Special Portland cements - Special Inorganic Cements - page 35

In handling mixes made with low-porosity cement, their particular rheology and setting
behavior must be taken into consideration. So far, low-porosity cements have been
produced and applied on an industrial scale only in the former Czechoslovakia (Skvara et
al., 1981).
2.17
SPECIAL APPROACHES IN PORTLAND CEMENT MANUFACTURE
In conventional production of Portland clinker a preground and adequately homogenized
blend of starting materials with the desired oxide composition, commonly called raw
meal, is fired in a rotary kiln at a maximum temperature of about 1450-1500 °C. Coal
or—less often—mineral oil or natural gas serve as sources of thermal energy. The non-
volatile inorganic constituents of the fuel combine with the raw meal and eventually
become constituents of the resulting clinker. In most production units a preheater is
installed in front of the rotary kiln in which the raw meal is preheated by the hot gases
leaving the kiln, for better fuel economy. In some units a precalciner is also installed
between the rotary kiln and preheater, in which the CaCO 3 present in the raw meal is
decomposed to CaO +CO 2 before it enters the kiln. The hot clinker leaving the kiln is
cooled down to acceptable temperatures, and finally is ground, together with added
gypsum or anhydrite, to the desired fineness.
In the production of the raw meal some non-conventional starting materials, instead of
or in addition to limestone, clay, marl, or quartz, are sometimes used. They may include
fly ash (Odler and Zhang, 1997), blast furnace slag (Blanco-Varela et al., 1988), and a
variety of other industrial by-products or wastes. Out of these, some may also contain
variable amounts of organic residues that supply part of the required thermal energy.
Additional amounts of energy may be saved if the starting mix contains distinct amounts
of decarbonated CaO, as is the case in some industrial slags or CaO-rich ashes.
It is possible to intensify the clinker mineral formation process by adding to the raw
meal small amounts of a mineralizer/fluxing agent (Gouda, 1980; Odler and Abdul-
Maula, 1980a, 1980b; Odler, 1991; Surana and Jeshi, 1990). Of these, calcium fluoride
and other fluorine compounds are the most effective and the most widely used (Klemm et
al., 1979; Gouda, 1980; Odler and Abdul-Maula, 1980a, 1980b; Blanco-Varela et al.,
1984; Surana and Jeshi, 1990; Ayed et al., 1992). Other known mineralizers/fluxes
include ZnO (Tsuboi et al., 1972; Knöfel, 1978; Gouda, 1980; Odler and Abdul-Maula,
1980b; Baeker et al., 1983; Kakali and Parissakis, 1995), MgO (Tsuboi, 1972; Gouda,
1980), and CuO (Odler and Abdul-Maula, 1980b; Kakali et al., 1996). A mineralizer/flux
considered especially effective is a combination of CaF 2 and CaSO 4 (Tong and Lin,
1986; Borgholm et al., 1995; Blanco-Varela et al., 1996; Odler and Zhang, 1996), or
even a combination of CaF 2 , CaSO 4 , and ZnO (Huang et al., 1992; Lu et al., 1992). It has
to be stressed, however, that mineralizers/fluxing agents added to the raw meal not only
accelerate the formation of Portland clinker, but also influence its structure and the
properties of the resultant cement (Odler and Abdul-Maula, 1980a, 1980b; Page et al.,
1986; Borgholm et al., 1995; Odler and Zhang, 1996).
To intensify the formation of clinker without the use of mineralizers/fluxes it has been