Materials for engineering, 3rd Edition - (Malestrom)

148
Materials for engineering
55% tricalcium silicate (C 3 S)
20% dicalcium silicate (C 2 S)
12% tricalcium aluminate (C 3 A)
8% tetracalcium aluminoferrite (C 4 AF)
3.5% hydrated calcium sulphate (gypsum) (C4SH 2)
1.5% minor constituents, CaO (C), K 2 O, Na 2 O.
The shorthand symbols have the following meaning:
Oxide CaO SiO 2 Al 2 O 3 Fe 2 O 3 H 2 O SO 3
Symbol C S A F H S
4.4.1 Setting and hardening
After water is added to the cement powder, the resulting cement paste can be
shaped and used for a period approaching two hours. Figure 4.9 is a calorimetric
curve showing the heat of evolution with time during the hydration of Portland
cement. During the period marked Stage I, a considerable amount of heat is
evolved (a peak of 200 W kg –1 after 30 s), which constitutes the stage of
setting. Setting ends when hardening starts and the cement paste begins to
solidify (Stage II). Figure 4.10 shows the change in compressive strength
with time for Portland cement: hardening is seen to start after an hour or so
and continues at a decreasing rate for a number of years.
The setting reactions can occur over a long period of time, as the different
constituents of the cement react at different rates, the rate of hydration being
in the approximate order:
10
Stage I
Stage II
Heat evolution(J kg –1 s –1 )
5
0
Setting
peak
Induction
period
Hardening
peak
15 min 2.4 hrs 1 day 10 days 100 days
Time (log scale)
4.9 Calorimetric curve showing the heat evolution with time during the
setting of Portland cement.