Leaching Processes and Evaluation Tests for Inorganic Constituent Release from Cement-Based Matrices - Stabilization and Solidification of Hazardous, Radioactive, and Mixed Wastes - page 238

When L/S ratios are large or infiltration rates are rapid, the Peclet number may
be greater than 10 and the rate of release is assumed to be controlled by mass
transport through a solid matrix. This scenario occurs with monolithic materials or
when granular material is overlain by low permeability covers or surrounded by
material with much greater hydraulic conductivity. Typically release is estimated by
evaluating mass transport into an infinite bath with long-term release limited by the
available content.

10.3.2.3 Groundwater Composition

In near-surface or above-ground applications, the solid material is most likely to be
contacted by rainwater or surface runoff. Although rainwater may contain sulfuric
acid due to industrialization, the total acidity of infiltrating rainwater is typically
much lower than the ANC of S/S materials. Other components (e.g., CO 2 , organic
matter, metals, salts) may be dissolved in the leachate as it percolates through the
soil zone. For subsurface application scenarios, the commonly anticipated leachant
is groundwater, which tends to be mineralogically “soft” in comparison to the high
ionic strength of the pore solution. In addition, biological activity, co-application
with other materials, reduction of Fe- and S-bearing species in soils or wastes, or
interaction with the surrounding environment may result in groundwater that contains
aggressive components.

10.3.2.3.1 Acids
The effects of acid attack on the chemical and morphological characteristics of
cement pastes have been well documented 69,74-77 and result in the formation of a
calcium depletion zone near the exposed surface surrounding a kernel of unreacted
cement paste. In the calcium depletion zone, porosity was significantly increased,
the Ca/Si was very low, and the pH showed a gradient between the surface and the
boundary of the re-mineralization zone.
Exposure of S/S materials to acids (e.g., through carbonation, chloride ingress,
or biological activity) reduces the buffering capacity and pH of the S/S matrix. In
turn, the solubility of metal ions in solution may be increased or decreased. 25 Organic
acids may contribute to uncertainty of the leaching test through complexation with
contaminants (e.g., lead acetate) or incomplete dissociation.
ANC should not be confused with acid resistance. 65,67 The kinetics of mineral
dissolution are significant in acid resistance and are not accounted for in the ANC.
The highest ANC materials (e.g., portland cement) usually contain large amounts
of free lime that are easily dissolved in acidic solutions, while matrices with a high
content of C-S-H and a low Ca/Si ratio (e.g., activated blast furnace and fly ash/lime)
have a lower ANC but exhibit greater resistance to acid attack over time by creating
a siliceous protective layer on mineral surfaces. 65

10.3.2.3.2 Chelation with Organic Molecules
Leachants containing organic acids may have a secondary effect on the local solubility
of constituents in that ligands introduced by the acid dissociation may form highly stable
and soluble metal chelants within the pore solution. Ethylenediaminetetraacetic acid