Environmental Engineering Reference
In-Depth Information
wastes. In the first way, pozzolan is blended with cement, then mixed with the waste
and water, if applicable, to stabilize/solidify the waste. In the second way, a poz-
zolanic material is mixed with the waste first to absorb certain contaminants, then
cement is added to stabilize/solidify the waste material. The use of some pozzolanic
materials as additives for S/S enhancement is discussed in Chapter 8. However,
pozzolanic reactions will happen in the same way as described above. As pozzolanic
reaction takes place, the contaminants absorbed on the pozzolan particles may be
partially released and leached out, depending on the nature of the pozzolan and
contaminants.
25
4.2.3.3
Portland Natural Pozzolan Cements
Several natural pozzolans such as zeolites, calcined clays, and volcanic ashes are
used for production of portland natural pozzolan cements.
26
No publication can be
identified on the direct use of blended or interground portland natural pozzolan
cements for S/S. However, zeolites and several clay minerals have been widely used
as sorbents for certain contaminants before portland cement is added. Chapter 7
provides detailed discussions on the use of these natural pozzolans as sorbents.
4.2.3.4
Portland Fly Ash Cement
Fly ash is a by-product of the combustion of pulverized coal in thermal power plants.
Fly ashes are heterogeneous fine powders consisting mostly of rounded or spherical
glassy particles. The composition of fly ash depends on the coal used, but also on
the various substances injected into the coal or gas stream to reduce gaseous pol-
lutants or to improve efficiency of particulate collectors. When limestone and dolo-
mite are used for desulfurization of the exit gases, CaO and MgO content in fly ash
will be increased. Conditioning agents such as sulfur trioxide, sodium carbonate and
bicarbonate, sodium sulfate, phosphorus, magnesium oxide, water, ammonia, and
triethylamine are often used to improve the collection efficiency.
Fly ash consists mainly of SiO
2
, Al
2
O
3
, Fe
2
O
3
, and CaO. According to ASTM
C618,
27
fly ash belongs to Type F if the (SiO
2
+Al
2
O
3
+Fe
2
O
3
) > 70%, and belongs
to Type C if 70% > (SiO
2
+Al
2
O
3
+Fe
2
O
3
) > 50%. The most abundant phase in fly
ashes is glass. Crystalline compounds usually account for 5 to 50% and include
quartz, mullite, hematite, spinel, magnetite, melilite, gehlenite, kalsilite, calcium
sulphate, and alkali sulfate.
28
High-calcium fly ash may contain appreciative amounts
of free CaO, C
3
A, C
2
S, CaSO
4
, MgO, and C
3
A.CaSO
4
. The x-ray diffraction (XRD)
technique is very useful in identifying these crystallized substances in fly ashes. A
broad diffraction halo, which is attributed to the glassy phase, always appears on
the XRD patterns of fly ashes.
Fly ashes are widely used to partially replace portland cement in concrete to
reduce materials cost and to improve some properties of the concrete.
29
The intro-
duction of fly ash will decrease the C/S ratio of C-S-H and increase the retention
of cationic contaminants in C-S-H. The pozzolanic reactions between lime and fly
ashes can be described using Equations 4.7 to 4.10.
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