Leaching Processes and Evaluation Tests for Inorganic Constituent Release from Cement-Based Matrices - Stabilization and Solidification of Hazardous, Radioactive, and Mixed Wastes - page 242

massive pours, and radioisotopic heating) and if the active aggregate phases are
present in the waste (e.g., soil remediation or including aggregate for filling and
closing empty tanks).

10.3.3.2 External Stresses

Alterations due to external stresses tend to be more dramatic and occur more readily
than those from internal stresses. External stresses may be caused by contact with
liquids and gases or by changes in environmental conditions (e.g., temperature).
Contact of an S/S matrix with a leachant leads to depletion of matrix constituents
70,105-114 and potential degradation through sulfate attack 101,115-123 or carbon-
ation. 8,28,124-142 When S/S materials are stored in unsaturated environments at relative
humidity < 100%, additional degradation may occur due to moisture transport. 143-
149 The above phenomena have been extensively studied with regard to structural
materials or S/S matrices, so only a brief overview is presented with regard to
leaching and durability effects.

10.3.3.2.1 Dissolution and Leaching
Dissolution of calcium hydroxide due to leaching results in (i) decreased pore water
alkalinity, (ii) reduction in buffering capacity, and (iii) a shift in pH-dependent
constituent solubility. Complete depletion of calcium hydroxide in cement paste has
been associated with a 13% increase in macroporosity 70,76,106 and a 70% decrease in
compressive strength of the matrix. 70,108 Mineral phase depletion is accelerated by
direct exposure to acidic leachants (e.g., acetic acid, nitric acid) 75-77 or to acid-
forming species (e.g., ammonium nitrate, 105,106 carbon dioxide. 69 )

10.3.3.2.2 Sulfate Attack
Sulfate attack is a complex process that primarily is attributed to ingress of sulfate
salts; however, the mechanism of degradation is not completely understood and
depends on the speciation of sulfate (e.g., Na 2 SO 4 , MgSO 4 , (NH 4 ) 2 SO 4 ). 101,150 Two
commonly assumed mechanisms of sulfate attack are the expansive reactions: (i)
sulfate ions and free lime combine to form gypsum, and (ii) gypsum and hydrated
calcium aluminate combine to form ettringite. 101,117-119 In the presence of magnesium
ions, these chemical reactions are accompanied by conversion of C-S-H to magne-
sium silica hydrates. 117,118 Confounding the above mechanism may be other mech-
anisms typically not considered “sulfate attack,” including hydration expansion of
sulfate salts and increased osmotic pressures due to sulfate salt crystalization. 101
The assumed cause of physical deterioration is mechanical pressure resulting
from differential expansion of precipitates in pores. 118,151 The presence of gypsum
and ettringite in the microcracks implies that the physical damage results from the
formation of expansive minerals, but both the physical damage and the crystals may
be caused by desiccation. 101 Physical degradation due to sulfate attack may manifest
in (i) loss of cohesion and strength, (ii) expansion and cracking, and (iii) scaling
and shelling of the surface in successive layers. 122,152,153 Chemical alteration is due
to consumption of calcium hydroxide, decalcification of C-S-H, and precipitation
of sulfate minerals. 101,118,154,155