Civil Engineering Reference
In-Depth Information
To obtain strengths that are still acceptable, even though reduced, it appears that the
amount of calcium carbonate in the raw meal should be reduced only to levels at which
some alite can still be formed in the burning process. It was found that such an approach
distinctly increases the strengths obtained, compared with clinkers in which this phase is
completely absent. As an alternative solution it was suggested that belite cements with
acceptable strength properties could be produced by blending an alite-free clinker, made
with a very low lime saturation factor, with limited amounts of another clinker, whose
composition corresponds to ordinary Portland cement and which may serve as a source of
the required alite (Ludwig and Pöhlman, 1986).
To improve the strength development of belite cements it has also been recommended
that small amounts of BaO should be added to the raw meal (Rajczyk and Nocun-
Wczelik, 1992; Rajczyk
et al.,
1992; Rajczyk, 1997). This oxide acts as a dopant for C
2
S
and increases its reactivity.
I
n addition to the C
3
S/C
2
S ratio, the compressive strength of belite cements also
depends on the Al
2
O
3
/Fe
2
O
3
ratio in the interstitial phase (Ikabata, 1997), though to a
lesser extent. Up to about 28 days the strength of the cement tends to increase with an
increasing value of this ratio.
Belite cements may be used in applications in which high early strength is not
essential. They also meet the requirements for cements with low hydration temperature.
3.3
ACTIVATED BELITE CEMENTS
Activated belite (or belitic) cements differ from “normal” belite cements by the presence
of a belite phase whose reactivity has been enhanced by specific measures. The main
purpose of such activation is to improve the strength development of the resultant
cement, especially at shorter hydration times.
It has been found that very effective activation may be achieved by combining an
increased alkali content of the raw meal and a very rapid cooling of the clinker formed, in
particular in the temperature range 1300-900°C (Stark
et al,
1979, 1986; Gies and
Knöfel, 1986; Lampe, 1986; Milke, 1992). In this way the
α
′
-C
2
S phase, which is more
reactive than
β
-C
2
S and exists in the resultant clinker at high temperatures, is at least
partially preserved in the cooled product. Moreover, defects are introduced into the
crystalline lattice, which increase the reactivity of the dicalcium silicate present even
further. To effectively prevent the
α
′
-C
2
S
→
β
-C
2
S conversion, cooling rates of up to 1000
K/min were found to be necessary. About 5% of alkali oxides and other foreign ions are
typically present in the stabilized
α
′
-C
2
S phase.
Another dopant that may be considered for increasing the reactivity of belite cements is
SO
4
2−
ions (Gies and Knöfel, 1987; Stark
et al.,
1987). Under these conditions the final
clinker contains the
β
-C
2
S rather than the
α
′
-C
2
S phase, even if high cooling rates have
been employed; however, the SO
4
2−
-doped form of
β
-dicalcium silicate is much more
reactive than its SO
4
2−
-free counterpart. The doped C
2
S typically contains about 3% each
of SO
3
and Al
2
O
3
in its crystalline lattice. If an SO
3
-doped belitic clinker that also
contains some alite is to be produced, the amount of SO
3
in the raw meal must not exceed
