Environmental Engineering Reference
In-Depth Information
FIGURE 4.1
Relationship between C/S ratio and equilibrium ph (based on reference 12).
4.2.2
P
ORTLAND
B
LAST
F
URNACE
S
LAG
C
EMENT
Blast furnace slag is a by-product from iron production. To control the iron quality,
the raw materials are carefully controlled and the range of chemical composition of
slag is fairly narrow for a specific ore and furnace operating conditions. However,
the chemical compositions of slag vary with different ores and furnace operating
conditions. It can be represented in the quaternary diagram CaO-SiO
2
-Al
2
O
3
-MgO
and may vary within a wide range: CaO — 30 to 50%, SiO
2
— 28 to 38%, Al
2
O
3
— 5 to 24%, MgO — 1 to 18%, S — 0.4 to 2.5%, Fe
2
O
3
— 0.3 to 3%, MnO —
0.2 to 3%, TiO
2
- < 4%, Na
2
O+K
2
O - < 2%.
Ground granulated blast furnace slag (GBFS) has been widely used as a portland
cement replacement due to its low cost and beneficial effect on some properties of
the corresponding concrete. The use of GBFS for S/S of wastes may also have
several beneficial functions: (1) it decreases the pH value of the initial pore solution
to ~11, which increases the precipitation of some heavy metals; (2) it lowers the
oxidation-reduction potential, which reduces the solubility of most radionuclides
and the corrosion of steel containers;
2
(3) it precipitates some metals as sulfides,
which are even more insoluble than the corresponding hydroxides; and (4) it reduces
the permeability of the waste form.
The standard oxidation potential, E
h
, measured for cement-slag mixes drops
precipitously to become reducing at slag contents above 60% by mass.
13-16
Slag-
based cement effectively stabilizes chromates even when the waste loading
approaches 90% by mass or higher,
17
which suggests that Cr(VI) is reduced to Cr(III).
In addition to providing a reducing environment, slag may precipitate metals as
sulfides that are even more insoluble than the corresponding hydroxides. Soluble
inorganic mercury compounds are not stabilized effectively in a portland cement
matrix. However, the use of more than 10% slag in the binder can greatly decrease
the extracted Hg during TCLP testing.
17
A slag-based waste form was designed to solidify/stabilize low-level radioactive
alkaline salt solution at the Savannah River Plant.
18
The use of blast furnace slag
together with portland cement and fly ash significantly reduced the release of
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