Microstructure and Microchemistry of Waste Forms - Stabilization and Solidification of Hazardous, Radioactive, and Mixed Wastes - page 214

presence of the waste. Evidence obtained from thermal analysis corroborates those
obtained by other techniques. For example, XRD would show in such cases the
presence of more unhydrated tri- and di-calcium silicates. MAS NMR would also
show more of these phases.
The pH of a cement-based waste form may well depend on the free calcium
hydroxide. If calcium hydroxide is present the pH will be buffered at around 12.6. The
amount of calcium hydroxide can be monitored by TG or XRD, the former being used
more. Apart from calcium hydroxide, ettringite (pH ca. 10.7) and the calcium silicate
hydrate (pH 9) can also buffer the pH of a cementitious system. The amount of ettringite
can be potentially quantified and its effect on the pH can be measured.

9.5.5 XRD

XRD is a widely used analytical tool in many branches of science and engineering.
It is typically used for phase identification. With careful sample preparation and
proper experimental conditions, however, the phase proportions can be quantified
and important information about particle size (diffracting domain size) can be
obtained. XRD is a relatively rapid technique and is widely available.
A finely powdered sample of a waste form can be scanned over a large angle
to obtain a graph of angle (2θ) versus diffraction intensity. A huge data base,
published by the Joint Committee on Powder Diffraction Standards-International
Center for Diffraction Data (JCPDS-ICDD) and containing XRD patterns of tens of
thousands of pure phases, can be used to match the peaks in an unknown pattern.
Figure 9.10 shows the diffraction patterns of a waste form. The basic waste form
contains an electroplating metal sludge (consisting of nickel, chromium, cadmium,
and mercury) in a Class C fly ash and calcium hydroxide matrix. An increasing
amount of zinc nitrate was added to the waste form. The diffraction pattern of the
sludge is also included in the figure for comparison. Aluminum powder was added
to all samples as an internal standard to correct for 2θ errors.

FIGURE 9.10 XRD of electroplating sludge (Ni, Cr, Cd, Hg) in a Class C fly ash-Ca(OH) 2
matrix with 0, 2, 5, and 8 wt% zinc nitrate (the untreated sludge XRD is shown for comparison).