Civil Engineering Reference
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generically known as c-S-H, the compound primarily responsible for the
binding properties exhibited by the material (cong and Kirkpatrick, 1996;
richardson and Groves,
1997; Taylor, 1997; García-Lodeiro et al.,
2012a).
The reaction mechanisms in alkaline cements differ from the mechanisms
observed in oPc, as discussed below.
Glukhovsky (1994) and Krivenko (1992, 1994) proposed a model for the
alkaline activation of Sio
2
- and cao-rich materials (such as blast furnace
slag) to describe the reaction sequence in which the alkaline cation (r
+
) acts
as a mere catalyser in the initial phases of hydration, via cationic exchange
with the ca
2+
ions. The mechanism proposed by these authors is summarised
below:
==Si—o
-
+ r
+
= ==Si—o—r
+ oH
-
= ==Si—o—r—oH
-
==Si—o—r
==Si—o—r—oH
-
+ ca
2+
= ==Si—o—ca—oH + r
+
These same authors believed that as the reactions advance, the alkaline
cations are taken up into the structure.
Fernández-Jiménez (2000) and Fernández-Jiménez et al.
(2003) reported
that the nature of the anion in the solution also plays an instrumental role in
activation, especially at early ages and in particular with regard to setting. The
model that describes the reaction mechanisms (based on a model proposed
by Glasser, 1990) is depicted in Fig. 17.2.
The slag particles first undergo chemical attack due to the high pH in
the liquid medium. The insoluble hydration products resulting from that
attack settle on the surface of the grains, forming a semi-protective layer or
barrier that prevents the reactions from proceeding at the desired pace. These
hydration products are amorphous and consequently undetected by X-ray
C-(A)-S-H
formation
Alkaline solution
Outer C-(A)-S-H
[Si
4+
]
[Ca
2+
]
[Al
3+
]
Blast furnace slag
[OH
-
]
Inner C-(A)-S-H
17.2
Theoretical model of the reaction mechanism in alkali-activated
slag.
