolan in question. Given the same fineness, then, and
the same clinker, pozzolan-free cements generally exhibit higher short-term
mechanical strength than cements with pozzolan additions, but in the medium
and long term, the latter have substantially higher strength than the former.
Moreover, portlandite fixation in the form of secondary tobermorite affords
cement paste greater chemical resistance or durability, primarily for two reasons.
On the one hand, portlandite is considerably more water soluble and vulnerable
to acid media than tobermorite, which is less liable to be leached or carried
away. This conversion from portlandite to tobermorite makes cement pastes
more resistant to attack by pure mountain and granitic soil water, which is
a powerful solvent, and to aggressive acid or carbonic solutions. Pure water
dissolves portlandite physically; acid solutions convert it chemically into soluble
calcium salts, while carbonic solutions turn it into soluble and leachable calcium
bicarbonates. The concomitant porosity weakens cement paste, mortars and
concretes. Pozzolanic action prevents or mitigates that effect.
Secondary tobermorite gel formation densifies cement paste, generally
lowering total porosity, while increasing the proportion of micropores at the
expense of mesopores and especially macropores, even where total porosity
rises slightly. This has very beneficial effects, since micropores are less
interconnected and more difficult to penetrate, making cement paste less
permeable to water and aggressive ionic solutions, primarily sulphates and
chlorides, and less permeable also to ion penetration by diffusion. Pozzolanic
cement paste is ten times less chloride ion-penetrable than Portland cement
without such additions (calleja, 1992).
