Environmental Engineering Reference
In-Depth Information
Types I through V, respectively, except for the allowance of up to 5% limestone in
Type 10 and Type 30 cements. The European cement standard EN197 is very
different from ASTM C150 or CAS-A5, and classifies cement into CEM I, CEM
II, CEM III, CEM IV, and CEM V. These do not correspond to the cement types in
ASTM C150. CEM I is portland cement and CEM II through V are blended cements.
EN197 also has strength classes and ranges.
In the presence of water, C
3
S and C
2
S in cement hydrate to form calcium silicate
hydrate gel (C-S-H gel) and Ca(OH)
2
. In the presence of calcium sulfate, C
3
A hydrates
to form calcium trisulfoaluminate hydrate (3CaO.Al
2
O
3
.3CaSO
4
.32H
2
O - AFt or ettring-
ite), or calcium monosulfoaluminate hydrate (3CaO.Al
2
O
3
.CaSO
4
.12H
2
O — AFm or
monosulfate). In the absence of calcium sulfate, C
3
A reacts with water and calcium
hydroxide to form tetracalcium aluminate hydrate [3CaO.Al
2
O
3
.Ca(OH)
2
.12H
2
O].
C
4
AF reacts with water to form calcium aluminoferrite hydrates
(6CaO.Al
2
O
3
.Fe
2
O
3
.12H
2
O). These hydration reactions can be expressed as follows:
3(3CaO.SiO
2
) + 6H
2
O → 3CaO.2SiO
2
.3H
2
O + 3Ca(OH)
2
(4.1)
2(2CaO.SiO
2
) + 4H
2
O → 3CaO.2SiO
2
.3H
2
O + Ca(OH)
2
(4.2)
3CaO.Al
2
O
3
+ 3CaSO
4
.2H
2
O + 26H
2
O → 3CaO.Al
2
O
3
.3CaSO
4
.32H
2
O (4.3)
3CaO.Al
2
O
3
.3CaSO
4
.32H
2
O + 2(3CaO.Al
2
O
3
) + 4H
2
O →
3(3CaO.Al
2
O
3
.CaSO
4
.12H
2
O)
(4.4)
3CaO.Al
2
O
3
+ 12H
2
O + Ca(OH)
2
→ 3CaO.Al
2
O
3
.Ca(OH)
2
.12H
2
O
(4.5)
4CaO.Al
2
O
3
.Fe
2
O
3
+ 10H
2
O + 2Ca(OH)
2
→ 6CaO.Al
2
O
3
.Fe
2
O
3
.12H
2
O (4.6)
A hardened cement paste is a heterogeneous multiphase system. At room temper-
ature, a fully hydrated portland cement paste consists of 50 to 60% C-S-H gel, 20 to
25% Ca(OH)
2
, 15 to 20% ettringite (or AFt) and AFm by volume. These minor
hydration products, such as Ca(OH)
2
, 3CaO·Al
2
O
3
·6H
2
O, and AFt, form in small
quantities depending on the composition of the cementing material and hydration
conditions. The detailed description on microstructural characteristics of hardened
cement pastes can be found in many topics,
7a,7b
and is summarized in Chapter 9.
4.2.1.2
S/S with Portland Cement
Portland cement has been used alone for S/S of many wastes.
1
C-S-H gel is the main
binding component and responsible for the mechanical strength of hardened pastes.
It also plays an important role in S/S of contaminants. Komarneni et al.
8
found that
poorly crystallized C-S-H formed below 100°C
has cation ion-exchange and ion-
absorption properties. The lower the C/S ratio of C-S-H is, the more amount of cation
the C-S-H can retain. Bhatty
9
proposed several mechanisms for the immobilization
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