Interactions between Wastes and Binders - Stabilization and Solidification of Hazardous, Radioactive, and Mixed Wastes - page 159

collected and used to construct neural network models of unconfined compressive
strength (UCS) as a function of mix composition. 28 The following ranking was
determined for UCS values predicted for addition of the contaminants, on an equimo-
lar basis: at 7 days, Cl ~ Cr(III) > NO 3 - ~ Cd > control > Zn ≥ Ni > Pb > Cu >>Ba;
at 28 days, Cl > Cr(III) > NO 3 - ~ control ≥ Zn ≥ Cd > Ni > Pb > Cu >> Ba. Application
of the best neural network to other data suggested that Cs is a retarder and Cr(VI)
has no effect. No trends could be discerned for Hg, K, Mn, Na, and SO 4 2- with the
available data.
The effects observed in the literature for calcium aluminate cements, 17,29-45 which
are less commonly used in waste management due to the superior properties of
calcium silicate-based cements, are presented separately in Table 7.3. Unfilled areas
in the tables indicate an absence of available information. Some of the effects are
discussed in more detail in the following notes, which refer to the superscripts in
Table 7.3:

K. There is agreement in the literature that Li salts, with the single exception
of Li 2 BO 2 , are the strongest accelerators for calcium aluminate cements
of all types and are capable of causing flash setting. Li salts are thought
to act by creating nucleation substrates for the hydration products.
L. However, for the other anions and cations in Table 7.3, varied effects on
calcium aluminate cement have been reported. Several authors have tried
to rank the effects of various cations and anions, but there is no agreement
among them. 34,38 Other authors have tried to come up with general state-
ments to describe the behavior, but these tend to be subject to exceptions.
For instance, Parker 29 stated that alkalis are accelerators and acids are
retarders of calcium aluminate cement, yet sulfuric acid is known to be
an accelerator; Sharp et al. 46 suggested that compounds that increase the
C/A ratio act as accelerators, yet CaCl 2 is known to be a retarder.
M. The possible diversity of the effects is illustrated by observations for
calcium sulfate by Bayoux and coworkers, 43 who found gypsum to have
no effect on setting time of calcium aluminate cement, whereas hemihy-
drate was an accelerator and anhydrite a retarder. The authors suggested
that the relations between the dissolution kinetics of these different forms
of calcium sulfate controls both the kinetics and the products of hydration.

N. It has been suggested that CH acts as an accelerator for calcium aluminates

by increasing the C/A ratio of the solution, and driving precipitation of
the calcium-rich phases C 2 AH 8 and C 4 AH 13 . 46 portland cement is thought
to be an accelerator because hydration of C 2 S and C 3 S generates CH. 7

Again, neural network analysis was used to construct models of setting time as a
function of mix composition using existing data for pure compound additions to
calcium aluminate cements from 17 references. 47 The following ranking of induction
times was determined, on an equimolar basis: Ba > Ca ≥ Sr ≥ Mg ≥ K ~ Na > control
>> Li and citrate ≥ Br - >> SO 4 2- > C1 - > control ≥ CO 3 2- ≥ OH - > PO 4 3- ~ NO 3 - >
A1O 2 - . It was also shown that the reproducibility of setting time measurement is poor,
which may account for some of the differences in the findings in the literature.