Civil Engineering Reference
In-Depth Information
SiO
2
Silica
fume
Metakaolin
Fly
ash
Blast furnace
slag
OPC
Al
2
O
3
CaO
17.1
Standard composition of the prime materials used to
manufacture alkaline cements on a CaO-SiO
2
-Al
2
O
3
diagram.
this material is shown in Fig. 17.1. The minority crystalline phases include
a solid solution of two melilite family minerals, gehlenite and akermanite,
which crystallise in the tetragonal system (Fernández-Jiménez, 2000; Shi
et
al.,
2006).
The main requirements to be met by such slag for use in activated slag
cement are listed below (Fernández-Jiménez 2000; Shi
et al.,
2006):
∑ it must be granulated or pelletised and have a vitreous phase content of
85-95%.
∑ it must exhibit structural disorder, since the lower the degree of
polymerisation in the glass, the higher is its hydraulic activity: the degree
of polymerisation depends on the (Sio
4
) tetrahedral and the al and mg
coordination in the vitreous phase of the slag.
∑ it must be pH-basic, i.e., have a cao + mgo/Sio
2
ratio of greater than
1. Basic slag has a higher hydraulic potential, since the lime content in
the slag controls its activation. That notwithstanding, acid slag may also
be alkali-activated.
∑ It must be ground to a specific surface of 400-600 m
2
/kg. Specific
surface plays an important role in the rate and intensity of the activation
reaction.
17.2.2 Reaction mechanisms
The hydration of the calcium silicates in Portland cement (c
3
S and c
2
S)
yields portlandite (ca(oH)
2
) and a non-crystalline calcium silicate hydrate,
