Civil Engineering Reference
In-Depth Information
diffraction (Xrd) or electron diffraction. Since these c-S-H-type gels, with
lower c/S ratios than in the product present in Portland cement pastes, also
contain al in their structure, they are actually c-(a)-S-H gels. This layer of
gel partially isolates the most anhydrous part of the slag particles. Total slag
hydration may, however, lead to the formation of pores that would favour the
mobility of dissolved ions, in turn inducing the formation of hydrotalcite-like
solids (mg
6
al
2
co
3
(oH)
16
·4H
2
o). nonetheless, hydration also redistributes
paste porosity, rendering the matrix denser and porosity more concentrated
to thereby reduce permeability.
17.2.3 Reaction products
The major reaction product in Portland cement hydration, a c-S-H-type gel,
is the substance primarily responsible for the mechanical properties of the end
material. Portlandite, ettringite and calcium monosulphoaluminate, among
others, form as secondary products (Taylor, 1997). The process is similar
in alkaline cements. The main reaction product is a c-(a)-S-H gel (whose
composition and structure vary with respect to the standard c-S-H generated
in oPc hydration), while a series of secondary products also form. The type
of secondary product generated depends on starting material composition,
activator type and concentration and curing conditions (cheng et al.,
1992;
Larosa et al.,
1992; roy et al.,
1994; Fernández-Jiménez, 2000; Fernández-
Jiménez et al.,
2003; Shi et al.,
2006; rodriguez et al.,
2008; Bernal
et al.,
2011a). Table 17.1 compares the most common reaction products forming
in normal Portland cement hydration to the products of (calcium-rich
and low calcium) alkaline cement activation (Wang and Scrivener,
1995;
Fernández-Jiménez et al.,
1996, 2003; Taylor, 1997; Palomo
et al.,
1999;
Fernández-Jiménez, 2000; Shi et al.,
2006; duxson et al.,
2007a; Provis and
van deventer,
2009; Winnefeld et al.,
2010; Puertas et al.,
2011).
Broadly speaking, the nature and composition of the reaction products
forming during the alkaline activation of materials are among the most
controversial aspects of this field of research and the areas most in need of
Table 17.1
Products precipitating in different types of binders
Binder type
OPC
Alkaline cement
(Na,K)
2
O-CaO-Al
2
O
3
-SiO
2
-H
2
O (Na,K)
2
O-Al
2
O
3
-SiO
2
-H
2
O
Primary
C-S-H
C-A-S-H
N-A-S-H
Secondary Ca(OH)
2
AF
m
AF
t
Hydrotalcite
[Mg
6
Al
2
CO
3
(OH)
16
·4H
2
O]
C
4
AH
13
Zeolites:
hydroxysodalite, zeolite
P, Na-chabazite, zeolite
Y, faujasite
CASH
8
C
4
AcH
11
C
8
A
2
cH
24
C = CaO
S = SiO
2
A = Al
2
O
3
N = Na
2
O H = H
2
O
C = CO
2
