Question

The phase relations in the system of CaO-SiO2 show the possibility of C3S formation at temperature range from 1250 to 2150 °C. However, to produce pure C3S phase, generally, over 1600 °C is suggested, which is considerably higher than the 1450 °C in an industrial kiln. Explain why it is possible to form C3S at this lower temperature in the industrial kiln.

Answer 1

In the burning zone, the chemical and physical changes of the material take place simultaneously; it is important with respect to the kinetics of clinkerization reactions and agglomeration processes. The exact temperature of melt formation depends on the chemical composition of the feed. Under certain conditions, melt may be formed at temperatures lower than -1250°C. For instance, quartz grains in the kiln feed will give rise to melt at 1200°C or lower. The surrounding CaO will partly agglomerate and form crystals of Belite, which can be identified in the clinker as Belite nests. Also, the alkalis, sulfates, and chlorides present in the kiln system will form liquid phases at lower temperatures than the proper clinker liquid. However, the largest amount of clinker melt is formed within a narrow temperature interval and over a short distance in the rotary kiln. The coating formed on the brick lining reflects this. In the upper transition zone or the entrance to the burning zone, the soft, relatively thin coating changes to a dark, clinker like, hard, thick coating within half a meter. So, the C₃S formation depending on the time and temperature treatment of the mix, and overall chemical composition of the kiln feed.

The partial melting causes the material to aggregate into lumps or nodules, typically of diameter 1–10 mm. This is called clinker.hence we prefer lower temperature.

Formation of the desired clinker minerals involves heating the rawmix through the temperature stages mentioned above. The finishing transformation that takes place in the hottest part of the kiln, under the flame, is the reaction of belite (Ca₂SiO₄) with calcium oxide to form alite (Ca₃O·SiO₅):

Ca₂SiO₄ + CaO → Ca₃SiO₅

Also abbreviated in the cement chemist notation (CCN) as:

C₂S + C → C₃S

Tricalcium silicate is thermodynamically unstable below 1250 °C, but can be preserved in a metastable state at room temperature by fast cooling: on slow cooling it tends to revert to belite (Ca₂SiO₄) and CaO.