Leaching Processes and Evaluation Tests for Inorganic Constituent Release from Cement-Based Matrices - Stabilization and Solidification of Hazardous, Radioactive, and Mixed Wastes - page 241

diffusion within the matrix 94,95 at temperatures > 20°C 96 , and (iii) decreasing
strength. 13,97 The effect of elevated temperature on mineralogy is somewhat ambig-
uous, since the stability of most mineral hydrates decreases with temperature,
whereas portlandite becomes more stable. 98 Blended cements, which typically do
not have high portlandite contents, have been shown to demonstrate increased
porosity and decreased degree of hydration when cured at high temperatures,
especially between 10 and 60°C. 49
At subzero temperatures, freezing and thawing may cause deterioration of
cement materials through (i) hydraulic pressures of capillary pore water freezing,
(ii) osmotic pressures in partially frozen pore water containing salts, or (iii) transport
of water from C-S-H gels. 16 If the resultant internal stresses exceed the tensile
strength 13 or fracture strength 16 of the material, pressures are relieved by cracking.

10.3.3 M ATERIAL A GING

Cement-based materials may be considered metastable in the sense that the proper-
ties of the material do not remain constant but change as a result of aging. Klich et
al. 99 point out that, morphologically, the same physical and chemical mechanisms
that degrade structural concretes are responsible for deterioration of solidified waste
matrices. Thus, significant advances in the development of S/S systems may be made
after consideration of the structural cements durability literature. In general, degra-
dation of cement materials results from internal and external stresses of the matrix.
Internal stresses are caused by chemical reactions occurring in the absence of outside
influences, whereas external stresses may be induced through interaction between
the cement and its surrounding environment.

10.3.3.1 Internal Stresses

Typically, internal reactions are kinetically limited and occur on a geological time
scale. 7 Thus, the associated changes in morphology, mineralogy, and leaching prop-
erties due to internal stresses are difficult to assess without accelerating the reaction
mechanism. 100 Degradation mechanisms that lead to internal stresses include (i)
internal chemical reactions, (ii) delayed ettringite formation (DEF), and (iii)
alkali-aggregate reactions.
The mineralogical nature of cementitious wastes continues to change beyond the
28 days considered standard for physical hardening of cement materials. Full hydration
of cement clinker may take years to complete, especially if reactants must diffuse
through siliceous gels surrounding clinker particles. The dissolution of waste materials
may limit incorporation into developing C-S-H or calcium sulfoaluminates. The uptake
of water, via continued hydration or salt crystallization, can result in cracking due to
osmotic pressures, 101 self-desiccation, 26 or autogenous shrinkage. 102,103
DEF 39 and alkali-silica reactions 26,54,104 are common causes for cracking of
concrete products and structures. DEF is related to high-temperature curing of the
products, and alkali-aggregate reactions require the presence of alkali-reactive
aggregates. Thus, they seldom happen to cement-solidified waste forms, but may
need to be considered for higher internal temperatures (e.g., high cement contents,