Page 99 - Handbook of Thermal Analysis of Construction Materials

82 Chapter 3 - Formation and Hydration

difference in reactivities of the clinkers produced by these two methods by
applying the DTA technique. [26] By conducting DTA of clinkers hydrated
for various periods, it was concluded that the rate of hydration, as deter-
mined from the peak intensities of Ca(OH) , was lower for clinkers made
2
in the rotary kiln.

4.0 SYNTHESIS OF CEMENT PHASES

DTA of binary, ternary, and quarternary systems of relevance to
clinkers has been studied. Mixtures containing various proportions of
oxides such as calcium oxide, aluminum oxide, ferric oxide, and silica are
heated to high temperatures in the DTA apparatus and the endothermal and
exothermal effects that develop are analyzed.
In the heating of a kaolin-lime mixtures, gehlenite (C AS) and a
2
small amount of β-C S have been detected at 900°C. At 1050°C, formation
2
of C A occurs. At the temperatures of 1100°C and 1400°C, C A and C S
12 7 3 3
[1]
are formed respectively. In the thermograms of Ca(OH) -SiO mixtures,
2
2
several endothermal and exothermal effects result that can be attributed to
the formation of β-C S, γ-C S, Ca(OH) , etc.
2
2
2
DTA has been applied to investigate the efficiency of silicate
materials to form C S when they are heated with CaO. In Fig. 6 thermo-
3
grams of mixtures of CaO and siliceous materials such as silica gel, quartz,
silica glass, α-CS or β-CS (synthetic glass), C S and γ-C S are given. [27]
2
3 2
The weak endotherms at about 1175K and 1225K may be attributed to C S
3
transition. The endothermal inflection at about 1700K corresponds to the
α´ to α-C S transition. By determining the peak intensities of the endother-
2
mal effects due to C S transitions, it is possible to assess the relative
3
amounts of C S formed in various mixes. The results show that the largest
3
amount of C S is formed in the mixture γ -C S+CaO and the lowest, in the
3
2
silica gel + CaO mixture.
It has been observed that the nature of the reactions in the system
CaO-SiO may not reflect that occurring in the kiln because of the preva-
2
lence of CO in the kiln. Hence, some work has been carried out on the
2
system containing CaCO and SiO . This mixture when heated shows
3 2
endothermal effects at 900 and 976°C that are attributed to triclinic-
[1]
monoclinic-trigonal transformations in C S. According to De Keyser’s
3
work, in the system CaCO -SiO , the formation of C S occurs at 808°C,
2
2
3
transformation of γ-C S to α´ at 912°C, and the transformation of α´-C S
2 2
to β-C S at 965°C. [1]
2