The setting and hardening of concrete are the result of chemical and physical processes that take place
between Portland cement and water, i.e. hydration. To understand the properties and behaviour of cement
and concrete some knowledge of the chemistry of hydration is necessary.
A) Hydration reactions of pure cement compounds
The chemical reactions describing the hydration of the cement are complex. One approach is to study the
hydration of the individual compounds separately. This assumes that the hydration of each compound
takes place independently of the others.
I. Calcium silicates
Hydration of the two calcium silicates gives similar chemical products, differing only in the amount of
calcium hydroxide formed, the heat released, and reaction rate.
2 C3S + 7 H → C3S2H4 + 3 CH 2 C2S + 5 H → C3S2H4 + CH
The principal hydration product is C3S2H4, calcium silicate hydrate, or C-S-H (non-stoichiometric). This
product is not a well-defined compound. The formula C3S2H4 is only an approximate description. It has
amorphous structure making up of poorly organized layers and is called glue gel binder. C-S-H is believed
to be the material governing concrete strength. Another product is CH - Ca(OH)2, calcium hydroxide. This
product is a hexagonal crystal often forming stacks of plates. CH can bring the pH value to over 12 and it
is good for corrosion protection of steel.
II. Tricalcium aluminate
Without gypsum, C3A reacts very rapidly with water:
C3A + 6 H → C3AH6
The reaction is so fast that it results in flash set, which is the immediate stiffening after mixing, making
proper placing, compacting and finishing impossible.
With gypsum, the primary initial reaction of C3A with water is :
C3A + 3 (C S H2) + 26 H → C6A S 3H32
The 6-calcium aluminate trisulfate-32-hydrate is usually called ettringite. The formation of ettringite slows
down the hydration of C3A by creating a diffusion barrier around C3 A. Flash set is thus avoided. Even
with gypsum, the formation of ettringite occurs faster than the hydration of the calcium silicates. It
therefore contributes to the initial stiffening, setting and early strength development. In normal cement
ettringite is not stable and will further react to form monosulphate (C4A S H18).
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