Civil Engineering Reference
In-Depth Information
∑
a phase known as hydrotalcite (mg
6
al
2
co
3
(oH)
16
·4Ho) has been detected
in slag activated with naoH (Fernández-Jiménez, 2000; Ben Haha
et
al., 2011) and waterglass (cheng et al., 1992). Hydrotalcite is a natural
mineral whose structure consists of layers of brucite (mg(oH)
2
) with
interstitial water molecules and co
3
2-
ions. it forms sub-microscopically,
making it difficult to identify with scanning electron microscopy (SEM).
These tiny hydrotalcite crystals are dispersed throughout the c-S-H gel.
Phases of this type have also been found in cement-slag blends.
∑
c
4
aH
13
-type phases have been detected in slag activated with naoH. These
phases can be identified as platelets 0.1-0.2 mm thick and approximately
1.5 mm in diameter (Larosa et al., 1992; Wang et al., 1995). other authors
(cheng et al., 1992) have observed carbonated phases such as c
4
acH
11
and c
8
a
2
cH
24
in slag pastes activated with naoH and ca(oH)
2
.
17.2.4 Cementitious gel structure
a detailed comparison between the c-S-H gel forming in alkaline cements
and the gel found in oPc follows. a full understanding of gel structure and
composition is vital, since this is the compound primarily responsible for the
physical-mechanical properties of the material and hence its performance
during its service life.
cementitious gels are not readily characterised due to their amorphous
nature. nonetheless, the information reported in recent years as a result of
the application of a number of characterisation techniques, most prominently
nuclear magnetic resonance, has proved to be very helpful for establishing
models able to explain and describe the structure of the various gels
formed.
Taylor based his well-known model for the c-S-H formed during oPc
hydration on the structure of defective tobermorite, also known as dreirketten
type chains (cong and Kirkpatrick,
1996; Taylor, 1997;
richardson, 2008;
Garcia-Lodiero et al.,
2012a) (see Fig. 17.3(a)). Perfect tobermorite consists
of two linear chains of silica tetrahedra arranged on either side of a central
sheet of cao. every fourth silicate in the linear chain is repeated; of the
three tetrahedra in each group, two are connected to the centre sheet of cao
by two oxygen bridges while the third, known as the bridging tetrahedron,
is not connected to the cao. on the nanostructural scale, then, tobermorite
consists of infinite linear chains of silica tetrahedra (Q
2
units), whereas in
C-S-H gel many of the bridging tetrahedra are missing, giving rise to finite
two-, three- or five-link chains (see Fig. 17.3(b)).
Schilling et al. (1994), in turn, proposed a model for the c-S-H gel generated
by the alkaline activation of cao-rich materials (such as blast furnace slag),
in which alT
T
is taken up into the structure, replacing a Si tetrahedron in a
bridging tetrahedron position (see Fig. 17.4). This model was subsequently
