General Guidelines for S/S of Wastes - Stabilization and Solidification of Hazardous, Radioactive, and Mixed Wastes - page 17

phase is aqueous. This suspension can then be mixed with cement for solidification.
Surfactants or emulsifiers can be used also to disperse organic waste material in an
aqueous phase and then combined with cement for solidification .

2.4.3 P HYSICAL E NCAPSULATION OF C ONTAMINATED W ASTE AND

S OILS

Some contaminants remain soluble despite best efforts at chemical fixation. In these
cases, the mechanical barrier of the physical cement matrix plays the dominant role
in preventing leaching of these contaminants and their subsequent release to the
environment. As discussed above, the permeability of a hardened solidified waste
depends on its pore structure. Thus, a dense matrix ensures a good mechanical barrier
to migration of dissolved constituents. The initial porosity is determined by the water
to cement ratio (W/C). Taylor discusses the effect of W/C on the structure and
properties of portland cement paste and equations to calculate volumes (total paste,
hydration product, unreacted cement, and nonevaporable water) and porosity (total
water, free water, and capillary) as a function of W/C. 23 As cement hydrates, hydra-
tion products fill the voids between cement particles. According to Powers, the
volume of hydration products is about 1.6 times the volume of its constituents. 24
Two types of pores form within the paste: gel pores and capillary pores. Gel pores
constitute about 28% of the total C-S-H gel volume with a size of 1.5 to 2.0 nm
(too small to permit the flow of water, with its molecular diameter of about 0.3 nm
combined with the strong hydrogen bonding between water molecules). The capillary
pores depend on the initial W/C and determine the movement of water, as shown in
Figure 2.3. 25 Figure 2.4 shows the effect of the cement paste age (W/C = 0.51) on
the d'Arcy's permeability coefficient, 26 and Table 2.2 shows the approximate age
required for discontinuity of the capillary pores in cement pastes as a function of
W/C. 25 At a W/C of 0.7, the continuous capillary pores within cement pastes are
never segmented.

2.4.4 L EACHABILITY OF C ONTAMINANTS FROM THE T REATED W ASTE

OR C ONTAMINATED M ATERIALS

Leaching happens when the contaminants in a treated waste form come in contact
with a leachant. The manner in which this contact occurs is determined by the
properties of the waste form such as water content, pore structure, homogeneity, and
d'Arcy's permeability or hydraulic conductivity. Leaching can take place through
convection or diffusion and depends on the physical and chemical fixation of con-
taminants. Chapter 10 discusses leaching mechanisms. One of the main goals of S/S
formulation design is a leach-resistant waste form.

2.4.5 D URABILITY OF THE T REATED W ASTE OR C ONTAMINATED

M ATERIALS

Direct durability concerns for treated hazardous wastes or contaminated materials
include stability after immersion in water, resistance to wetting/drying cycles, resistance