Civil Engineering Reference
In-Depth Information
aggregate that contains SiO
2
in a reactive form (such as opal, flint, chert, some silicate
minerals, volcanic glasses, and even sufficiently strained or microcrystalline quartz) an
interaction between the alkaline hydroxide present in the pore solution and the SiO
2
of
the aggregate may take place, resulting in an expansion of the material and eventual
cracking. Such a process is called
alkali-silica (or silicate) reaction
. Alkaline
hydroxides of the pore solution may also react with some dolomitic rocks containing—in
addition to dolomite—clay minerals, and the resulting
dedolomitization reaction
may
also cause expansion and cracking. A necessary condition for such reactions to occur is a
sufficiently high hydroxide content in the pore solution caused by a high alkali content of
the clinker. (For more details see Chapter 22.)
To avoid expansion and concrete damage even in the presence of alkali-sensitive
aggregates the content of equivalent NaO (defined as Na
2
O
e
= Na
2
O+0.66K
2
O) in the
concrete mix should not exceed 4 or even 3 kg/m
3
. To meet this criterion a special
Portland cement with particularly low alkali content must be employed. In such a cement
the equivalent Na
2
O content should not exceed 0.6%.
Low-alkali Portland cement may be produced by using starting materials with
sufficiently low alkali contents; however, in most instances such materials are not
available. In such situations some of the alkali metals present in the starting materials
must be removed in the course of manufacture.
In the course of clinker burning a part of the alkalis present in the material become
volatilized in the sintering zone, in which temperatures of the raw mix of around 1450 °C
are reached. The volatility of alkalis is especially high if they can volatize in the form of
sulfates: of those, potassium sulfate exhibits a significantly greater volatility than sodium
sulfate. The volatilized constituents are swept back with the hot gases, and if the kiln is
equipped with a preheater, most of the volatilized sulfates become reabsorbed by the cold
raw meal introduced into the system, and reenter the kiln. The fraction of alkali metals
that leave the system with the hot gases as gas or dust is normally negligible, and thus the
alkali content of the produced clinker corresponds roughly to that of the raw meal.
If a reduction of the alkali content in the clinker is required, part of the hot gases
carrying the alkaline compounds must be diverted though a bypass as they are leaving the
rotary kiln and before they enter the preheater or precalciner. The dust of the bypass
gases, rich in alkalis, is then filtered out and disposed of. The fraction of hot gases that
must be diverted through the bypass system depends on the quantities of alkalis that have
to be removed: in most instances a diversion of 10-20% of the gases through the bypass
is sufficient to reduce the alkali content of the clinker to an acceptable level. The process
is associated with a loss of thermal energy, as heat cannot be recovered from the hot
gases that have been diverted.
To increase the amounts of alkalis undergoing volatilization in the sintering zone, and
thus to reduce the alkali content of the produced clinker even further, small amounts of
calcium chloride may be added to the raw meal (Puscasu and Dimitrescu, 1994). Under
these conditions additional amounts of alkalis escape in the form of potassium and
sodium chloride, which are substances with an even greater tendency to volatilize than
the corresponding sulfates.
To reduce the adverse effect of alkalis on the free lime content and ultimate strength, it
has been recommended that gypsum should be added to the raw meal in amounts of up to
