Page 94 - Handbook of Thermal Analysis of Construction Materials

Section 3.0 - Clinkerization 77

Ferrogenous minerals have been used as fluxes to form the melt and
to facilitate the clinkerization at lower temperatures. The types of minerals
used include hematite, magnetite, goethite, limonite, siderite, and ankerite.
Thermal methods may be used to identify them.
Mineralizers are added to the raw feed to accelerate the kinetics of
reactions by modifying the solid and liquid state sintering. The temperature
of decomposition of calcium carbonate is lowered in the presence of
mineralizers. In the synthesis of C S and C S, DTA has shown that some
3
2
phosphates, carbonates, sulfates, and chlorides decrease the decarbonation
temperature and that of the formation of the silicates. [20] Calcium fluoride
acts both as a flux and a mineralizer in promoting the formation the
tricalcium silicate phase. DTA thermograms have been applied to study the
effect of mineralizers on the melting behaviors and crystallization tempera-
tures of clinker. Early formation of liquid can be established by thermal
techniques. [5]

3.0 CLINKERIZATION

The chemical processes that are involved in the formation of
clinker are very complex. Thermal Analysis, XRD, and IR techniques have
been applied for studying the reactions during the calcination of cement raw
materials. The rates of reactions are influenced by many factors resulting
in variations in temperatures at which the processes occur. Only approxi-
mate temperatures are assigned for the reactions. At a temperature of about
100°C evaporation of water takes place, at > 500°C, combined water from
calcium hydroxide is expelled, and at > 800°C, calcium carbonate and
magnesium carbonate decompose with the formation of CA and C F and
2
C S begins to form. Between 800 and 900°C, C A is formed, and at 900–
12
2
7
1000°C, C A and C AF begin to appear and calcium carbonate is com-
3 4
pletely decomposed. The production of both C A and C AF is completed
3
4
at about 1000–1200°C and the formation of C S reaches the maximum. At
2
1200–1280°C, liquid appears, with the occurrence of a substantial amount
of C S between 1200 and 1450°C. [11] Co-existence of phases is also well
3
documented. It has been found that the reactivity of clay minerals with
calcium carbonate increases in the order: muscovite > montmorillonite >
chlorite > illite > kaolin, and the reactivity of silica with CaO increases in
the order: quartz > chalcedony > opal > cristobalite > tridymite-silica (from
feldspar) > silica (from mica) > silica (from glassy slag). A comprehensive