Civil Engineering Reference
In-Depth Information
from lafarge) was carried out in early 2011 (Walenta and Comparet, 2011).
Clinkering in a Portland industrial kiln was proven with lower operating
temperatures (1225-1300 °C). Furthermore, the reduction of Co
2
emissions
of 25-30% for BCSAF cement was also proven when compared with the
emissions of CeM (I) type oPC cement. Temperature in the clinkering
zone was a key parameter as too low temperatures gave under burnt binder
with high free lime and C
12
A
7
contents, and too high temperatures may
give kiln blockage, loss of grindability and C
4
A
3
S
decomposition with
high So
2
emissions. However, optimum clinkering temperature resulted
in no
x
emissions being much lower than those of oPC fabrication,
and So
2
measured industrial emissions at the same level than for oPC
production.
18.4 Hydration of calcium sulfoaluminate cements
In previous sections, yeelimite-containing cements have been mainly
classified as CSA or BCSAF. However, from the point of view of hydration
the first process is common in both types of binder. Once water is added to
cement, the first effect that occurs is the wetting of the cement and the start
of yeelimite dissolution. However, only part of this exothermic process is
usually recorded in a calorimeter owing to the time required for the mixing
procedure and stabilization time, point (1) in Fig. 18.4. Immediately after
wetting, the following reaction takes place:
C
4
A
3
S
+ 2 C
S
H
2
+ 34 H Æ C
6
A
S
3
H
32
+ 2 AH
3
[18.1]
Chemical reaction [18.1] corresponds to the formation of ettringite (C
6
A
S
3
H
32
),
also known as AFt, by the reaction of C
4
A
3
S
with soluble calcium sulfate and
water. This reaction also takes place with anhydrite or bassanite although at
different rates. Formation of ettringite will continue while calcium sulfates
are present; signal (2) in Fig. 18.4. If there is not enough sulfates to react
with yeelimite phase, reaction [18.2] will start:
C
4
A
3
S
+ (16 +
x
) H Æ C
4
A
S
H
(10+
x
)
+ 2AH
3
x
= 0,2,4
[18.2]
reaction [18.2] corresponds to the hydration of yeelimite to form an AFm-
type phase with different water contents, depending on the time of hydration,
water-to-cement W/C ratio and initial cement phase assemblage. This is also
an exothermic reaction which takes place once calcium sulfates are consumed,
signal (3) in Fig. 18.4 (odler, 2000). These two reactions may take place
in the early hydration of CSA and BCSAF cements. This is shown in Fig.
18.5 where cryo-scanning electron microscopy photographs of BCSAF pastes
are shown. Pastes from BCSAF cements with 20 wt% of gypsum showed
a lot of acicular-hexagonal AFt crystals even at very early hydration ages,
reaction [18.1]. Conversely, pastes from BCSAF cements with (only) 5 wt%
