Environmental Engineering Reference
In-Depth Information
FIGURE 9.7
The effect of the reducing agent FeSO
4
on the XANES spectra of chromate (10
wt% sodium chromate) in portland cement.
The use of ettringite for the removal of toxic anions such as borate
30
and
selenate
31
has been suggested. It is expected that when ettringite crystallizes from
a waste stream containing these anions they will replace the sulfate ions in the crystal
structure. However, Myneni et al.
32
showed that more of the waste anions were
sorbed on the surface of the ettringite than were incorporated within the crystal
structure.
EXAFS and XANES have been traditionally applied to bulk specimens. It is
possible to focus the synchrotron X-ray beam to a very small area, a few micrometers
across, and obtain speciation information from such an area. Thus, spatial variation
in speciation can be studied. It is well established by now that the addition of
granulated blast furnace slag (which contains +2 Fe and -2 S) to the waste form
can reduce the oxidation state of various toxic elements.
33
For example, the oxidation
state of chromium can be reduced from hexavalent to trivalent. Bajt et al.
34
used a
synchrotron-based X-ray microprobe to show that this is indeed the case. With the
presently available synchrotron sources, the spot size of the focused X-ray beam
can be as small as 1 μm, and the detection limit for chromium can be 10 ppm.
9.5.4
T
HERMAL
A
NALYSIS
In thermal analysis, a small amount of a material is heated at a programmed rate,
and the change in its properties over temperature is monitored.
35
A wide range of
thermal analysis techniques, such as thermogravimetry (TG), differential thermal
analysis (DTA), and differential scanning calorimetry (DSC), are used by cement
chemists. TG, for example, is routinely applied to quantify the amount of calcium
hydroxide in hydrated portland cement.
Cement hydration reactions are often affected by the presence of a waste, usually
retarded. This process can be monitored by thermal analysis and often quantified.
Figure 9.8 shows the pozzolanic reaction of a Class F fly ash when mixed with Type
I portland cement in 1:1 proportion, with a water to solids (W/S) ratio of 0.5. A
significant amount of calcium hydroxide is present by 7 days in the hydrated mixture.
The amount of calcium hydroxide is lower in the year-old specimen, and no calcium
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