Interactions between Wastes and Binders - Stabilization and Solidification of Hazardous, Radioactive, and Mixed Wastes - page 153

development of compressive strength,” hardening as “significant development of
compressive strength.” Thus, interferences with cement hydration may result in the
following bulk effects:

• Acceleration/activation of setting or hardening (including flash setting, in

which the matrix loses its plasticity immediately upon mixing)

• False setting (in which matrix plasticity is lost quickly upon mixing, but

can be recovered by additional mixing)

• Altered water demand

• Retardation of setting or hardening (including complete inhibition of

hydration)

• Altered strength development (including matrix disruption)

• Altered pore solution composition

From a use standpoint, acceleration of setting and false setting may cause
handling difficulties, leading to equipment failure and poor workability. Increased
water demand, which may be caused by acceleration of setting or false setting, or
simply by altered rheological characteristics of the mix, can result in a more porous
structure and poor durability, as can retardation. Altered strength development is
often thought of as taking the form of increased or decreased early strength, but
later strength may also be affected, and even a product that appears to set and gain
strength normally at first can deteriorate rapidly later.
Numerous examples of interactions of impurities with cement, in either S/S or
cement and concrete products, may be found in the literature, and have also been
discussed in a number of review articles. 2,3,5-10 Findings from the literature are
summarized in the following sections, for inorganic compounds, organic compounds,
and other impurities. These results have been generated by experiments using pure
compounds; numerous case studies also exist in the literature for treatment of real
wastes. Because of the heterogeneity and complex structure and composition of real
wastes, it is more difficult to generalize about these, but an attempt to address this
issue is made in Section 7.2.5.
It should be noted that many of the observations in the literature are based on
studies of the physical manifestations of setting and strength development, or some-
times on measurements of heat evolved by hydration reactions, but it is stressed that
the effects discussed above do not simply change the rate of setting or hardening,
but often alter the hydration products. At the simplest level, retardation decreases
the quantities of the hydration products at a given time, whereas acceleration
increases them. However, impurities in cement pastes may also alter the proportions
of the hydration products, change the Ca/Si ratio of the C-S-H, create solid solutions
with the hydration products, or create entirely new hydration products. Several recent
studies have shown effects of wastes and contaminants on leachate pH and S/S
product acid neutralization capacity, which are indicative of changes to the hydration
products. 11-14 Such changes can have important consequences for immobilization of
contaminants by all of the mechanisms discussed in the following Section 7.3, but
this is an aspect of S/S of which there have been few investigations.