Civil Engineering Reference
In-Depth Information
required. A potential problem in large-scale production may be the partial thermal
dissociation of CaSO
4
present in the raw meal, which may require appropriate measures
to control the emission of SO
3
. On the other hand, the emission of both CO
2
and NO
x
will be significantly reduced (Takuma
et al.,
1994).
To obtain sulfobelite cement the clinker that is produced must be ground to a specific
surface area similar to that of ordinary Portland cement. Gypsum or anhydrite must be
added only if the amount of anhydrite in the clinker is insufficient. The grindability of
sulfobelitic clinkers is better than that of typical Portland cement clinkers.
The initial stage of hydration of sulfobelite cement is dominated by a fast formation of
ettringite. A significant fraction of the sulfoaluminate phase is consumed within a few
hours of hydration (Mudbhatkal
et al.,
1986; Wang
et al.,
1992a; Kasselouri
et al.,
1995;
Ali
et al.,
1997). About 60-70% of it is consumed within the first 24 hours, and most but
not all within 28 days. Generally, the degree of hydration increases with increasing
amount of calcium sulfate in cement (Wang
et al.,
1992a). The initial hydration of the
cement may be accelerated by the presence of sulfate ions in the liquid phase. Aluminate
and calcium ions delay hydration by creating a retarding layer, which may be considered
a co-precipitate of aluminum and calcium hydroxide (Palon and Majling, 1996).
T
he C
2
(A,F) phase also hydrates very rapidly, yielding the AFt phase as a product of
reaction. A large fraction of the ferrite phase is consumed within the first day of
hydration (Kasselouri
et al.,
1995).
Initially, the hydration of C
4
A
3
progresses mainly according to equation (4.4) (section
4.1). However, if the dissolution rate of the calcium sulfate cannot keep pace with that of
the C
4
A
3
, the hydration may also progress partly according to reaction (4.3). Aluminum
hydroxide is formed simultaneously.
If non-reacted calcium sulfate is still present, after all the C
4
A
3
and C
2
(A,F) have
been consumed, the formation of ettringite may continue, though at a slower rate, in a
reaction between AH
3
, formed as a by-product in the hydration of C
4
A
3
, and calcium
sulfate and calcium hydroxide, formed in the hydration of C
2
S:
(4.9)
In cements in which the calcium sulfate content has been too low for complete
conversion of all the Al
2
O
3
present in the original cement to ettringite, a formation of the
AFm phase may get under way in a reaction between ettringite and AH
3
, after all the
calcium sulfate has been used up:
(4.10)
For optimum formation of ettringite, the amount of calcium sulfate that is dissolved in the
liquid phase must match the dissolution rate of C
4
A
3
and C
2
(A,F). This may be
controlled by the amount of calcium sulfate present, its form, and its degree of dispersion.
