Leaching Processes and Evaluation Tests for Inorganic Constituent Release from Cement-Based Matrices - Stabilization and Solidification of Hazardous, Radioactive, and Mixed Wastes - page 243

10.3.3.2.3 Moisture Transport
In the absence of thermal gradients, the change in moisture content of hygroscopic,
porous materials (e.g., cement, concrete) has been described empirically as non-
linear moisture transport 156-160 and also using a two-regime mechanism. 148,161 Liquid
water moves under the influence of capillary and osmotic pressures, while water in
the vapor phase is transferred due to gradients in relative humidity. The primary
physical effect of moisture transport is cracking of the matrix due to local desiccation.
Perhaps more importantly, drying facilitates cement degradation by allowing reactive
gases (e.g., CO 2 , O 2 ) to penetrate into the pore system through the vapor phase.
10.3.3.2.4 Carbonation
Carbonation is arguably the most common chemical degradation process, with a
strong potential to influence physical and chemical properties of S/S materials, 162-
164 because of resultant changes in pore water pH, constituent speciation, and pore
structure. The carbonation reaction may be caused by attack of carbonic acid under
saturated conditions; 69,140 however, changes in physical and chemical properties are
more closely associated with atmospheric carbonation under drying condi-
tions. 135,163,165-167 Atmospheric carbon dioxide diffuses into the pore vapor, dissolves
into the receding pore solution film, and reacts with aqueous cations, producing
carbonate precipitates and water. 26,28 The primary product of the carbonation reaction
is calcium carbonate due to the high calcium content of the cement pore water;
however, other metal cations have been observed. 8
The rate of atmospheric carbonation may be limited by the rate of moisture
transport or the diffusion of atmospheric CO 2 through the pore vapor space. 28,168
Cement mineralogy plays an additional role in that penetration of a carbonate front
depends on the availability of “carbonatable” ions (i.e., slag cements produce less
calcium hydroxide, leading to greater carbonate penetration). 169 Thus, the carbon-
ation effect may be controlled by optimization of supplementary cementing mate-
rials. 168,169
The deleterious effects of atmospheric carbonation include (i) neutralization of
the system to pH values < 10, 8,132 (ii) decalcification and polymerization of the C-
S-H, 26,69 (iii) decomposition of ettringite, 137,167 and (iv) decreased retention of oxy-
anionic species. 8,134,170 Conversely, carbonation may benefit S/S treatment through
(i) conversion of easily soluble calcium hydroxide into more stable calcium carbon-
ate, (ii) densification of the microstructure (e.g., pore blocking, capping, or diameter
reduction), 132,165 (iii) marked increases in compressive strengths, 26,134,135,171 and (iv)
enhanced retention of metal cations in some S/S materials. 134,135,165,167

10.4 OVERVIEW OF LEACHING TESTS

In general, leaching tests measure leaching or release from a representative sample
of the subject matrix (sample) during contact with a liquid phase (leachant) under
a set of controlled leaching conditions (e.g., contact time, liquid-to-solid ratio, pH,
leachant composition). The liquid solution that results from a leaching procedure
(leachate) is analyzed for physical and chemical properties in order to determine
information on constituent release from the material.