Interactions between Wastes and Binders - Stabilization and Solidification of Hazardous, Radioactive, and Mixed Wastes - page 152

e.g., CaCl 2 or gypsum, that are used as additions and admixtures in conventional
cement and concrete applications (see also Chapter 8), but also contain many dif-
ferent compounds that would not otherwise be blended with cement, including
harmful contaminants. In the context of this chapter, all components of wastes are
considered “impurities,” i.e., potentially reactive materials that would not ordinarily
be present in a commercial cement.
In S/S, interaction of wastes with binders is of interest from two main stand-
points:

1. Interferences of impurities with cement hydration, including setting and

strength development, and matrix durability

2. Immobilization of contaminants

These aspects are linked, in the sense that development of a strong, durable
matrix of low permeability is usually important for contaminant immobilization, and
is therefore a goal of S/S treatment. The focus of this chapter is on both beneficial
and deleterious interactions that can occur between waste components and hydraulic
binders (“cement”).

7.2 INTERFERENCES WITH BINDER HYDRATION

7.2.1 I NTRODUCTION

The detailed process of cement hydration still remains a subject of intensive study
and differs for different types of cement, as discussed in Chapter 4. In general, it
proceeds by dissolution of the anhydrous cement phases from the surface of the
cement particles in the mixing water, followed by precipitation of hydration products
to build a strong cohesive matrix. Impurities may alter normal cement hydration at
different stages; therefore, different compounds have different, and sometimes mul-
tiple, mechanisms of interaction with the binder. By changing the chemistry of the
evolving pore solution with respect to composition, pH, and ionic strength, impurities
can change:

• The solubility of the anhydrous phases

• The dissolution kinetics of the anhydrous phases

• The nucleation rate of the hydration products

• The growth rate of the hydration products

• The composition of the hydration products

• The morphology of the hydration products

These effects are often concentration dependent and can vary according to the
curing conditions. When impurities with different interference mechanisms as pure
compounds are combined, as is likely to occur in both waste utilization and solidi-
fication applications, the overall effect can be difficult to predict. 3
Setting and hardening are physical manifestations of the chemical process of
hydration of cements. Taylor 4 defines setting as “stiffening without significant