Page 98 - Handbook of Thermal Analysis of Construction Materials

Section 3.0 - Clinkerization 81

Another application of DTA/TG is to assess the quality of the
unhydrated cement by identifying and estimating the amounts of CH, H O,
2
and carbonates. Only minimum amounts of these (formed by aeration)
should be present in the raw cement for it to be of good quality. The quality
of oil cements has been evaluated applying this approach. [19]
In the production of sulfoaluminate cement from raw materials
such as limestone, bauxite, and anhydrite, DTA has been used to monitor
the reactions that take place when these mixes are heated to a temperature
of 1025°C. By applying DTA and XRD it was found that the dehydration
and decomposition of bauxite occurs at 530°C. At about 900°C calcium
carbonate is decomposed to CaO and it reacts with α-SiO to form C AS.
2
2
At a higher temperature, C AS reacts with CaSO to form calcium
2 4
sulfoaluminate and α-C S. [22] Thermal analysis has also been applied to
2
investigate the reactions occurring in the formation of a clinker from
calcium carbonate mixed with CMS , C AS , and NAS . [23]
3
2
6
3
There has been a continued interest in producing cement clinker at
lower temperatures than what has been the normal practice. A study of
binary and ternary systems containing limestone, alkaline basalt and
fluorite has been conducted to study the possibility of obtaining belite and
alite at lower temperatures. [24] The reaction was followed by DTA, DTG,
and TG. Several endothermal and exothermal peaks were obtained. De-
carbonation occurred at about 650°C, and a peak at 1145°C could be linked
to the formation of C A and gehlenite. The peak at 1170°C was attributed
7
12
to the formation of belite, and that at 1235°C to the formation of alite.
Extensive work has been carried out on the utilization of waste and
by-products in concrete industry. The utilization of cement kiln dust is an
example of a waste material that is also a large source of pollution at the
cement plant. In an investigation, a mixture of kiln dust and kaolin was fired
up to 1250°C and the resulting phases were studied by DTA and XRD. [25]
Several peaks (two exothermal and five endothermal) were obtained in
DTA. The DTA examination revealed that the dust consists of dolomitic
limestones with some alkalis and quartz. At 1000°C, β-C S formed as the
2
main phase with some C A and C AF and at 1100°C spurrite decompo-
4
12
7
sition occurred. At 1250°C gehlenite was formed. This study was useful in
assessing the temperature to which the dust should be calcined so that the
resultant product could be utilized for making porcelain.
Although most cement clinkers are manufactured utilizing the
rotary kilns, in some countries vertical kilns have been used. In addition to
the mechanism of clinkering, the heating and cooling schedules in these two
types of kilns are different. A study was undertaken to determine the