Microstructure and Microchemistry of Waste Forms - Stabilization and Solidification of Hazardous, Radioactive, and Mixed Wastes - page 217

Most IR observations are made in transmission with potassium bromide pellets;
the diffuse total reflectance method requires less difficulty in sample preparation,
however. The moisture content of the powder should be as low as possible, which
otherwise makes KBr (potassium bromide) pellet preparation difficult and dominates
the spectrum. Powdered samples are usually dried in an oven at 105°C. In cemen-
titious materials, this process would dehydrate ettringite and gypsum and will also
affect other low-thermal-stability hydrated phases. An alternative to this drying
process would be drying over P 2 O 5 and silica gel in a desiccator.
Figure 9.11 shows the FTIR spectra of a series of lime-Class C fly ash (LFA)
mixtures containing a heavy metal (Ni, Cr, Cd, and Hg) sludge and increasing amounts
of an interference, copper nitrate. The quartz (from the Class C fly ash) bands are strong
in the lime-fly ash mixture without the sludge and any interference. The 796- to 781-
cm -1 doublet of quartz can be usually traced through all the samples even after the
sludge and interference were added. In hydrated cements, the silicate (Si-O-Si anti-
symmetric stretching, γ3 ) band is typically present as seen around 970 cm -1 . Such a band
is present in the LFA, with sludge but with or without copper nitrate at 968 cm -1 . The
position of the band stays constant to up to 5% copper nitrate concentration, indicating
that the copper nitrate is not affecting the degree of polymerization of the calcium
silicate hydrate. The band cannot be observed in the presence of 8% by weight of copper
nitrate. There is a band at 943 cm -1 , which, if it corresponds to the Si-O-Si band in the
C-S-H, would indicate a remarkable change in its structure, getting less polymerized.
Even after drying at 105°C, the broad water band around 3500 cm -1 stays strong with
8% copper nitrate, indicating that moisture is strongly sorbed to the binder. In all other
specimens this band is significantly reduced. The peak for molecular water around 1640
cm -1 is prominent when the sludge and the copper nitrate are present. This peak
maintains the same intensity with up to 5% copper nitrate, but is particularly strong for
8% copper nitrate by weight.
FIGURE 9.11 FTIR spectra of a heavy metal (Ni, Cr, Cd, Hg) sludge in lime-Class C fly
ash for 0, 2, 5, and 8 wt% Cu(NO 3 ) 2 (the spectra for lime-fly ash alone is shown for
comparison).