Civil Engineering Reference
In-Depth Information
cement and the quality of the aggregate. In contrast, low-cement castables (LCC) contain
only about 5-8% of calcium aluminate cement, plus silica fume and reactive alumina.
These measures lead to a significant improvement of the refractoriness of the material. In
recent developments the cement content has been pushed even lower, to 3-5% (ultra-low
cement castables, ULCC), and silica fume has been replaced by ultrafine refractory
alumina. High-performance refractory concretes have in many cases replaced refractory
bricks, and their use is expanding.
In insulating refractory concretes the conventional aggregates are replaced by
lightweight heat-resistant aggregates such as expanded vermiculite, expanded perlite,
expanded chamotte, or pumice. These concretes are widely used to limit the heat losses of
industrial kilns and high-temperature installations.
In addition to high temperature, the refractory concrete to be produced must also be
resistant to possible chemical corrosion. Just as in applications at ordinary temperatures,
porosity—apart from the chemical nature of the solid phases—is the main factor that
controls the chemical resistance of refractory concrete. The same general principles as
outlined for the production of a good non-refractory, low-temperature concrete for civil
engineering applications must also be maintained in the production of refractory concrete.
One corrosive agent to which refractory concrete may be exposed during service is
carbon monoxide (CO). To ensure an acceptable resistance of hydrated calcium
aluminate cement to this agent, the cement must not contain iron oxide, which may
undergo chemical reduction to bivalent iron under these conditions. In situations in which
hydrogen may come into contact with concrete based on calcium aluminate cement, the
silica content of the binder may be critical, and thus should be kept low.
To improve the performance of calcium aluminate cement in high-temperature
concrete formulations, it has been suggested that it should be combined with water glass
(Krivenko
et al.,
1997). A binder of this type yields colloidal gibbsite (AH
3
) and
hexagonal calcium aluminate hydrates (CAH
10
and C
2
AH
8
) as products of hydration at
ambient temperature. Upon heating, these phases lose their chemically bound water, and
this process is followed by the formation of gehlenite (C
2
AS) at 800-1000°C. The
residual strength increases with increasing silicate modulus of the water glass
(SiO
2
/Na
2
O), and ranges between 60% and 100% of the initial strength.
23.3
PHOSPHATE-BONDED REFRACTORY CONCRETES
Phosphate-bonded refractory concretes are combinations of a phosphate binder and a
refractory aggregate. The hardening of the system takes place as the result of a chemical
reaction between these two constituents. To control the setting behavior small amounts of
a “setter” (such as magnesium chromate) may also be added to the mix.
Based on the temperature at which the setting and hardening takes place, one has to
distinguish between air-setting and heat-setting types of refractory concrete. In air-setting
concretes the setting/hardening process takes place at ambient temperature, and mixes of
this type may also be employed for other than high-temperature applications. In heat-
setting mixes an elevated temperature is needed to achieve setting, and the use of such
