Environmental Engineering Reference
In-Depth Information
described above, has indicated that C
3
AH
6
is the most stable hydration product of
aluminous cement.
There are many laboratory research projects using high-alumina cements for S/S
of wastes.
65-67
Shen
67
compared three cements — portland cements, high-alumina
cement, and alkali-activated slag cements — for immobilization of cesium. After
28 days of curing at room temperature, the three waste forms were tested for Cs
leachability at 25 and 70°C. At 25°C, high-alumina cement showed the lowest, and
portland cement the highest leachability, which may be attributed to the lower
solubility of Al(OH)
3
compared with that of Ca(OH)
2
. As the temperature was
increased to 70°C, the cesium leachability increased drastically due to the conversion
of hydration produced. Actually, it was found that the Sr leachability from high-
alumina cement-solidified wastes is lower than that from portland cement-solidified
waste forms.
68
However, high-alumina cement-solidified waste forms showed better
thermal integrity and stability than portland cement-solidified waste forms at high
temperatures, especially at above 600°C.
68
High-alumina cements are highly recom-
mended for incorporating wastes by hot pressing.
The addition of some additives into high-alumina cement can change its hydra-
tion products and increase the efficacy of immobilization of contaminants. Amor-
phous silica was added to calcium aluminate cement to give a composition similar
to zeolites and clay minerals.
69
Laboratory test results indicated that silica-adjusted
high-alumina cement gave lower leachability for Ca, Sr, and the rare earths, but is
higher for Na, Mg, and U. Another study noticed that the addition of silica fume to
high-alumina cement resulted in the formation of zeolitic phases, which drastically
reduces the leaching of cesium.
70,71
Analysis of
27
Al and
29
Si using solid NMR and
microanalysis with an electronic microscope indicated the formation of chabazite
with the following composition: Ca
1.38
Cs
1.38
Al
5.24
Si
6.76
O
24
.nH
2
O.
72
The lab research
also confirmed that an increase of curing temperature accelerates the formation of
chabazite and decreases the leaching of Cs very significantly.
With the adjustment of chemical composition of high-alumina cement, trisul-
foaluminate — C
3
A.3CaSO
4
.32H
2
O (AFt or ettringite), which is a member of an
isostructural group of compounds of prismatic or needle morphology — can form.
Ettringite can be a material tolerant of many substitutions,
73-79
as discussed in detail
in Chapter 7.
Calcium aluminate was used to stabilize municipal solid waste incineration fly
ash containing 10.5% chlorides (in alkali chlorides), 8.3% SO
3
(mainly as anhydrite
and some as gypsum), and 19.3% CaO.
80
The mixing proportion was 84.5% fly ash
and 15.5% calcium aluminate at a water-to-cement ratio of 2.7. After 4 hours of
hydration, most anhydrite was consumed and ettringite reached the maximum. After
complete consumption of sulfate, monochlorocalcium aluminate starts to form, until
the calcium aluminate is completely consumed, after about 2 days.
The main concern with the use of ettringites for waste management is their stability.
Thermodynamic calculation indicates that AFt is stable only below 65°C.
81
In the
laboratory, it was noticed that AFt was destroyed after a heat treatment of 16 hours
at 80°C.
82
After the heat treatment, the aluminate and sulfate were generally present
in the C-S-H gel, but some monosulfate and very small amounts of hydrogamet
were also present; some sulfate was found in the pore solution.
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