Civil Engineering Reference
In-Depth Information
these chemical admixtures do not have the same effect on alkali-activated
fly ash systems as on Portland cement pastes. As most of these admixtures
decompose in the highly alkaline media needed to activate fly ash, they are
ineffective.
17.3.6 Durability
Alkali-activated fly ash cements perform extremely well not only in terms
of strength, but also of durability. material durability is closely related to
mineralogical and microstructural composition. The chief durability problems
in oPc pastes, mortars and concretes involve the presence of calcium. in
alkali-activated fly ash cements, by contrast, the main reaction product formed
is a three-dimensional aluminosilicate hydrate (non-calcium n-a-S-H gel),
regarded as a pre-zeolite (Palomo et al., 2004), which is clearly different
from the c-S-H gel formed in oPc pastes. durability, then, must necessarily
differ in oPc and alkaline cements.
Sulphate and seawater resistance
The aggressive chemicals to which portland cement mortars and concretes are
most commonly exposed are sulphates or ions found in sea water (Johansen
et al.,
1995; allahverdi and Škvára,
2001; rostami and Brendley,
2003;
Bakharev,
2005; Fernández-Jiménez et al.,
2007; Pacheco-Torgal et al.,
2012).
Fernández-Jiménez
et al.
(2007) monitored the resistance to these attacks in fly
ash mortars activated with different alkalis (n: naoH, W: na
2
Sio
3
+ naoH).
Generally speaking, they observed no significant decay in these materials as
a result of the action of these agents. in fact, mechanical strength continued
to develop in the mortars studied regardless of the medium in which they
were immersed (see Fig. 17.12). after immersion in aggressive media for one
year the mortars showed no visible signs of decay. nonetheless, microscopic
analysis revealed a gel richer in silicon and with some magnesium in the
specimens exposed to sea spray, while the samples immersed in a sodium
sulphate solution were found to contain sodium sulphate. in both cases,
however, these decay products were detected only sporadically.
Bakharev (2005) also studied the performance of alkali-activated ash mortars
when immersed in sodium sulphate and magnesium sulphate solutions. This
author found that mortar stability depended largely on the intrinsic order
of the aluminosilicate gel (n-a-S-H gel) constituents, observing that the
greater the degree of crystallinity in the gel, the greater was its stability in
aggressive media. Like Fernández-Jiménez
et al. (2007), Bakharev found that
n-a-S-H gels prepared with naoH performed better than the ones prepared
with sodium silicate as the activator.
