Environmental Engineering Reference
In-Depth Information
hexagonal platelet structure of Ca(OH)
2
, and consists of layers of C
4
(A,F), with
interlayers of X.H
y
(see also Chapter 9). It appears from the general formulae that
AFt and AFm minerals may take up a variety of anion impurities without changing
their structures. Exploitation of this characteristic for immobilization of anions, such
as Cl
-
, NO
3
-
, and SO
4
2-
, and metalloid oxyanions, such as AsO
3
3-
, AsO
4
3-
, CrO
4
2-
,
MoO
4
2-
, SeO
3
2-
, SeO
4
2-
, TeO
4
2-
, VO
4
3-
, and WO
4
2-
, in ettringite has been proposed
89-
91
and increasingly followed up with research. CrO
4
2
—AFt has been shown to form
readily,
90,92-94
and both CrO
4
2
—AFt and CrO
4
2
—AFm have been observed in cement-
based products containing real wastes.
95
Uptake of SeO
4
2- 96
and AsO
4
3-
97,98
by
ettringite has also been demonstrated, and Zhang and Reardon
99
demonstrated high
uptake of four oxyanions by ettringite, with the order of preference B(OH)
4
-
> SeO
4
2-
> CrO
4
2-
> MoO
4
2-
; uptake of these oxyanions by hydrocalumite (a form of AFm)
was even higher, but did not follow the same order. Johnson
77
reports that synthesis
of AFt containing SO
3
2-
, NO
3
-
, SeO
4
2-
, and CO
3
2-
is only possible using a laboratory
method that does not resemble conditions in cements, whereas MoO
4
2
—AFt and
Cl—AFt could not be formed at all. However, MoO
4
2
—AFm has been synthesized
100
and Cl—AFm, Friedel's salt, is commonly found in hardened cement pastes and
concretes. However, solubility of chlorides remains high in cement-based products
as elsewhere.
AFt in particular is also known to take up other trivalent cations, such as Cr
3+
,
Ni
3+
, and Co
3+
in place of Al or Fe.
88
C
3
A may also react to produce hydrogarnet,
C
3
AS
x
H
6-2x
,
101
which is known to take up Fe, and has been shown to take up Cr
3+
.
102
Gougar et al.
88
also review reports that divalent cations, such as Ba
2+
, Cd
2+
, Co
2+
,
Hg
2+
, Ni
2+
, Pb
2+
, Sr
2+
, and Zn
2+
, can replace Ca
2+
in AFt minerals, but the evidence
for this is not strong. Johnson
77
reports several studies that indicate surface sorption
is also an important mechanism for uptake of oxyanions by AFt and AFm. It should
be noted that ettringite has been shown to have poor acid neutralization capacity
and resistance to acid attack,
11,67
which limits its long-term durability in the envi-
ronment, and the usefulness of immobilization mechanisms based on AFt. Studies
of iodine uptake in calcium aluminate cement blended with Ca(OH)
2
and CaSO
4
showed transformation of 4CaO.Al
2
O
3
.13H
2
O to 3CaO.Al
2
O
3
.CaI
2
.12H
2
O.
103,104
Examination of Cs retention by the hydrated CaO-SiO
2
-Al
2
O
3
system indicated
that uptake was greatest in a matrix low in Al
2
O
3
and nearly equimolar in SiO
2
and
CaO content, which was attributed to the formation of zeolitic phases.
105
Experiments
with stabilization of an alkaline low-level radioactive waste simulant using cement,
coal fly ash, and attapulgite clay also showed formation of zeolites and concluded
that contaminants were incorporated in these phases.
79
However, other work indicates
that alumina-substituted calcium silicate hydroxy hydrate selectively takes up Cs.
106
7.4
SUMMARY
The components of wastes have been demonstrated to have significant effects on prop-
erties related to the handling, durability, and leaching of cement-based products, which
must be taken into account in formulation development. Contaminant leachability is
controlled by both physical and chemical mechanisms. An improved understanding of
chemical immobilization mechanisms is gradually developing, but many of the details
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