Civil Engineering Reference
In-Depth Information
17.3.3 Reaction products
The alkaline activation of silica- and alumina-rich (and low-calcium) materials
such as fly ash and metakaolin leads to the precipitation of the main reaction
product, an amorphous alkaline aluminosilicate hydrate (m
n
-(Sio
2
)-(alo
2
)
n
·
w
H
2
o) known as n-a-S-H gel (Fernández-Jiménez et al.,
2006a). This
product also goes by the name alkaline inorganic polymer. its silicon and
aluminium tetrahedra are distributed at random, forming a three-dimensional
skeleton (see Fig. 17.6) (Palomo
et al.,
2004; Fernández-Jiménez and Palomo,
2005b). The secondary reaction products in this type of systems are zeolites
such as hydroxysodalite, zeolite P, na-chabazite, zeolite Y and faujasite
(criado, 2007).
17.3.4 Cementitious gel structure
The structure of the gels forming in the alkaline activation of low-calcium,
silicoaluminous materials differs substantially from the gels formed in the
activation of calcium-rich cements. These n-a-S-H gels are characterised
by a three-dimensional structure in which the Si is found in a variety of
environments, with a predominance of Q
4
(3Al) and Q
4
(2al) units. The Si
4+
and al
3+
cations are tetrahedrally coordinated and joined by oxygen bonds.
The negative charge on the alo
4
-
group is offset by the presence of alkaline
cations (typically na
+
and K
+
).
n-a-S-H gel formation entails a series of stages that can be summarised
OH
-
Si
Al
Na
+
17.6
Plan view projection of the three-dimensional structure of a
N-A-S-H gel
(
Criado, 2007
).
