Leaching Processes and Evaluation Tests for Inorganic Constituent Release from Cement-Based Matrices - Stabilization and Solidification of Hazardous, Radioactive, and Mixed Wastes - page 235

and elevates pore water pH towards a value of 14. 7,8,26 As shown in Chapter 7, the
solubility of amphoteric metal hydroxide cations passes through a minimum in the
pH range 9 to 11, rising sharply with pH thereafter. Thus, pore water conditions
dominated by dissolution of pure Ca(OH) 2 are not optimal for the fixation of many
priority pollutants.
C-S-H consists of amorphous gel-like phases, which comprise more than 60
wt% of cement paste, 29 and has a variable stoichiometry with different calcium to
silica ratios depending on the cement compositions and the hydration conditions. 32-35
Due to its high surface area and amorphous properties, several researchers report
that C-S-H is a principle mineral phase for adsorption of metal cations and
anions. 29,32,35-37 However, the retention properties of C-S-H gel vary with Ca/Si ratio
in that the surface charge of C-S-H gel goes through a point of 0 net charge at a
Ca/Si ratio of approximately 1:2. 7 Therefore, anions adsorbed to the positively
charged surface of C-S-H with a high Ca/Si ratio may be released in favor of cation
adsorption to the negative surface charge of C-S-H with a low Ca/Si ratio during
decalcification of C-S-H gels.
Calcium sulfoaluminates, such as ettringite (3CaO•Al 2 O 3 •3CaSO 4 •32H 2 O) and
monosulfate (3CaO•Al 2 O 3 •CaSO 4 •12H 2 O), coexist to varying degrees at different
stages of the hydration/aging process. 38,39 Chapter 7 discusses the incorporation of
cations 29,35,40 and oxyanions 7,29,32,41-43 as well as the stability of calcium sulfoaluminates.
Supplemental cementing materials such as blast furnace slag, fly ash, and silica
fume are often used in S/S treatment recipes to modify hydration products and matrix
properties. Blended cement matrices exhibit finer, less continuous pore structure, 44,45
higher compressive strength, 46-49 and increased capacity for contaminant adsorption
and incorporation. 45,50,51 Chapter 4 discusses the hydration and microstructure of
blended cements containing blast furnace slag, fly ash, and silica fume.

10.3.1.2 Hydraulic Conductivity

Hydraulic conductivity is a material property representing the ability of a fluid to
flow through a porous material. The hydraulic properties of a material primarily
depend on the capillary porosity, pore-size distribution, and connectivity of the pore
structure. 52-55 In turn, the pore structure is a function of the water/cement ratio, the
presence of hygroscopic admixtures, and curing conditions. 56 The Katz-Thompson
model 57 uses percolation theory to correlate water permeability to the pore structure;
however, conflicting research has found this model is either valid 58,59 or invalid 58,60
for predicting measured permeability in cement-based materials.
Experimentally determined values of water permeability for mature portland
mortar range from 10 -12 to 10 -13 m/s, while blended cements tend to exhibit even
lower values. 26,44,58,61 Stegemann and Côté 12 state that for low permeability materials
(< 10 -9 m/s), infiltration of water is negligible and the rate of constituent release is
expected to be limited by the diffusion of species to the surface. Thus, it can be
expected that the rate-limiting release mechanism for undamaged monolithic S/S
materials in most disposal scenarios will be mass transport of constituents through
the solidified material.