Microstructure and Microchemistry of Waste Forms - Stabilization and Solidification of Hazardous, Radioactive, and Mixed Wastes - page 219

that some of the Cr VI entered the C-S-H structure and increased its degree of
polymerization. NMR independently corroborated that the degree of polymerization
of the C-S-H increased. Electron microscopy showed that the presence of Cr VI also
changed the physical characteristics of the C-S-H, making it more permeable.

9.6.2 S TRUCTURAL C HARACTERIZATION OF A S OLIDIFIED S YNTHETIC

W ASTE F ORM

A synthetic waste containing arsenic, cadmium, chromium, lead, and phenol was
stabilized/solidified in a suite of unknown binders. The principal aim of the study
was to assess the efficacy of different binders by leaching tests. Microstructural and
phase characterization was a secondary objective. Still, such studies can provide
additional constraints about the behavior of the waste forms. EDX microanalysis of
some of the waste forms are shown in Figure 9.12. All the analyses came from waste
form specimens, which were coated with carbon, not gold, so that the gold M peak
did not interfere with the sulfur K peak. The analysis shows that the principal
elements present in a μm-volume region are sulfur and calcium. A minor amount
of silicon is detected, while a trace amount of chromium is also visible. The major
elements and their proportions indicate that the analysis came from a calcium sulfate.
Since EDX does not detect water, whether the calcium sulfate is gypsum or anhydrite
could not be confirmed. The XRD pattern of the same waste form is shown in Figure
9.13D. The pattern matches with that of gypsum, and no other peaks are present. A
gypsum-based binder was thus used to stabilize/solidify the waste. The other EDX
patterns shown in Figure 9.12 have calcium and silicon as the principal elements,
which is typical of cementitious material-based patterns. SEM of the waste form
from which the analysis B was obtained had abundant diatom. Diatomaceous earth
was thus used in this waste form. The XRD pattern of the waste form indicated the
presence of opal. The analysis B also has more silicon than calcium, which is unlike
portland cement. This observation also points to the fact that there is an additional

FIGURE 9.12 EDX microanalysis of waste forms of synthetic waste (As, Cd, Cr, Pb, phenol).