Environmental Engineering Reference
In-Depth Information
29
Si spectra contain peaks, usually overlapping ones, that correspond to Si in five
different environments.
16
In describing these spectra the symbol Q represents a Si
atom surrounded by four oxygen atoms. A superscript following the Q shows the
number of other Q units attached to the Si atom under study. The principal trans-
formation occurring during cement hydration, as followed by NMR, is the formation
of chains containing Q
1
(chain-terminating) and Q
2
(chain-lengthening) units starting
from the orthosilicate ions (Q
0
) present in the cement clinker. The loss of Q
0
units
as a function of time is a measure of the overall degree of hydration of the cement
clinker and agrees well with calorimetric and other measurements of degree of
hydration.
17
It is well known that under acidic conditions, silicate species condense
to relatively long-chain polymers, while under basic conditions, which is what
pertains in cement hydration, the polymers are low-molecular-weight oligomers.
This is obvious in the NMR spectra from the relative proportions of Q
1
and Q
2
units.
The Si peaks are usually broad and often show shoulders, and the interpretation has
sometimes been made that the shoulders represent bonding of the type Si-O-M,
where M is a metal or metalloid atom taking the place of Si in the silicate oligomers.
Fly ash, kiln dust, and related pozzolanics undergo hydration reactions similar
to those in portland cement, and Si NMR, although more complex, is applicable in
these cases as well. Addition of wastes, even in relatively low concentration, can
have a major effect on these hydration reactions, altering the proportions of the Q
n
components, retarding or accelerating the reactions, or even completely inhibiting
silicate matrix formation.
18
In such cases
29
Si NMR is an extremely useful diagnostic
tool for determining whether effective cement-setting reactions have occurred.
Hydration of aluminate phases occurs more rapidly than that of the silicate
phases, and NMR spectroscopy can distinguish the chemical shift differences
between tetrahedral 4-coordinate and octahedral 6-coordinate Al atoms, as shown
initially for hydration of tricalcium aluminate.
15
In most samples of cement clinker
and other pozzolanic binders both 4- and 6-coordinate Al are present initially, and
different lots from different sources show wide variations. Nevertheless, hydration
almost always converts the mixture essentially completely to 6-coordinate Al. There
are, however, exceptions to this. A notable example is the reversion of 6-coordinate
Al to 4-coordinate after long curing times in a case of arsenic immobilized in portland
cement/fly ash mixtures.
19
The reversion is also associated with increased arsenic
leachability.
As curing takes place in the common binders that are formed via hydration reactions,
water distributes itself into a number of different environments that are characterized
by different nuclear magnetic spin-lattice relaxation times.
20,21
Again, the process can
be followed over time as water (or hydroxide) appears in three main phases (C-S-H,
calcium hydroxide, and pore waters with freely exchangeable protons), with a small
amount of a fourth. This process is sensitive to the presence of waste materials in the
curing matrix. For instance, a retardation of matrix formation in the presence of Cd
2+
is obvious in smaller proportions of the magnetization fraction arising from protons in
C-S-H and CH, and larger proportions from exchangeable protons.
22
Most NMR studies have focused on the effects of wastes on the matrix of a
cement or related material, and these effects are relevant to waste containment and
expected long-term stability of the waste form. However, NMR can also be used to
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