Hydraulic Cement Systems for Stabilization/Solidification - Stabilization and Solidification of Hazardous, Radioactive, and Mixed Wastes - page 53

of metal ions by C-S-H, as discussed in Chapter 7. Successful treatment of metal-
bearing wastes by S/S may involve the following:

1. Control of excess acidity by neutralization

2. Destruction of metal complexes

3. Control of oxidation state

4. Conversion to insoluble species

5. Formation of a solid with solidification reagents

The portland cement system is a high-alkaline and porous material and can
address requirements 1, 4, and 5. In many cases, these requirements are enough to
stabilize/solidify some hazardous wastes. Compared with other binders, portland
cement has advantages of availability, low cost, and simple operation. However, it
also has some disadvantages that need to be addressed in some cases.
One concern is the final pH of the system. Chapter 7 provides detailed descrip-
tions of how pH affects the solubility and leachability of some heavy metals in waste
forms. The solubility of amphoteric metals, such as Cd, Cr, Cu, Pb, Ni, and Zn,
varies with pH, and the optimum pH range to precipitate them is about 10. Thus, it
seems that the pH of portland cement is not ideal for precipitating heavy metals; the
pH of the waste forms solidified with portland cement containing very low alkali
content and very high alkali content can be very different, which in turn can have a
significant effect on the leachability of heavy metals.
Portland cement contains some alkali; the amount is dependent on the raw mate-
rials and production process used. Usually, the pH value of the pore solution of a
hardened portland cement is over 12.5, maybe up to 13.5 due to the presence of
alkalis. A survey on characteristics of North American portland cements in 1994
indicates that the alkali content of portland cement can be as low as 0.05% by mass
and as high as 1.2% by mass. 10 Cement can release almost all its alkalis to the pore
solution, and the pore solution of concrete consists mainly of alkali hydroxide after
28 days of hydration. 11 Thus, the alkalis in cements have an important effect on the
pH of pore solution of solidified waste forms.
The relationship between the C/S ratio of C-S-H and equilibrium pH is shown
in Figure 4.1. There are two plateaus for the equilibrium pH. One corresponds to
around pH 12, where the C/S ratio is greater than 1.0. The other plateau corresponds
to pH 10, where the C/S ratio of C-S-H varies from 0.05 to 0.6. Typically, the C/S
ratio of C-S-H from the hydration of portland cement ranges from 1.4 to 1.7, which
is associated with a pH that is not ideal for the retention of heavy metals. To produce
a porewater with a pH value that minimizes solubility of metals, the C/S ratio of
C-S-H in a hydrated cement paste should be lower than 1.
Another potential disadvantage of using pure portland cement is that many
inorganic and organic contaminants interfere with the hydration and setting of
portland cement, as discussed in Chapter 7. Furthermore, portland cement cannot
destroy metal complexes or control the oxidation state of metals as needed. Thus,
portland cement cannot be directly used as an S/S agent alone when these issues
are concerns. The following sections discuss some portland cement-based cementing
systems containing various additives, which have also been used for S/S of wastes.