Materials for engineering, 3rd Edition - (Malestrom)

Glasses and ceramics 149
50
Reaction complete
Compressive strength (MPa)
40
30
20
10
Induction
period
Hardening reactions
Setting reactions
0
15 min 2.4 hrs 1 day 10 days 100 days
Time (log scale)
4.10 Change in compressive strength with time during the setting of
Portland cement.
C 3 A > C 3 S > C 4 AF > C 2 S
The rapid hydration of C 3 A would lead to ‘flash setting’ of the cement,
due to the formation of calcium aluminate hydrates:
C 3 A + 21H → C 4 AH 13 + C 2 AH 8
which then convert readily to other hydrates. This is avoided by the addition
of gypsum (C4SH 2 ) which combines with C 3 A to form sulpho-aluminate
hydrates such as ettringite:
C A + C SH + 26 H → C ASH (ettringite)
3 4 2 6 32
These are in the form of very fine needle-like crystals, which form slowly
around the aluminate, and flash set is avoided. The rate of hydration of C 3 A
then becomes similar to that of C 3 S and it is these hydration reactions
proceeding together which are mainly responsible for the initial high rate of
heat evolution in the cement paste.
The hydration reactions of the two calcium silicates are very similar:
2C 3 S + 7H → C 3 S 2 H 4 + 3CH
2C 2 S + 5H → C 3 S 2 H 4 + 3CH
C 3 S 2 H 4 (calcium silicate hydrate) is referred to as C-S-H gel, and it forms
as a poorly crystalline material in the size range of colloidal matter (less than
1 µm in any dimension). The surfaces of the cement grains become coated
with silicate and aluminate gels, causing a drastic deceleration in the reaction
rate. The cohesion of the cement paste during the setting period is due to gel-