Leaching Processes and Evaluation Tests for Inorganic Constituent Release from Cement-Based Matrices - Stabilization and Solidification of Hazardous, Radioactive, and Mixed Wastes - page 231

This chapter provides an overview of the leaching mechanisms, factors that
control constituent leaching from cement-based materials, and leaching evaluation
tests applicable to S/S materials. The discussion is limited primarily to inorganic
species in S/S waste placed in a disposal release scenario; however, with few
limitations, similar release mechanisms and environmental factors may apply for
organic contaminants in S/S waste forms, alternative applications of waste-cement
matrices (e.g., above-ground placements, beneficial reuse of wastes), and the broader
class of cementitious materials in general.

10.2 LEACHING MECHANISMS

In its most basic form, leaching involves contact of a solid matrix with a liquid
phase (leachant) into which constituents are released and transport, producing a
liquor (leachate). 6 The rate of constituent release to the environment depends on the
physical and chemical mechanisms of leaching including (i) geochemical control of
equilibrium, (ii) sorption and desorption, and (iii) mass transport of constituents into
the environment.

10.2.1 G EOCHEMICAL C ONTROL

Geochemical control of leaching occurs at the interface between solid and liquid
where the dissolution of mineral phases and solubility of constituents in the liquid
phase are coupled, pH-dependent processes. The release of a constituent in equilib-
rium with a liquid bath of fixed volume may be limited by the solid through
dissolution of solid mineral phases or by the liquid phase through constituent solu-
bility under the chemical conditions of the liquid phase (e.g., pH, temperature, or
the presence of other ions).
Although solubility may be affected by the rates of dissolution, local equilibrium
over short intervals of time is often assumed between solid phases and liquid
constituents. The magnitude of solubility is influenced by liquid properties including
pH and the presence of dissolved organic matter, complexing agents, or other ions.
For example, it is typically assumed that dissolution of portlandite, Ca(OH) 2 , pro-
vides the high alkalinity of cement pore solution, although cement pore expression
analysis indicates that the pore water pH is higher than the dissolution pH of
portlandite. The presence of alkalis, regardless of their speciation (e.g., NaCl, KOH),
raises the pore water pH and depresses the solubility of calcium. 7,8
When the solid phase limits the amount of constituent leached into the liquid
bath, the release is “availability-limited.” 5,9,10 Here availability is the maximum labile,
or leachable, fraction of a constituent under a particular set of chemical conditions.
Since this parameter is operationally defined (i.e., based on the determining chemical
conditions), it is usually determined at release conditions designed to maximize
release of oxyanions (e.g., AsO 4 2- , SeO 3 - ) and cations (Pb 2+ , Na + , Ca 2+ , Cd 2+ , Cs + ).
The conditions under which availability is determined should reflect the field con-
ditions that would realistically result in maximal release in order to best represent
leaching potentials. 5,9