Environmental Engineering Reference
In-Depth Information
10.3.1.3
Acid Neutralization Capacity
ANC, a material parameter representing the ability of a matrix to resist a decrease
in pH, is a key retention characteristic of S/S materials. Resistance to pH change
is important, since the solubility of inorganic constituents and the kinetics of
mineral dissolution often are pH dependent. Typically, ANC of a material is
evaluated by plotting the steady-state pH response* as a function of predetermined
acid additions. Plateaus in the curve indicate zones of pH buffering due to mineral
phase dissolution.
The pH stability of pure mineral solids
62,63
and single-component cement
systems
64
have been resolved from ANC plateaus. Pure calcium oxide-silica-water
systems exhibit variable neutralization behavior with plateaus at approximately 12.3,
11.9, and 9.9, depending on their Ca/Si ratio.
65
Calcium hydroxide is often consid-
ered the primary contributor to the ANC of cement-based materials, and the ANC
plateau at pH 12.3 is typically associated with neutralization of free lime. However,
differential acid neutralization analysis
66,67
indicates the calcium hydroxide only
accounts for about 25% of the ANC to a pH of 10.
67
Above pH 11.6, C-S-H, ettringite,
monosulfate, hydrogarnet, brucite, and hydrotalcite also may contribute to ANC, but
below a pH of 9, the solid matrix is generally degraded to the point that these solid
phases do not exist.
68
In comparison to pure systems, the addition of waste or mineral
additives lowers the levels of pH plateaus or changes the number and magnitude of
the buffering zones.
65
Thus, analysis of ANC curves for cements with mixed mineral
phases and waste additives is quite complex, and accurate direct resolution of mineral
phases has not been possible.
67
A significant portion of the high acid resistance of cement materials is due to
the presence of low solubility C-S-H rather than the relatively more soluble calcium
hydroxide. C-S-H gel does not dissolve in the same manner as crystalline minerals
like calcium hydroxide, maintaining acid resistance by decalcifying to calcium
hydroxide and a siliceous gel of a lower Ca/Si ratio according to the stoichiometry
presented by Bonen and Sarkar.
69
x
CaO·SiO
2
·
n
H
2
O +
y
H
2
O → (
x
-
y
)CaO·SiO
2
·
n
H
2
O +
y
Ca(OH)
2
(10.1)
The decalcification of C-S-H typically occurs slowly, either co-currently or after
depletion of the calcium hydroxide phase,
69,70
and provides both buffering capacity
and long-term acid resistance.
10.3.2
R
ELEASE
C
ONDITIONS
Several release scenario parameters influence the rate of constituent leaching from
solidified materials in disposal placements. These factors include (i) rate and flow
regime of infiltrating fluid, (ii) the composition of the liquid phase in contact with
the solidified waste, (iii) the geometry and physical properties of the applied S/S
* Since the dissolution reactions within cement materials are irreversible, “steady-state pH response”
refers to the conditions during which no net change in pH occurs.
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