Leaching Processes and Evaluation Tests for Inorganic Constituent Release from Cement-Based Matrices - Stabilization and Solidification of Hazardous, Radioactive, and Mixed Wastes - page 240

matter and other biologically active groundwaters tend to be depleted of oxygen
content and are likely to remain either reducing or only weakly oxidizing. 7,88 Fur-
thermore, reduced waste materials (e.g., anaerobic sediments, mining wastes, slags)
may also control the redox potential of S/S materials. This implies that a high pH,
low E h environment may be both obtainable and sustainable (i.e., re-oxidation pre-
vented) for many disposal scenarios. 7

10.3.2.4 Particle Size of the Disposed Fill

For granular materials that may be treated using S/S technologies (e.g., soils, ash
residues, slags), the average particle size and particle size distribution of the waste
material play important roles in determining the limiting release mechanism. Smaller
waste particles have a large surface area exposed to a leachant and a characteristic
distance (i.e., particle radius) for mass transfer. Thus, with all other conditions equal,
smaller particles should release constituents more rapidly than larger particles.
One significant advantage of S/S material is that encapsulation in a monolithic
matrix minimizes the surface area of the waste that is exposed to the environment.
Therefore, particle size of the waste itself is usually only a concern for physical
(e.g., aggregate-sized particles, rather than chemical reasons. Of course, the mono-
lithic attributes of S/S materials decrease with age-induced degradation of the phys-
ical structure and are only valid as long as the material durability holds. In one study,
aged S/S materials were broken down into a granular or soil-like consistency due
to exposure to an aggressive environment. 89 In this scenario, the surface-to-volume
ratio of the S/S material (i.e., the particle size distribution of the degraded zone)
becomes an important release parameter.

10.3.2.5 Temperature

Often, S/S materials are deposited deep in the subsurface where temperatures are
relatively constant at geographically localized values. For example, average temper-
atures encountered in municipal solid waste landfills range from 10 to 45°C while
temperatures for deep-well hydrofracture injection deposition range from 8 to
15°C. 12 Recent initiatives into beneficial reuse of treated wastes require S/S materials
to be placed in shallow burial and near-grade applications where temperatures may
vary both diurnally and seasonally with ambient conditions.
Under certain conditions, the temperature of the S/S material may be higher
than that of the surrounding environment. Sources of internal heat generation come
from hydration reactions or waste characteristics. Generation of heat due to cement
hydration may be significant for massive concrete elements; 90 however, S/S materials
usually are poured on a smaller scale. In addition, the heat of hydration is relatively
short-lived in comparison to long-term release assessment intervals. S/S materials
containing high concentrations of radioactive wastes may maintain high temperatures
(e.g., > 70°C) due to radioactive decay of nuclides retained in the cement. 91,92
High temperatures influence leaching properties by (i) accelerating ongoing
matrix reactions (e.g., hydration, adsorption, and dissolution) via Arrhenius effects
at temperatures ~60°C, 93 (ii) increasing the rates of constituent and moisture