Civil Engineering Reference
In-Depth Information
C
2
S + (
x
+ 2 -
y
) H Æ C
y
SH
x
+ (2 -
y
) CH [18.9]
reaction [18.9] corresponds to the direct hydration of belite to form the
amorphous gel C
y
SH
x
and portlandite. However in BCSA pastes, belite
coexists with aluminum-rich amorphous hydrates, promoting the formation
of stratlingite (Palou et al., 2005; Cuberos et al., 2010; Gartner and Macphee,
2011):
C
2
S + AH
3
+ 5H Æ C
2
ASH
8
[18.10]
This reaction consumes the amorphous AH
3
formed by the hydration reactions
of aluminum-rich phases. The presence of stratlingite has been confirmed
in BCSAF pastes by XrPd and ATd techniques (Cuberos et al., 2010;
Aranda et al., 2011; Morin et al., 2011). This reaction may play a key role
in the increase of early-ages mechanical strengths, and it is produced at a
larger pace if high temperature polymorphs of belite are stabilized. reaction
[18.9] may take place at later ages, if aluminum hydroxide is depleted. The
inset in Fig. 18.4 shows ATd signals of a BCSAF paste hydrated for 6
months (Morin et al., 2011). These data confirm the presence of CSH gel
and the # symbol highlights the absence of the endothermic effect of AH
3
decomposition.
There are other reactions which have to be taken into account. Some
studies have confirmed the presence of katoite phases, also known as siliceous
hydrogarnet, with C
3
ASH
4
being the stoichiometry of one member of the
series, C
3
(A,F)S
x
H
6-2
x
(iron-free,
x
= 1). A/F ratio is still unknown in these
katoites and
x
is probably not greater than 1 (Gartner and Macphee, 2011).
The proposed formation mechanism is detailed below:
C
2
S + 1/2C
4
AF + 5H Æ C
3
(A,F)SH
4
+ CH [18.11]
Reaction [18.11] justifies the consumption of ferrite and releases to solution
portlandite, which is not detected by Xrd or ATd. Consequently, this
portlandite may be consumed by stratlingite as follows, producing larger
quantities of katoite:
C
2
ASH
8
+ CH Æ C
3
ASH
4
[18.12]
The inset in Fig. 18.4 shows the endothermic effect of siliceous hydrogarnet
or katoite decomposition and the asterisk symbol is highlighting the absence
of stratlingite after 6 months of hydration (Morin et al., 2011). Katoite
formation by reaction [18.12] may be of interest for the durability of mortars
and concretes based on BCSAF as portlandite is consumed. Finally, the effect
of temperature on the hydration of CSA cements at very early ages has also
been studied (Zhang and Glasser, 2002).
The understanding of the hydration reactions is key for tailoring the
properties of CSA and BCSAF mortars and concretes, but the microstructure
development is also of the utmost importance. Very recently, the early
