Environmental Engineering Reference
In-Depth Information
chromium, technetium, and nitrate in the waste due to three mechanisms: by decreas-
ing the permeability of the waste form, reducing Cr
6+
and Tc
7+
to Cr
3+
and Tc
4+
, and
precipitating them in the form of Cr(OH)
3
and TcO
2
.
Akhter et al.
19
used various combinations of Type I portland cement, Class F fly
ash, blast furnace slag, lime, and silica fume as additives to stabilize/solidify soils
containing heavy metals at concentrations ranging from 10,000 to 12,200 mg/kg. A
blended cement consisting of 85% GBFS and 15% portland cement was used to
stabilize steelmaking flue dust in two consecutive pelletization stages.
20
The first
involves pelletizing the steelmaking flue dust and the second involves coating these
pellets with the blended cement. The results showed that a 2-mm thin coating could
stabilize the dust classified as a hazardous product.
4.2.3
P
ORTLAND
P
OZZOLAN
C
EMENT
4.2.3.1
Introduction
According to ASTM Standard Specification C618, a pozzolan is defined as “siliceous
or siliceous and aluminous materials which in themselves possess little or no cemen-
titious value but will, in finely divided form and in the presence of moisture,
chemically react with calcium hydroxides at ordinary temperatures to form com-
pounds possessing cementitious properties.”
4.2.3.2
Hydration and Microstructure of Portland Pozzolan
Cements
When portland pozzolan cement contacts water, portland cement hydrates first; the
lime released from the hydration of portland cement reacts with pozzolan to form
new products depending on the composition of the pozzolan and hydration condi-
tions. The main hydration product in a lime-pozzolan mixture is C-S-H with a C/S
ratio less than 1.5, depending on the local concentration of reactants.
23
The aluminate
in the pozzolan may yield a variety of hydrates: calcium aluminate hydrate (C
4
AH
19
),
gehlenite hydrate (C
2
ASH
8
), AFt (C
3
A.3CaSO
4
.32H
2
O), and AFm
(C
3
A.CaSO
4
.12H
2
O). Carbon dioxide (CO
2
) can be combined in tetracalcium alu-
minate hydrate (C
3
A.CaCO
3
.12H
2
O).
24
The pozzolanic reactions can be generally
expressed as follows:
Ca(OH)
2
+ SiO
2
+ (n-1)H
2
O → xCaO.SiO
2
.nH
2
O
(4.7)
Ca(OH)
2
+ Al
2
O
3
+ (n-1)H
2
O → xCaO.Al
2
O
3
.nH
2
O
(4.8)
Ca(OH)
2
+ Al
2
O
3
+ SiO
2
+ (n-1)H
2
O → 3CaO.Al
2
O
3
.2SiO
2
.nH
2
O
(4.9)
(1.5-2.0)CaO.SiO
2
.aq + SiO
2
→ (0.8-1.5)CaO.SiO
2
.aq
(4.10)
Because of the chemical nature of pozzolans, they can be very effective sorbents
for some contaminants. Pozzolans may be used in two different ways for S/S of
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