Environmental Engineering Reference
In-Depth Information
Figure 4.4 shows the leached fraction of Cs
+
in portland cement and alkali-
activated slag cement pastes containing 0.5% CsNO
3
after 28 days of moist curing
at 25°C. The results indicate that the Cs
+
in portland cement pastes shows a much
higher leached fraction than that in alkali-activated slag cement pastes at the same
temperature. As the temperature increases from 25 to 70°C, the leached fraction of
Cs
+
in both pastes escalates. The leached fraction of Cs
+
in portland cement pastes
at 25°C is even higher than that from alkali-activated slag cement pastes at 70°C.
The calculation using Arrhenius' Equation indicated that the Cs
+
leaching activation
energy of portland cement pastes is 18.96 kJ/mol compared with 25.19 kJ/mol for
alkali-activated slag cement pastes. The lower leached fraction and higher leaching
activation energy of Cs
+
in alkali-activated slag cement pastes than in portland
cement pastes can be attributed to the less porous structure and lower C/S ratio in
C-S-H. A partial replacement of slag with metakaolin will increase the porosity of
the hardened cement pastes. However, it decreases the leached fraction of Cs
+
and
Sr
2+
in the hardened pastes.
93
The authors attribute the decrease in the leached fraction
to the formation and adsorption properties of (Al+Na) substituted C-S-H and the
self-generated zeolite precursor.
Many substances can significantly interfere with the hydration of cement, as
discussed in Chapter 7. This is the basic principle for the use of different cement
chemical admixtures such as retarders, accelerators, and superplasticizers to obtain
some special properties of cements and concrete. It has been reported that heavy
metals show much less interference with the hydration of alkali-activated slag
cements than with portland cement. Shi et al.
94,95
investigated the S/S of electrical
AFD with portland cement and an alkali-activated slag cement using an adiabatic
calorimeter. The AFD is from the production of a specialty steel, which contains a
high concentration of a variety of heavy metals. When 30% AFD is added, it retarded
the hydration of the cement, but did not show an obvious effect on the hydration of
cement at later ages. As the AFD content is increased from 30% to 60%, it retarded
the hydration of portland cement very significantly, and the solidified waste forms
did not show measurable strength after 6 months of hydration. When AFD is mixed
with alkali-activated slag-based cement, the early hydration of the cement was
retarded more obviously as the AFD content increased. However, the cement con-
tinued to hydrate with time-released more heat as the AFD content increased. It
seems that the presence of AFD retarded the early hydration, but was beneficial to
the later hydration of the slag.
14,15,96
Several heavy metals, such as Zn
2+
, Pb
2+
, Cd
2+
, and Cr
6+
, were stabilized in
NaOH-, Na
2
CO
3
-, and sodium silicate-activated slag cements.
97,98
These alkali-
activated slag cements could immobilize these heavy metals very well regardless of
activator. Cho et al.
99
investigated the leachability of Pb
2+
and Cr
6+
immobilized in
NaOH and sodium silicate-activated slag cement pastes. They noticed that the leach-
ability of Pb
2+
and Cr
6+
in alkali-activated slag cement pastes varied with curing
conditions, but was very small. There is a very good relationship between the
diffusion coefficient of Cr
6+
and the pore volume with a radius less than 5 nm.
The other advantage of alkali-activated slag cement is its dense structure and
stability at high temperatures. Under hydrothermal conditions, the main hydration
products of portland cement are C
2
SH(A) and Ca(OH)
2
, which give the cement paste
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