Civil Engineering Reference
In-Depth Information
igneous rock under tropical conditions, and is rich in alumina; it may also contain
variable amounts of iron. For the production of high-grade calcium aluminate cements
with CaO+Al
2
O
3
contents of up to 95%, calcined alumina in combination with high-
purity lime or limestone must be employed.
C
alcium aluminate cement with a lower or intermediate Al
2
O
3
content cannot be
produced in rotary or shaft kilns of the type common in the manufacture of Portland
clinker. The temperature range between incipient melting and complete fusion of the raw
mixes is too narrow to permit successful clinkerization, in which a melt and solid phases
must coexist. Moreover, the viscosity of calcium aluminate (-ferrite) melts is significantly
lower than that of the calcium silicate-aluminate-ferrite melts formed when Portland
clinker is produced.
C
alcium aluminate cements with lower Al
2
O
3
contents and distinct amounts of Fe
2
O
3
are commonly produced by complete fusion of a blend of limestone and bauxite at
temperatures of 1450-1600°C in a modified shaft kiln. At these temperatures the whole
material converts to a melt, and is collected at the bottom of the kiln. The melted material
is tapped off, and crystallization of the melt takes place in the course of cooling outside
the kiln. The cooled product appears as a gray, finely grained compact rock. The burning
is usually done under oxidizing conditions, but burning under reducing conditions is also
possible.
Subsequently, the high-alumina clinker is crushed and ground to a fine powder in a ball
mill. The clinker is very hard to grind, and the process is associated with a high power
consumption. Unlike Portland cement, no set controlling constituents, such as gypsum,
need to be interground with the burnt material.
H
igh-Al
2
O
3
calcium aluminate cements are produced by solid-state sintering of
calcined alumina and lime or limestone in rotary kilns. In a solid-state reaction between
calcium oxide and aluminum oxide Ca
2+
, together with O
2−
, migrates into the Al
2
O
3
to
form calcium aluminates. Of these C
12
A
7
has the highest growth velocity, irrespective of
the C/A ratio of the starting material, and other phases are formed as secondary products
from this phase and the excess oxide (Scian
et al.,
1987). In addition to calcium
aluminate phases, the resultant product may also contain distinct amounts of residual
non-reacted alumina.
10.5
COMPOSITION OF CALCIUM ALUMINATE CEMENT
Table 10.1 shows the range of chemical composition of calcium aluminate cement. The
Al
2
O
3
content of this binder may vary over a wide range, depending on the existing phase
composition. The CaO content may also vary widely, and it declines with increasing
amount of Al
2
O
3
. Distinct amounts of silica and iron oxide are usually present in cements
with lower alumina contents. Bivalent iron is typical for cements produced under
reducing conditions. TiO
2
, MgO, SO
3
, and alkali oxides are usually present only in small
amounts, and are of little significance.
The phases present in cement produced by a fusion process are formed by
crystallization from the melt. However, equilibrium conditions are not necessarily
