Materials for engineering, 3rd Edition - (Malestrom)

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Materials for engineering
to-gel contact on adjoining grains, aided to an increasing extent by the
interlocking of the ettringite crystals as indicated in Fig. 4.11(a).
C 4 AF acts as a flux in the cement kiln, but its hydration products play
little part in the setting and hardening processes. It does, however, contribute
colour to the cement.
The hardening process starts after about three hours. The envelope of gel
around the cement grains also acts as a semi-permeable membrane for water.
Water is drawn through the gel because of the high concentration of calcium
inside and an osmotic pressure builds up until it is relieved by the envelope
bursting. The ruptured gel then peels away from the grain forming gel foils
and fibrils, and tubules of C 3 A (Fig. 4.11(b)). The grain can now hydrate
further, corresponding to the second heat of evolution shown in Fig. 4.9, and
a multitude of fibres grow and multiply. Hardening corresponds to the
interlocking of these fibres, causing the paste to become rigid. Crystalline
plates of Ca(OH) 2 (CH), known as portlandite, also form and hardening
continues as further fibres and crystals form (Fig. 4.11(c)).
4.4.2 Control of the rate of hydration of cement
The rate of cement hydration is affected by the ambient temperature. In cold
conditions, accelerators (e.g. calcium chloride) are added to concrete to
increase the kinetics of hydration, so that, for example, the cement sets
before frost can damage it. In hot environments, a retarder is used (e.g.
sugar) to reduce the setting speed by inhibiting the growth of C-S-H gel. The
use of such additives enables the setting and hardening of the cement to be
controlled so that the cement can be used in a wider range of applications. If
it is essential for the cement to set quickly and to exhibit a rapid initial gain
in strength, the kinetics can be increased as above, whereas if a cement is
required to set slowly, it can be slowed down by the appropriate additive.
4.4.3 Structure–property relationships in cement and
concrete
The elastic modulus
Concrete is a composite material (Fig. 4.8). The Young’s modulus of gravel
(a) (b) (c)
4.11 Illustrating the setting and hardening of Portland cement,
showing (a) gel-to-gel contact, (b) growth and interlocking of fibres
and (c) growth and interlocking of fibres and crystals.