Environmental Engineering Reference
In-Depth Information
Since it is clear from the foregoing that cements respond, often dramatically, to
small additions of impurities, it is possible to use additives and admixtures to adjust
the hydration characteristics of an S/S product to mitigate undesirable effects of
waste components. This is discussed further in Chapter 8.
7.2.2
E FFECTS OF I NORGANIC C OMPOUNDS ON C EMENT
H YDRATION
Table 7.1 and Table 7.2 summarize the effects observed for a number of inorganic
and heavy metal compounds, respectively, on cement hydration, as described in the
literature. 2-6,8,9,12,15-24 The terms “acceleration” and “retardation” have been applied
to describe effects on setting, hardening, or both, as this is how they are used in the
literature. It should be noted that this is a somewhat misleading practice, as an
impurity may cause, for example, initial acceleration, followed by delayed hardening,
or vice versa. Conflicting reports about the effects of different impurities may be at
least partly attributable to this practice.
The information in Table 7.1 and Table 7.2 reflects the preponderance of inves-
tigations concerning portland cements in the literature, but may sometimes also be
true for other cement types, or blended cement systems. For example, it has been
shown that metal cations such as Zn 2+ , Pb 2+ , Cd 2+ , and Cr 6+ also decrease the strength
of alkali-activated slag cement. 25 Some of the effects are discussed in more detail
in the following notes, which refer to the superscripts in the tables:
A. As the most reactive phases in portland cement, and those most important
for setting and initial strength development, C 3 A and C 3 S are accelerators,
and also tend to be the focus of action by other accelerators and retarders.
Other reactive calcium aluminates or calcium aluminate cements are also
accelerators for portland cement, and vice versa. These species act as
activators or accelerators for pozzolans. Cement kiln dust or lime may
also be used as activators for pozzolans.
B. Alkali carbonates illustrate the unpredictable nature of some compounds
that interfere with the setting of cement. Low proportions of alkali car-
bonates (< 0.1%) have been found to retard the setting of portland cement;
an increased amount results in flash setting, and further increased amounts
may have no effect on setting, whereas flash setting occurs also at very
high proportions. The interference mechanism is not well understood, but
it has been suggested that the effect of carbonates is due in part to the
production of thaumasite [Ca 6 (Al,Si) 2 (SO 4 ) 2 (CO) 3 .(OH,O) 12 .24H 2 O],
rather than ettringite [Ca 6 Al 2 (SO 4 ) 3 (OH) 12 .26H 2 O].
C. In general, sodium and potassium salts are thought to raise the pH and
cause precipitation of amorphous CH that interferes with C 3 A hydration,
and yet many alkali salts act as accelerators of portland cement. They
may be added as activators to cements containing pozzolanic materials
(Chapter 4), where they increase the solubility of the unhydrated phases.
Some salts, such as NaCl, accelerate at low concentrations, but retard at
very high concentrations.
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