Civil Engineering Reference
In-Depth Information
possible, the maximum acceptable value being
w/c
=0.40. The cement content should not
be less than 400 kg/m
2
. In this way the strength loss due to conversion may be minimized
and a sufficiently low porosity of the cement paste may be secured, even after
conversion.
3. It has to be kept in mind that intensive hydration heat liberation will start within
hours after mixing. This may be especially critical in situations where dissipation of the
heat is hampered by the large mass of the concrete. In thin, non-loadbearing members,
cooling by spraying with cold water for at least 2-3 days may be sufficient to control this
problem, whereas in the case of mass concrete a cooling system must be used. On the
other hand, the danger of damaging the fresh concrete mix by frost, even at very low
ambient temperatures, is very low if calcium aluminate cement is used as binder.
Legal restrictions, high price, and concerns about long-term performance limit the
widespread use of calcium aluminate cement for construction purposes. Its use may be
attractive in applications where rapid strength development is required, as in the
production of prefabricated concrete elements. In applications in which only a limited
service time is planned, calcium aluminate cement may be used without concern.
10.11.2
High-temperature applications
The fact that hardened calcium aluminate cement pastes preserve much of their strength
up to high temperature makes this cement suitable for use in high-temperature
applications. In principle, calcium aluminate cement systems may be used up to the
temperature at which a liquid phase starts to be formed in the material, and that depends
on the composition of the cement. In general, the temperature of melt formation increases
with increasing alumina content of the cement, whereas its iron content affects this
temperature unfavorably. Thus a white, high-Al
2
O
3
, low-(Fe
2
O
3+
FeO) cement should be
used for applications in which the material will be exposed to particularly high
temperatures. Even higher service temperatures may be achieved by the use of barium
rather than calcium aluminate cement, but cements of this type are not commercially
available at present.
In addition to the binder, the maximum allowable service temperature of refractory
concrete is determined by the aggregate employed. Thus an aggregate capable of
sustaining the maximum anticipated temperature must be selected.
10.11.3
Calcium aluminate cement based rapid-setting/hardening binder
By combining calcium aluminate cement with 5-20% of Portland cement or Portland
cement with 5-20% of calcium aluminate cement a binder may be obtained that exhibits
a very short setting time and rapid strength development. Of these two alternatives the
latter is more widely used, not least for economic reasons.
The rate of setting and hardening may be controlled by the mutual ratio of both
cements; it increases with increasing proportion of the minor constituent, until a plateau
is reached. Further acceleration may be obtained if a small amount of a lithium salt, such
