Civil Engineering Reference
In-Depth Information
T
he hydration of the ferrite phase is much slower than that of CA and C
12
A
7
, and this
reaction scarcely affects the setting and initial strength development of calcium aluminate
cements. C
3
(A,F)H
6
is formed as the final product, with C
2
(A,F)H
x
and C
4
(A,F)H
x
as
intermediates.
Dicalcium silicate in calcium aluminate cements does not yield a C-S-H phase and free
calcium hydroxide, as occurs in Portland cement. Instead, strätlingite (also called
gehlenite hydrate, C
2
AH
8
) is formed as the product of hydration.
The total heat of hydration of calcium aluminate cement is in the range 450-500 J/g,
and is similar to that of Portland cement. However, 70-90% of it is liberated within the
first 24 hours (at 20°C), making dissipation of the heat into the environment more
difficult than in situations where Portland cement is employed, where the liberation of
hydration heat takes place much more slowly. This may be critical, especially in the
erection of massive structures, where a significant rise of temperature may take place
shortly after mixing. Maximum temperatures of up to 80°C may be reached in the bulk of
the concrete structure. This in turn may accelerate the “conversion” of the calcium
aluminate hydrates formed. Wet curing has to be employed to prevent superficial
dehydration and dusting of the hardened concrete.
The chemical shrinkage associated with the complete hydration of calcium aluminate
cement amounts to about 10-12 ml per 100 g of cement. This value is greater than that
common in Portland cement (about 5 ml/100 g).
Calcium aluminate cement reacts in the course of hydration with more water than
Portland cement. In a Ciment Fondu cement, the type most widely employed, a
water/cement ratio of about 0.7 is needed to attain complete hydration. This amount of
water is higher than that needed to obtain satisfactory rheology of the fresh concrete mix,
in most instances. If no conversion were to take place in the hardened paste such a high
water/cement ratio would be acceptable, as the high water addition would still result in a
relatively low porosity of the hardened material. However, a distinct liberation of
combined water, associated with an increase of porosity, takes place upon conversion of
the primary formed CAH
10
to C
3
AH
6
. In mixes with lower water/cement ratios the
amount of water present may not be sufficient for complete hydration. Nevertheless,
acceptable strengths may still be attained, owing to the low porosity of the incompletely
hydrated paste. As will be shown below, such pastes exhibit a lower strength loss in the
course of subsequent conversion than those made with a higher
w/c
.
The microstructure of the hydrated calcium aluminate cement paste depends on the
initial water/cement ratio, hydration time, and hydration temperature (Halse and Pratt,
1986; Scrivener and Capmas, 1995). In secondary electron imaging on fractured surfaces
the CAH
10
phase appears as hexagonal prisms or fine needles, typically up to 10 µm long
and 2 µm wide. C
2
AH
8
is visualized in the form of small hexagonal platelets. C
3
AH
6
appears in the form of spheroidal particles. Well-recognizable crystal forms can be
detected, especially in pastes made with higher water/cement ratios and relatively high
porosities, whereas at low
w/c
the paste appears in SEM studies as a rather compact
material without readily distinguishable morphological features.
