Civil Engineering Reference
In-Depth Information
1 µm
1 µm
10 µm
1 µm
18.5
Cryo-scanning electron microscopy study for BCSAF pastes.
Top images are taken for a BCSAF-5wt% C
S
H
2
paste after 260 min of
hydration: (left) photograph showing acicular-hexagonal AFt crystals;
(right) photograph showing thin-layered AFm crystals. Bottom
images are taken for a BCSAF-20wt% C
S
H
2
paste: (left) photograph
after 30 min of hydration showing already a lot of ettringite; (right)
photograph after 260 min of hydration showing well-developed
hexagonal AFt crystals.
C
4
AF + 16H Æ 2C
2
(A
0.5
F
0.5
)H
8
[18.6]
C
12
A
7
+ 51H Æ 6C
2
AH
8
+ AH
3
[18.7]
2CA + 11H Æ C
2
AH
8
+ AH
3
[18.8]
Most of these reactions also yield aluminum or iron hydroxides (AH
3
or
FH
3
) which are initially amorphous as they cannot be directly detected by
X-ray powder diffraction (XRPD), but its occurrence may be confirmed by
thermal analysis techniques (Cuberos et al., 2010; Martín-Sedeño et al.,
2010; Morin et al., 2011). later, these aluminum-rich amorphous phases
may crystallize as gibbsite.
In BCSAF, hydration of C
2
S is of special interest. Independently of the
rate of reaction of a- and b-polymorphs, the hydration reaction would be
the same as that occurring in oPC:
