Environmental Engineering Reference
In-Depth Information
The stability of AFt is also dependent on the pH value of the environment.
Ettringite was found to be stable up to 60°C in the resulting solution of pH 11.2.
83,84
In another study, Gabrisovil et al.
85
demonstrated that, under nonequilibrium condi-
tions, the boundary for the disappearance of AFt is pH = 10.7 and for the monosul-
fate, pH = 11.6. Therefore, only gypsum and aluminate sulfate are stable at pH
values below 10. A laboratory study has indicated that AFt-based waste form is dis-
solved very quickly in acidic environments.
86
Although no published information is
available on the stability of those nonsulfate ettringites, it can be expected that they
behave very similarly. The conversion or decomposition of ettringites will release the
immobilized heavy metals. Precautions should be taken when ettringites are considered
for fixation of contaminants.
4.5
ALKALI-ACTIVATED SLAG CEMENTS
Alkali-activated slag cements usually consist of two basic components: a ground
granulated slag and an alkaline activator(s). The former shows little or no cementi-
tious behavior under normal conditions, but may give very high strengths in the
presence of an alkaline activator(s). Typical examples are GBFS and granulated
phosphorus slag (GPS). NaOH, Na
2
CO
3
, Na
2
O.nSiO
2
, and Na
2
SO
4
are the most
common and economical activators.
Although many research papers have been published on the hydration, micro-
structure, and properties of alkali-activated slag cements, the hydration mechanism
of alkali-activated slag cements is still not very clear. The main hydration product
of alkali-activated slag cement is C-S-H with a low C/S ratio. The minor hydration
products such as C
4
AH
13
, C
2
ASH
8
, and C
3
A.CaCO
3
.11H
2
O may appear depending
on the nature of slag and alkaline activators used. The detailed information on
microstructure and properties of alkali-activated slag cements can be found in several
publications.
3,88-90
When a proper alkaline activator is used, alkali-activated slag
cement can show much higher early and later strength than even ASTM Type III
portland cement. It was noticed that, for a given water-to-cement or water-to-slag
ratio, alkali-activated slag cement exhibits a much less porous structure than portland
cement.
4
Since the main hydration product of alkali-activated slag cement is C-S-H
with a low C/S ratio, and no Ca(OH)
2
forms, alkali-activated slag cements show
much better resistance in aggressive environments than portland cement. The former
also has much better stability at high temperatures in dry and wet conditions than
the latter. This means that alkali-activated slag cement is a better cement for S/S of
wastes than portland cement.
Figure 4.3 represents the mercury intrusion pore structure of portland cement
and alkali-blast furnace slag cement pastes cured 28 days at 25°C. It can be seen
that the alkali slag cement paste has not only a lower porosity, but also a finer pore
structure than portland cement paste. The alkali slag cement pastes contain mainly
pores with r < 100 Å, which can restrict the flow of liquid or diffusion of ions in
the pastes. This means that hydrated alkali-activated slag can have much finer pore
structure and lower porosity than portland cement pastes, which indicates that the
former is a better physical barrier than the latter.
91,92
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